SERVER-65002 Change 'unstable' boolean to 'stability' enum and allow 'internal' stability

27 files changed, 957 insertions, 574 deletions

diff --git a/buildscripts/idl/idl/ast.py b/buildscripts/idl/idl/ast.py
index 840ee8689da..8b4e4de90fc 100644
--- a/buildscripts/idl/idl/ast.py
+++ b/buildscripts/idl/idl/ast.py
@@ -207,7 +207,7 @@ class Field(common.SourceLocation):
         self.chained = False  # type: bool
         self.comparison_order = -1  # type: int
         self.non_const_getter = False  # type: bool
-        self.unstable = None  # type: Optional[bool]
+        self.stability = None  # type: Optional[str]
         self.default = None  # type: str
         self.type = None  # type: Type
         self.always_serialize = False  # type: bool
diff --git a/buildscripts/idl/idl/binder.py b/buildscripts/idl/idl/binder.py
index c9f669c9b47..84966494572 100644
--- a/buildscripts/idl/idl/binder.py
+++ b/buildscripts/idl/idl/binder.py
@@ -1005,7 +1005,9 @@ def _bind_field(ctxt, parsed_spec, field):
     ast_field.constructed = field.constructed
     ast_field.comparison_order = field.comparison_order
     ast_field.non_const_getter = field.non_const_getter
-    ast_field.unstable = field.unstable
+    # Ignore the 'unstable' field since it's deprecated by the 'stability' field and only there at parsing level
+    # to provide compatibility support.
+    ast_field.stability = field.stability
     ast_field.always_serialize = field.always_serialize
 
     ast_field.cpp_name = field.name
diff --git a/buildscripts/idl/idl/errors.py b/buildscripts/idl/idl/errors.py
index 47cd8ecf3f4..3fcf0828249 100644
--- a/buildscripts/idl/idl/errors.py
+++ b/buildscripts/idl/idl/errors.py
@@ -112,7 +112,7 @@ ERROR_ID_NON_CONST_GETTER_IN_IMMUTABLE_STRUCT = "ID0069"
 ERROR_ID_FEATURE_FLAG_DEFAULT_TRUE_MISSING_VERSION = "ID0070"
 ERROR_ID_FEATURE_FLAG_DEFAULT_FALSE_HAS_VERSION = "ID0071"
 ERROR_ID_INVALID_REPLY_TYPE = "ID0072"
-ERROR_ID_UNSTABLE_NO_API_VERSION = "ID0073"
+ERROR_ID_STABILITY_NO_API_VERSION = "ID0073"
 ERROR_ID_MISSING_REPLY_TYPE = "ID0074"
 ERROR_ID_USELESS_VARIANT = "ID0076"
 ERROR_ID_ILLEGAL_FIELD_ALWAYS_SERIALIZE_NOT_OPTIONAL = "ID0077"
@@ -128,6 +128,8 @@ ERROR_ID_DUPLICATE_ACCESS_CHECK = "ID0087"
 ERROR_ID_DUPLICATE_PRIVILEGE = "ID0088"
 ERROR_ID_EMPTY_ACCESS_CHECK = "ID0089"
 ERROR_ID_MISSING_ACCESS_CHECK = "ID0090"
+ERROR_ID_STABILITY_UNKNOWN_VALUE = "ID0091"
+ERROR_ID_DUPLICATE_UNSTABLE_STABILITY = "ID0092"
 
 
 class IDLError(Exception):
@@ -882,13 +884,13 @@ class ParserContext(object):
             location, ERROR_ID_INVALID_REPLY_TYPE,
             ("Command '%s' has invalid reply_type '%s'" % (command_name, reply_type_name)))
 
-    def add_unstable_no_api_version(self, location, command_name):
+    def add_stability_no_api_version(self, location, command_name):
         # type: (common.SourceLocation, str) -> None
-        """Add an error about a command with 'unstable' but no 'api_version'."""
+        """Add an error about a command with 'stability' but no 'api_version'."""
         # pylint: disable=invalid-name
         self._add_error(
-            location, ERROR_ID_UNSTABLE_NO_API_VERSION,
-            ("Command '%s' specifies 'unstable' but has no 'api_version'" % (command_name, )))
+            location, ERROR_ID_STABILITY_NO_API_VERSION,
+            ("Command '%s' specifies 'stability' but has no 'api_version'" % (command_name, )))
 
     def add_missing_reply_type(self, location, command_name):
         # type: (common.SourceLocation, str) -> None
@@ -967,6 +969,23 @@ class ParserContext(object):
         self._add_error(location, ERROR_ID_MISSING_ACCESS_CHECK,
                         'Command "%s" has api_version != "" but is missing access_check.' % (name))
 
+    def add_stability_unknown_value(self, location):
+        # type: (common.SourceLocation) -> None
+        """Add an error about a field with unknown value set to 'stability' option."""
+        # pylint: disable=invalid-name
+        self._add_error(
+            location, ERROR_ID_STABILITY_UNKNOWN_VALUE,
+            "Field option 'stability' has unknown value, should be one of 'stable', 'unstable' or 'internal.'"
+        )
+
+    def add_duplicate_unstable_stability(self, location):
+        # type: (common.SourceLocation) -> None
+        """Add an error about a field specifying both 'unstable' and 'stability'."""
+        # pylint: disable=invalid-name
+        self._add_error(location, ERROR_ID_DUPLICATE_UNSTABLE_STABILITY, (
+            "Field specifies both 'unstable' and 'stability' options, should use 'stability: [stable|unstable|internal]' instead and remove the deprecated 'unstable' option."
+        ))
+
 
 def _assert_unique_error_messages():
     # type: () -> None
diff --git a/buildscripts/idl/idl/generator.py b/buildscripts/idl/idl/generator.py
index 0adbeaedd28..a4ea20a4a33 100644
--- a/buildscripts/idl/idl/generator.py
+++ b/buildscripts/idl/idl/generator.py
@@ -217,7 +217,7 @@ class _FastFieldUsageChecker(_FieldUsageCheckerBase):
         self._writer.write_line('usedFields.set(%s);' % (_gen_field_usage_constant(field)))
         self._writer.write_empty_line()
 
-        if field.unstable:
+        if field.stability == 'unstable':
             self._writer.write_line(
                 'ctxt.throwAPIStrictErrorIfApplicable(%s);' % (bson_element_variable))
             self._writer.write_empty_line()
diff --git a/buildscripts/idl/idl/parser.py b/buildscripts/idl/idl/parser.py
index 870f6142f68..b4d47c2c0c1 100644
--- a/buildscripts/idl/idl/parser.py
+++ b/buildscripts/idl/idl/parser.py
@@ -349,7 +349,11 @@ def _parse_field(ctxt, name, node):
     field.name = name
 
     _generic_parser(
-        ctxt, node, "field", field, {
+        ctxt,
+        node,
+        "field",
+        field,
+        {
             "description":
                 _RuleDesc('scalar'),
             "cpp_name":
@@ -371,8 +375,11 @@ def _parse_field(ctxt, name, node):
                 _RuleDesc('mapping', mapping_parser_func=_parse_validator),
             "non_const_getter":
                 _RuleDesc("bool_scalar"),
+            # Allow both 'unstable' and the new alternative 'stability' options to support IDL compatibility tests with old IDLs.
             "unstable":
                 _RuleDesc("bool_scalar"),
+            "stability":
+                _RuleDesc("scalar"),
             "always_serialize":
                 _RuleDesc("bool_scalar"),
         })
@@ -418,6 +425,22 @@ def _parse_fields(ctxt, node):
         fields.append(field)
         field_name_set.add(first_name)
 
+    for field in fields:
+        if field.unstable is not None and field.stability is not None:
+            ctxt.add_duplicate_unstable_stability(field)
+
+        # Convert the deprecated 'unstable' option to the new 'stability' option to keep support for the IDL compatibility tests.
+        if field.unstable is not None:
+            if field.unstable:
+                field.stability = 'unstable'
+            else:
+                field.stability = 'stable'
+            field.unstable = None
+
+        if field.stability is not None and field.stability not in ("stable", "unstable",
+                                                                   "internal"):
+            ctxt.add_stability_unknown_value(field)
+
     return fields
 
 
@@ -853,8 +876,8 @@ def _parse_command(ctxt, spec, name, node):
 
     if not command.api_version:
         for field in command.fields:
-            if field.unstable:
-                ctxt.add_unstable_no_api_version(field, command.name)
+            if field.stability is not None and field.stability != 'stable':
+                ctxt.add_stability_no_api_version(field, command.name)
 
     spec.symbols.add_command(ctxt, command)
 
diff --git a/buildscripts/idl/idl/syntax.py b/buildscripts/idl/idl/syntax.py
index 1e24a445df2..83245954cc1 100644
--- a/buildscripts/idl/idl/syntax.py
+++ b/buildscripts/idl/idl/syntax.py
@@ -466,6 +466,7 @@ class Field(common.SourceLocation):
         self.validator = None  # type: Validator
         self.non_const_getter = False  # type: bool
         self.unstable = None  # type: Optional[bool]
+        self.stability = None  # type: Optional[str]
         self.always_serialize = False  # type: bool
 
         # Internal fields - not generated by parser
diff --git a/buildscripts/idl/idl_check_compatibility.py b/buildscripts/idl/idl_check_compatibility.py
index a436a098ce5..8dda48b6462 100644
--- a/buildscripts/idl/idl_check_compatibility.py
+++ b/buildscripts/idl/idl_check_compatibility.py
@@ -208,6 +208,18 @@ IGNORE_STABLE_TO_UNSTABLE_LIST: List[str] = [
     # The 'runtimeConstants' field is a legacy field for internal use only and is not documented to
     # users.
     'delete-param-runtimeConstants',
+    # The 'isTimeseriesNamespace' field is sent from mongos to shards for internal use.
+    'collMod-param-isTimeseriesNamespace',
+    'createIndexes-param-isTimeseriesNamespace',
+    'dropIndexes-param-isTimeseriesNamespace',
+    'listIndexes-param-isTimeseriesNamespace',
+    'insert-param-isTimeseriesNamespace',
+    'update-param-isTimeseriesNamespace',
+    'delete-param-isTimeseriesNamespace',
+    'findAndModify-param-isTimeseriesNamespace',
+    # The 'needsMerge' and 'fromMongos' fields of aggregation are sent from mongos to shards for internal use.
+    'aggregate-param-needsMerge',
+    'aggregate-param-fromMongos',
 ]
 
 # Once a field is part of the stable API, either by direct addition or by changing it from unstable
@@ -321,13 +333,13 @@ class FieldCompatibility:
     """Information about a Field to check compatibility."""
 
     def __init__(self, field_type: Optional[Union[syntax.Enum, syntax.Struct, syntax.Type]],
-                 idl_file: syntax.IDLParsedSpec, idl_file_path: str, unstable: Optional[bool],
+                 idl_file: syntax.IDLParsedSpec, idl_file_path: str, stability: Optional[str],
                  optional: bool) -> None:
         """Initialize data members and hand special cases, such as optionalBool type."""
         self.field_type = field_type
         self.idl_file = idl_file
         self.idl_file_path = idl_file_path
-        self.unstable = unstable
+        self.stability = stability
         self.optional = optional
 
         if isinstance(self.field_type, syntax.Type) and self.field_type.name == "optionalBool":
@@ -358,6 +370,11 @@ class ArrayTypeCheckResult(Enum):
     FALSE = 2
 
 
+def is_unstable(stability: Optional[str]) -> bool:
+    """Check whether the given stability value is considered as unstable."""
+    return stability is not None and stability != 'stable'
+
+
 def get_new_commands(
         ctxt: IDLCompatibilityContext, new_idl_dir: str, import_directories: List[str]
 ) -> Tuple[Dict[str, syntax.Command], Dict[str, syntax.IDLParsedSpec], Dict[str, str]]:
@@ -465,7 +482,8 @@ def check_reply_field_type_recursive(ctxt: IDLCompatibilityContext,
     # bson_serialization_type. For all other errors, we check that the old field is stable
     # before adding an error.
     if not isinstance(new_field_type, syntax.Type):
-        if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+        if not is_unstable(
+                old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
             ctxt.add_new_reply_field_type_enum_or_struct_error(
                 cmd_name, field_name, new_field_type.name, old_field_type.name,
                 new_field.idl_file_path)
@@ -496,12 +514,16 @@ def check_reply_field_type_recursive(ctxt: IDLCompatibilityContext,
                                                           new_field.idl_file_path)
 
         # If serializer is changed, it's a potential breaking change.
-        if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST and old_field_type.serializer != new_field_type.serializer:
+        if not is_unstable(
+                old_field.stability
+        ) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST and old_field_type.serializer != new_field_type.serializer:
             ctxt.add_reply_field_serializer_not_equal_error(
                 cmd_name, field_name, new_field_type.name, new_field.idl_file_path)
 
         # If deserializer is changed, it's a potential breaking change.
-        if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST and old_field_type.deserializer != new_field_type.deserializer:
+        if not is_unstable(
+                old_field.stability
+        ) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST and old_field_type.deserializer != new_field_type.deserializer:
             ctxt.add_reply_field_deserializer_not_equal_error(
                 cmd_name, field_name, new_field_type.name, new_field.idl_file_path)
 
@@ -517,10 +539,10 @@ def check_reply_field_type_recursive(ctxt: IDLCompatibilityContext,
                 if old_variant_type.name == new_variant_type.name:
                     # Check that the old and new version of each variant type is also compatible.
                     old = FieldCompatibility(old_variant_type, old_field.idl_file,
-                                             old_field.idl_file_path, old_field.unstable,
+                                             old_field.idl_file_path, old_field.stability,
                                              old_field.optional)
                     new = FieldCompatibility(new_variant_type, new_field.idl_file,
-                                             new_field.idl_file_path, new_field.unstable,
+                                             new_field.idl_file_path, new_field.stability,
                                              new_field.optional)
                     check_reply_field_type(ctxt,
                                            FieldCompatibilityPair(old, new, cmd_name, field_name))
@@ -528,7 +550,8 @@ def check_reply_field_type_recursive(ctxt: IDLCompatibilityContext,
 
             else:
                 # new_variant_type was not found in old_variant_types.
-                if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+                if not is_unstable(old_field.stability
+                                   ) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
                     ctxt.add_new_reply_field_variant_type_not_subset_error(
                         cmd_name, field_name, new_variant_type.name, new_field.idl_file_path)
 
@@ -536,7 +559,8 @@ def check_reply_field_type_recursive(ctxt: IDLCompatibilityContext,
         # Since enums can't be part of variant types, we don't explicitly check for enums.
         if isinstance(new_field_type,
                       syntax.VariantType) and new_field_type.variant_struct_type is not None:
-            if old_field_type.variant_struct_type is None and not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+            if old_field_type.variant_struct_type is None and not is_unstable(
+                    old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
                 ctxt.add_new_reply_field_variant_type_not_subset_error(
                     cmd_name, field_name, new_field_type.variant_struct_type.name,
                     new_field.idl_file_path)
@@ -546,7 +570,8 @@ def check_reply_field_type_recursive(ctxt: IDLCompatibilityContext,
                                    new_field.idl_file, old_field.idl_file_path,
                                    new_field.idl_file_path)
 
-    elif not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+    elif not is_unstable(
+            old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
         if isinstance(new_field_type, syntax.VariantType):
             ctxt.add_new_reply_field_variant_type_error(cmd_name, field_name, old_field_type.name,
                                                         new_field.idl_file_path)
@@ -565,7 +590,7 @@ def check_reply_field_type(ctxt: IDLCompatibilityContext, field_pair: FieldCompa
     field_name = field_pair.field_name
     array_check = check_array_type(ctxt, "reply_field", old_field.field_type, new_field.field_type,
                                    field_pair.cmd_name, 'type', old_field.idl_file_path,
-                                   new_field.idl_file_path, old_field.unstable)
+                                   new_field.idl_file_path, is_unstable(old_field.stability))
     if array_check == ArrayTypeCheckResult.INVALID:
         return
 
@@ -591,9 +616,8 @@ def check_reply_field_type(ctxt: IDLCompatibilityContext, field_pair: FieldCompa
     if isinstance(old_field_type, syntax.Type):
         check_reply_field_type_recursive(ctxt, field_pair)
 
-    elif isinstance(
-            old_field_type, syntax.Enum
-    ) and not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+    elif isinstance(old_field_type, syntax.Enum) and not is_unstable(
+            old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
         if isinstance(new_field_type, syntax.Enum):
             check_subset(ctxt, cmd_name, field_name, new_field_type.name, new_field_type.values,
                          old_field_type.values, new_field.idl_file_path)
@@ -606,7 +630,8 @@ def check_reply_field_type(ctxt: IDLCompatibilityContext, field_pair: FieldCompa
             check_reply_fields(ctxt, old_field_type, new_field_type, cmd_name, old_field.idl_file,
                                new_field.idl_file, old_field.idl_file_path, new_field.idl_file_path)
         else:
-            if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+            if not is_unstable(
+                    old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
                 ctxt.add_new_reply_field_type_not_struct_error(
                     cmd_name, field_name, new_field_type.name, old_field_type.name,
                     new_field.idl_file_path)
@@ -652,8 +677,10 @@ def check_reply_field(ctxt: IDLCompatibilityContext, old_field: syntax.Field,
     new_field_optional = new_field.optional or (new_field_type
                                                 and new_field_type.name == "optionalBool")
     ignore_list_name: str = cmd_name + "-reply-" + new_field.name
-    if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
-        if new_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+    if not is_unstable(
+            old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+        if is_unstable(
+                new_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
             ctxt.add_new_reply_field_unstable_error(cmd_name, new_field.name, new_idl_file_path)
         if new_field_optional and not old_field_optional:
             ctxt.add_new_reply_field_optional_error(cmd_name, new_field.name, new_idl_file_path)
@@ -668,14 +695,15 @@ def check_reply_field(ctxt: IDLCompatibilityContext, old_field: syntax.Field,
                                                               new_idl_file_path)
 
     # A reply field may not change from unstable to stable unless explicitly allowed to.
-    if old_field.unstable and not new_field.unstable and ignore_list_name not in ALLOWED_STABLE_FIELDS_LIST:
+    if is_unstable(old_field.stability) and not is_unstable(
+            new_field.stability) and ignore_list_name not in ALLOWED_STABLE_FIELDS_LIST:
         ctxt.add_unstable_reply_field_changed_to_stable_error(cmd_name, new_field.name,
                                                               new_idl_file_path)
 
     old_field_compatibility = FieldCompatibility(old_field_type, old_idl_file, old_idl_file_path,
-                                                 old_field.unstable, old_field.optional)
+                                                 old_field.stability, old_field.optional)
     new_field_compatibility = FieldCompatibility(new_field_type, new_idl_file, new_idl_file_path,
-                                                 new_field.unstable, new_field.optional)
+                                                 new_field.stability, new_field.optional)
     field_pair = FieldCompatibilityPair(old_field_compatibility, new_field_compatibility, cmd_name,
                                         old_field.name)
 
@@ -699,9 +727,9 @@ def check_reply_fields(ctxt: IDLCompatibilityContext, old_reply: syntax.Struct,
                         and resolved_old_chained_type.name == resolved_new_chained_type.name):
                     # Check that the old and new version of each chained type is also compatible.
                     old = FieldCompatibility(resolved_old_chained_type, old_idl_file,
-                                             old_idl_file_path, unstable=False, optional=False)
+                                             old_idl_file_path, stability='stable', optional=False)
                     new = FieldCompatibility(resolved_new_chained_type, new_idl_file,
-                                             new_idl_file_path, unstable=False, optional=False)
+                                             new_idl_file_path, stability='stable', optional=False)
 
                     check_reply_field_type(
                         ctxt, FieldCompatibilityPair(old, new, cmd_name, old_reply.name))
@@ -724,14 +752,14 @@ def check_reply_fields(ctxt: IDLCompatibilityContext, old_reply: syntax.Struct,
 
                 break
 
-        if not new_field_exists and not old_field.unstable:
+        if not new_field_exists and not is_unstable(old_field.stability):
             ctxt.add_new_reply_field_missing_error(cmd_name, old_field.name, old_idl_file_path)
 
     for new_field in new_reply_fields or []:
-        # Check that all fields in the new IDL have specified the 'unstable' field.
-        if new_field.unstable is None:
-            ctxt.add_new_reply_field_requires_unstable_error(cmd_name, new_field.name,
-                                                             new_idl_file_path)
+        # Check that all fields in the new IDL have specified the 'stability' field.
+        if new_field.stability is None:
+            ctxt.add_new_reply_field_requires_stability_error(cmd_name, new_field.name,
+                                                              new_idl_file_path)
 
         # Check that newly added fields do not have an unallowed use of 'any' as the
         # bson_serialization_type.
@@ -742,7 +770,8 @@ def check_reply_fields(ctxt: IDLCompatibilityContext, old_reply: syntax.Struct,
 
         if newly_added:
             allow_name: str = cmd_name + "-reply-" + new_field.name
-            if not new_field.unstable and allow_name not in ALLOWED_STABLE_FIELDS_LIST:
+            if not is_unstable(
+                    new_field.stability) and allow_name not in ALLOWED_STABLE_FIELDS_LIST:
                 ctxt.add_new_reply_field_added_as_stable_error(cmd_name, new_field.name,
                                                                new_idl_file_path)
 
@@ -782,7 +811,8 @@ def check_param_or_command_type_recursive(ctxt: IDLCompatibilityContext,
     # before adding an error.
 
     if not isinstance(new_type, syntax.Type):
-        if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+        if not is_unstable(
+                old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
             ctxt.add_new_command_or_param_type_enum_or_struct_error(
                 cmd_name, new_type.name, old_type.name, new_field.idl_file_path, param_name,
                 is_command_parameter)
@@ -815,20 +845,23 @@ def check_param_or_command_type_recursive(ctxt: IDLCompatibilityContext,
                 cmd_name, new_type.name, new_field.idl_file_path, param_name, is_command_parameter)
 
         # If serializer is changed, it's a potential breaking change.
-        if (not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST
+        if (not is_unstable(old_field.stability)
+                and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST
             ) and old_type.serializer != new_type.serializer:
             ctxt.add_command_or_param_serializer_not_equal_error(
                 cmd_name, new_type.name, new_field.idl_file_path, param_name, is_command_parameter)
 
         # If deserializer is changed, it's a potential breaking change.
-        if (not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST
+        if (not is_unstable(old_field.stability)
+                and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST
             ) and old_type.deserializer != new_type.deserializer:
             ctxt.add_command_or_param_deserializer_not_equal_error(
                 cmd_name, new_type.name, new_field.idl_file_path, param_name, is_command_parameter)
 
     if isinstance(old_type, syntax.VariantType):
         if not isinstance(new_type, syntax.VariantType):
-            if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+            if not is_unstable(
+                    old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
                 ctxt.add_new_command_or_param_type_not_variant_type_error(
                     cmd_name, new_type.name, new_field.idl_file_path, param_name,
                     is_command_parameter)
@@ -845,17 +878,18 @@ def check_param_or_command_type_recursive(ctxt: IDLCompatibilityContext,
                         old_variant_type.name == new_variant_type.name:
                         # Check that the old and new version of each variant type is also compatible.
                         old = FieldCompatibility(old_variant_type, old_field.idl_file,
-                                                 old_field.idl_file_path, old_field.unstable,
+                                                 old_field.idl_file_path, old_field.stability,
                                                  old_field.optional)
                         new = FieldCompatibility(new_variant_type, new_field.idl_file,
-                                                 new_field.idl_file_path, new_field.unstable,
+                                                 new_field.idl_file_path, new_field.stability,
                                                  new_field.optional)
                         check_param_or_command_type(
                             ctxt, FieldCompatibilityPair(old, new, cmd_name, param_name),
                             is_command_parameter)
                         break
                 else:
-                    if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+                    if not is_unstable(old_field.stability
+                                       ) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
                         # old_variant_type was not found in new_variant_types.
                         ctxt.add_new_command_or_param_variant_type_not_superset_error(
                             cmd_name, old_variant_type.name, new_field.idl_file_path, param_name,
@@ -871,12 +905,14 @@ def check_param_or_command_type_recursive(ctxt: IDLCompatibilityContext,
                         new_field.idl_file_path, is_command_parameter)
 
                 # If old type has a variant struct type and new type does not have a variant struct type.
-                elif not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+                elif not is_unstable(old_field.stability
+                                     ) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
                     ctxt.add_new_command_or_param_variant_type_not_superset_error(
                         cmd_name, old_type.variant_struct_type.name, new_field.idl_file_path,
                         param_name, is_command_parameter)
 
-    elif not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+    elif not is_unstable(
+            old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
         check_superset(ctxt, cmd_name, new_type.name, new_type.bson_serialization_type,
                        old_type.bson_serialization_type, new_field.idl_file_path, param_name,
                        is_command_parameter)
@@ -894,7 +930,7 @@ def check_param_or_command_type(ctxt: IDLCompatibilityContext, field_pair: Field
         ctxt, "command_parameter" if is_command_parameter else "command_namespace",
         old_field.field_type, new_field.field_type, field_pair.cmd_name,
         field_name if is_command_parameter else "type", old_field.idl_file_path,
-        new_field.idl_file_path, old_field.unstable)
+        new_field.idl_file_path, is_unstable(old_field.stability))
     if array_check == ArrayTypeCheckResult.INVALID:
         return
 
@@ -921,9 +957,8 @@ def check_param_or_command_type(ctxt: IDLCompatibilityContext, field_pair: Field
         check_param_or_command_type_recursive(ctxt, field_pair, is_command_parameter)
 
     # Only add type errors if the old field is stable.
-    elif isinstance(
-            old_type, syntax.Enum
-    ) and not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+    elif isinstance(old_type, syntax.Enum) and not is_unstable(
+            old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
         if isinstance(new_type, syntax.Enum):
             check_superset(ctxt, cmd_name, new_type.name, new_type.values, old_type.values,
                            new_field.idl_file_path, field_pair.field_name, is_command_parameter)
@@ -938,7 +973,8 @@ def check_param_or_command_type(ctxt: IDLCompatibilityContext, field_pair: Field
                 ctxt, old_type, new_type, cmd_name, old_field.idl_file, new_field.idl_file,
                 old_field.idl_file_path, new_field.idl_file_path, is_command_parameter)
         else:
-            if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+            if not is_unstable(
+                    old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
                 ctxt.add_new_command_or_param_type_not_struct_error(
                     cmd_name, new_type.name, old_type.name, new_field.idl_file_path,
                     field_pair.field_name, is_command_parameter)
@@ -996,9 +1032,9 @@ def check_command_params_or_type_struct_fields(
                         and resolved_old_chained_type.name == resolved_new_chained_type.name):
                     # Check that the old and new version of each chained type is also compatible.
                     old = FieldCompatibility(resolved_old_chained_type, old_idl_file,
-                                             old_idl_file_path, unstable=False, optional=False)
+                                             old_idl_file_path, stability="stable", optional=False)
                     new = FieldCompatibility(resolved_new_chained_type, new_idl_file,
-                                             new_idl_file_path, unstable=False, optional=False)
+                                             new_idl_file_path, stability="stable", optional=False)
                     check_param_or_command_type(
                         ctxt, FieldCompatibilityPair(old, new, cmd_name, old_struct.name),
                         is_command_parameter=False)
@@ -1029,16 +1065,17 @@ def check_command_params_or_type_struct_fields(
 
                 break
         allow_name: str = cmd_name + "-param-" + old_field.name
-        if not new_field_exists and not old_field.unstable and allow_name not in allow_list:
+        if not new_field_exists and not is_unstable(
+                old_field.stability) and allow_name not in allow_list:
             ctxt.add_new_param_or_command_type_field_missing_error(
                 cmd_name, old_field.name, old_idl_file_path, old_struct.name, is_command_parameter)
 
     # Check if a new field has been added to the parameters or type struct.
     # If so, it must be optional.
     for new_field in new_struct_fields or []:
-        # Check that all fields in the new IDL have specified the 'unstable' field.
-        if new_field.unstable is None:
-            ctxt.add_new_param_or_command_type_field_requires_unstable_error(
+        # Check that all fields in the new IDL have specified the 'stability' field.
+        if new_field.stability is None:
+            ctxt.add_new_param_or_command_type_field_requires_stability_error(
                 cmd_name, new_field.name, new_idl_file_path, is_command_parameter)
 
         newly_added = True
@@ -1048,14 +1085,15 @@ def check_command_params_or_type_struct_fields(
 
         if newly_added:
             allow_stable_name: str = cmd_name + "-param-" + new_field.name
-            if not new_field.unstable and allow_stable_name not in ALLOWED_STABLE_FIELDS_LIST:
+            if not is_unstable(
+                    new_field.stability) and allow_stable_name not in ALLOWED_STABLE_FIELDS_LIST:
                 ctxt.add_new_param_or_type_field_added_as_stable_error(
                     cmd_name, new_field.name, new_idl_file_path, is_command_parameter)
 
             new_field_type = get_field_type(new_field, new_idl_file, new_idl_file_path)
             new_field_optional = new_field.optional or (new_field_type
                                                         and new_field_type.name == 'optionalBool')
-            if not new_field_optional and not new_field.unstable:
+            if not new_field_optional and not is_unstable(new_field.stability):
                 ctxt.add_new_param_or_command_type_field_added_required_error(
                     cmd_name, new_field.name, new_idl_file_path, new_struct.name,
                     is_command_parameter)
@@ -1083,12 +1121,14 @@ def check_command_param_or_type_struct_field(
     """Check compatibility between the old and new command parameter or command type struct field."""
     # pylint: disable=too-many-arguments
     ignore_list_name: str = cmd_name + "-param-" + new_field.name
-    if not old_field.unstable and new_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+    if not is_unstable(old_field.stability) and is_unstable(
+            new_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
         ctxt.add_new_param_or_command_type_field_unstable_error(
             cmd_name, old_field.name, old_idl_file_path, type_name, is_command_parameter)
 
     # A command param or type field may not change from unstable to stable unless explicitly allowed to.
-    if old_field.unstable and not new_field.unstable and ignore_list_name not in ALLOWED_STABLE_FIELDS_LIST:
+    if is_unstable(old_field.stability) and not is_unstable(
+            new_field.stability) and ignore_list_name not in ALLOWED_STABLE_FIELDS_LIST:
         ctxt.add_unstable_param_or_type_field_to_stable_error(
             cmd_name, old_field.name, old_idl_file_path, is_command_parameter)
 
@@ -1100,22 +1140,26 @@ def check_command_param_or_type_struct_field(
                                                 and old_field_type.name == "optionalBool")
     new_field_optional = new_field.optional or (new_field_type
                                                 and new_field_type.name == "optionalBool")
-    if old_field.unstable and not new_field.unstable and not new_field_optional and new_field.default is None:
+    if is_unstable(old_field.stability) and not is_unstable(
+            new_field.stability) and not new_field_optional and new_field.default is None:
         ctxt.add_new_param_or_command_type_field_stable_required_no_default_error(
             cmd_name, old_field.name, old_idl_file_path, type_name, is_command_parameter)
 
-    if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST and old_field_optional and not new_field_optional:
+    if not is_unstable(
+            old_field.stability
+    ) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST and old_field_optional and not new_field_optional:
         ctxt.add_new_param_or_command_type_field_required_error(
             cmd_name, old_field.name, old_idl_file_path, type_name, is_command_parameter)
 
-    if not old_field.unstable and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
+    if not is_unstable(
+            old_field.stability) and ignore_list_name not in IGNORE_STABLE_TO_UNSTABLE_LIST:
         check_param_or_type_validator(ctxt, old_field, new_field, cmd_name, new_idl_file_path,
                                       type_name, is_command_parameter)
 
     old_field_compatibility = FieldCompatibility(old_field_type, old_idl_file, old_idl_file_path,
-                                                 old_field.unstable, old_field.optional)
+                                                 old_field.stability, old_field.optional)
     new_field_compatibility = FieldCompatibility(new_field_type, new_idl_file, new_idl_file_path,
-                                                 new_field.unstable, new_field.optional)
+                                                 new_field.stability, new_field.optional)
     field_pair = FieldCompatibilityPair(old_field_compatibility, new_field_compatibility, cmd_name,
                                         old_field.name)
 
@@ -1148,9 +1192,9 @@ def check_namespace(ctxt: IDLCompatibilityContext, old_cmd: syntax.Command, new_
         old_type = get_field_type(old_cmd, old_idl_file, old_idl_file_path)
         if new_namespace == common.COMMAND_NAMESPACE_TYPE:
             new_type = get_field_type(new_cmd, new_idl_file, new_idl_file_path)
-            old = FieldCompatibility(old_type, old_idl_file, old_idl_file_path, unstable=False,
+            old = FieldCompatibility(old_type, old_idl_file, old_idl_file_path, stability="stable",
                                      optional=False)
-            new = FieldCompatibility(new_type, new_idl_file, new_idl_file_path, unstable=False,
+            new = FieldCompatibility(new_type, new_idl_file, new_idl_file_path, stability="stable",
                                      optional=False)
 
             check_param_or_command_type(ctxt,
diff --git a/buildscripts/idl/idl_compatibility_errors.py b/buildscripts/idl/idl_compatibility_errors.py
index 150b3783aec..6b50aabfeb2 100644
--- a/buildscripts/idl/idl_compatibility_errors.py
+++ b/buildscripts/idl/idl_compatibility_errors.py
@@ -118,9 +118,9 @@ ERROR_ID_REPLY_FIELD_SERIALIZER_NOT_EQUAL = "ID0073"
 ERROR_ID_COMMAND_DESERIALIZER_NOT_EQUAL = "ID0074"
 ERROR_ID_COMMAND_PARAMETER_DESERIALIZER_NOT_EQUAL = "ID0075"
 ERROR_ID_REPLY_FIELD_DESERIALIZER_NOT_EQUAL = "ID0076"
-ERROR_ID_NEW_REPLY_FIELD_REQUIRES_UNSTABLE = "ID0077"
-ERROR_ID_NEW_PARAMETER_REQUIRES_UNSTABLE = "ID0078"
-ERROR_ID_NEW_COMMAND_TYPE_FIELD_REQUIRES_UNSTABLE = "ID0079"
+ERROR_ID_NEW_REPLY_FIELD_REQUIRES_STABILITY = "ID0077"
+ERROR_ID_NEW_PARAMETER_REQUIRES_STABILITY = "ID0078"
+ERROR_ID_NEW_COMMAND_TYPE_FIELD_REQUIRES_STABILITY = "ID0079"
 ERROR_ID_NEW_REPLY_CHAINED_TYPE_NOT_SUBSET = "ID0080"
 ERROR_ID_NEW_COMMAND_PARAMETER_CHAINED_TYPE_NOT_SUPERSET = "ID0081"
 ERROR_ID_NEW_COMMAND_CHAINED_TYPE_NOT_SUPERSET = "ID0082"
@@ -1101,29 +1101,29 @@ class IDLCompatibilityContext(object):
                         ("The generic reply field '%s' was removed from the new definition of the "
                          "generic_argument.idl file") % (field_name), file)
 
-    def add_new_reply_field_requires_unstable_error(self, command_name: str, field_name: str,
-                                                    file: str) -> None:
-        """Add an error that a new reply field requires the 'unstable' field."""
+    def add_new_reply_field_requires_stability_error(self, command_name: str, field_name: str,
+                                                     file: str) -> None:
+        """Add an error that a new reply field requires the 'stability' field."""
         self._add_error(
-            ERROR_ID_NEW_REPLY_FIELD_REQUIRES_UNSTABLE, command_name,
+            ERROR_ID_NEW_REPLY_FIELD_REQUIRES_STABILITY, command_name,
             ("The new definition of '%s' has reply field '%s' that requires specifying a value "
-             "for the 'unstable' field") % (command_name, field_name), file)
+             "for the 'stability' field") % (command_name, field_name), file)
 
-    def add_new_param_or_command_type_field_requires_unstable_error(
+    def add_new_param_or_command_type_field_requires_stability_error(
             self, command_name: str, field_name: str, file: str,
             is_command_parameter: bool) -> None:
         # pylint: disable=invalid-name
-        """Add an error that a new param or command type field requires the 'unstable' field."""
+        """Add an error that a new param or command type field requires the 'stability' field."""
         if is_command_parameter:
             self._add_error(
-                ERROR_ID_NEW_PARAMETER_REQUIRES_UNSTABLE, command_name,
+                ERROR_ID_NEW_PARAMETER_REQUIRES_STABILITY, command_name,
                 ("The new definition of '%s' has parameter '%s' that requires specifying a value "
-                 "for the 'unstable' field") % (command_name, field_name), file)
+                 "for the 'stability' field") % (command_name, field_name), file)
         else:
             self._add_error(
-                ERROR_ID_NEW_COMMAND_TYPE_FIELD_REQUIRES_UNSTABLE, command_name,
+                ERROR_ID_NEW_COMMAND_TYPE_FIELD_REQUIRES_STABILITY, command_name,
                 ("The new definition of '%s' has command type field '%s' that requires specifying "
-                 "a value for the 'unstable' field") % (command_name, field_name), file)
+                 "a value for the 'stability' field") % (command_name, field_name), file)
 
     def add_unstable_reply_field_changed_to_stable_error(self, command_name: str, field_name: str,
                                                          file: str) -> None:
diff --git a/buildscripts/idl/tests/compatibility_test_fail/abort/invalid_command_parameter_type/invalid_command_parameter_type.idl b/buildscripts/idl/tests/compatibility_test_fail/abort/invalid_command_parameter_type/invalid_command_parameter_type.idl
index fae1960e79b..43fab71d7f7 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/abort/invalid_command_parameter_type/invalid_command_parameter_type.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/abort/invalid_command_parameter_type/invalid_command_parameter_type.idl
@@ -44,4 +44,4 @@ commands:
         fields:
             invalidParameter:
                 type: None
-                unstable: false
+                stability: stable
diff --git a/buildscripts/idl/tests/compatibility_test_fail/abort/invalid_reply_field_type/invalid_reply_field_type.idl b/buildscripts/idl/tests/compatibility_test_fail/abort/invalid_reply_field_type/invalid_reply_field_type.idl
index 0df2972aed3..c38a2ad5432 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/abort/invalid_reply_field_type/invalid_reply_field_type.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/abort/invalid_reply_field_type/invalid_reply_field_type.idl
@@ -38,7 +38,7 @@ structs:
         fields:
             invalidReplyField:
                 type: None
-                unstable: false
+                stability: stable
 commands:
     replyFieldTypeInvalid:
         description: "This command aborts because its reply field type is invalid"
diff --git a/buildscripts/idl/tests/compatibility_test_fail/abort/valid_command_parameter_type/valid_command_parameter_type.idl b/buildscripts/idl/tests/compatibility_test_fail/abort/valid_command_parameter_type/valid_command_parameter_type.idl
index f772ac2a480..3777373960b 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/abort/valid_command_parameter_type/valid_command_parameter_type.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/abort/valid_command_parameter_type/valid_command_parameter_type.idl
@@ -44,4 +44,4 @@ commands:
         fields:
             invalidParameter:
                 type: string
-                unstable: false
+                stability: stable
diff --git a/buildscripts/idl/tests/compatibility_test_fail/abort/valid_reply_field_type/valid_reply_field_type.idl b/buildscripts/idl/tests/compatibility_test_fail/abort/valid_reply_field_type/valid_reply_field_type.idl
index 22feea211fb..23c609227a6 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/abort/valid_reply_field_type/valid_reply_field_type.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/abort/valid_reply_field_type/valid_reply_field_type.idl
@@ -38,7 +38,7 @@ structs:
         fields:
             invalidReplyField:
                 type: string
-                unstable: false
+                stability: stable
 
 commands:
     replyFieldTypeInvalid:
diff --git a/buildscripts/idl/tests/compatibility_test_fail/new/compatibility_test_fail_new.idl b/buildscripts/idl/tests/compatibility_test_fail/new/compatibility_test_fail_new.idl
index 434ac4a2c2f..f286dbc0ddd 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/new/compatibility_test_fail_new.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/new/compatibility_test_fail_new.idl
@@ -148,7 +148,15 @@ structs:
         fields:
             unstableNewField:
                 type: string
-                unstable: true
+                stability: unstable
+
+    InternalNewFieldReply:
+        description: "This reply contains a field that is stable in the old command but is
+                      internal in the new command."
+        fields:
+            internalNewField:
+                type: string
+                stability: internal
 
     StableNewReplyField:
         description: "This struct contains a reply field that is unstable in the old command but is
@@ -174,7 +182,7 @@ structs:
             optionalNewField:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     MissingNewFieldReply:
         description: "This reply contains a field that exists in the old command but is
@@ -186,7 +194,7 @@ structs:
         fields:
             requiredNewField:
                 type: string
-                unstable: false
+                stability: stable
 
     EnumNotSubsetReply:
         description: "This reply contains an enum field where the new enum values is not a subset
@@ -194,7 +202,7 @@ structs:
         fields:
             enumNotSubsetReplyField:
                 type: NewReplyFieldEnumNotSubset
-                unstable: false
+                stability: stable
 
     NotEnumFieldReply:
         description: "This reply contains a field that is an enum type in the old command but
@@ -202,7 +210,7 @@ structs:
         fields:
             notEnumReplyField:
                 type: string
-                unstable: false
+                stability: stable
 
     NotStructFieldReply:
         description: "This reply contains a field that is a struct type in the old command but
@@ -210,7 +218,7 @@ structs:
         fields:
             notStructReplyField:
                 type: string
-                unstable: false
+                stability: stable
 
     EnumOrStructFieldReply:
         description: "This reply contains a field that is a non-enum or struct type in the old
@@ -218,7 +226,7 @@ structs:
         fields:
             EnumOrStructReplyField:
                 type: NewReplyFieldEnumNotSubset
-                unstable: false
+                stability: stable
 
     BsonNotSubsetReply:
         description: "This reply contains a field type where the new bson_serialization_type
@@ -226,7 +234,7 @@ structs:
         fields:
             bsonNotSubsetReplyField:
                 type: intToIntString
-                unstable: false
+                stability: stable
 
     BsonNotSubsetReplyTwo:
         description: "This reply contains a field type where the new bson_serialization_type
@@ -234,7 +242,7 @@ structs:
         fields:
             bsonNotSubsetReplyFieldTwo:
                 type: intStringToIntStringBool
-                unstable: false
+                stability: stable
 
     OldBsonSerializationTypeAnyReply:
         description: "This reply contains a field whose old type has a bson_serialization_type
@@ -242,7 +250,7 @@ structs:
         fields:
             oldBsonSerializationTypeAnyReplyField:
                 type: oldBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     NewBsonSerializationTypeAnyReply:
         description: "This reply contains a field whose new type has a bson_serialization_type
@@ -250,7 +258,7 @@ structs:
         fields:
             newBsonSerializationTypeAnyReplyField:
                 type: newBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     BsonSerializationTypeAnyReply:
         description: "This reply contains a field whose old and new type have a bson_serialization_type
@@ -258,14 +266,14 @@ structs:
         fields:
             bsonSerializationTypeAnyReplyField:
                 type: bsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     BsonSerializationTypeWithVariantAnyStruct:
         description: "This struct contains a field whose old and new variant types have a bson_serialization_type
                         that contains 'any' that is not compatible"
         fields:
             bsonSerializationTypeAnyStructField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [oldBsonSerializationTypeAny, newBsonSerializationTypeAny]
 
@@ -274,7 +282,7 @@ structs:
                       bson_serialization_type that contains 'any' that is not compatible"
         fields:
             bsonSerializationTypeAnyStructField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<oldBsonSerializationTypeAny>, array<newBsonSerializationTypeAny>]
 
@@ -284,7 +292,7 @@ structs:
         fields:
             cppTypeNotEqualReplyField:
                 type: bsonSerializationTypeAnyCppTypeNotEqual
-                unstable: false
+                stability: stable
 
     SerializerNotEqualReply:
         description: "This reply contains a field whose old and new type
@@ -293,7 +301,7 @@ structs:
         fields:
             serializerNotEqualReplyField:
                 type: bsonSerializationTypeAnySerializerNotEqual
-                unstable: false
+                stability: stable
 
     DeserializerNotEqualReply:
         description: "This reply contains a field whose old and new type have
@@ -302,14 +310,14 @@ structs:
         fields:
             deserializerNotEqualReplyField:
                 type: bsonSerializationTypeAnyDeserializerNotEqual
-                unstable: false
+                stability: stable
 
     NewlyAddedBsonSerializationTypeAnyReply:
         description: "This reply contains a newly added field whose type has a bson_serialization_type
                         that contains 'any' that is not explicitly allowed"
         fields:
             newlyAddedBsonSerializationTypeAnyReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [string, bsonSerializationTypeAny]
 
@@ -320,7 +328,7 @@ structs:
         fields:
             oldBsonSerializationTypeAnyUnstableReplyField:
                 type: oldBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     NewBsonSerializationTypeAnyUnstableReply:
         description: "This reply contains a field that is unstable in the old version
@@ -329,7 +337,7 @@ structs:
         fields:
             newBsonSerializationTypeAnyUnstableReplyField:
                 type: newBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     BsonSerializationTypeAnyUnstableReply:
         description: "This reply contains a field that is unstable in the old version
@@ -338,7 +346,7 @@ structs:
         fields:
             bsonSerializationTypeAnyUnstableReplyField:
                 type: bsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     BsonSerializationTypeWithVariantAnyUnstableReply:
         description: "This reply contains a field whose old and new variant types have a bson_serialization_type
@@ -347,7 +355,7 @@ structs:
             bsonSerializationTypeWithVariantAnyUnstableReplyField:
                 type:
                     variant: [oldBsonSerializationTypeAny, newBsonSerializationTypeAny]
-                unstable: false
+                stability: stable
 
     CppTypeNotEqualUnstableReply:
         description: "This reply contains a field whose old and new type have a
@@ -356,7 +364,7 @@ structs:
         fields:
             cppTypeNotEqualReplyUnstableField:
                 type: bsonSerializationTypeAnyCppTypeNotEqual
-                unstable: false
+                stability: stable
 
     SerializerNotEqualUnstableReply:
         description: "This reply contains a field whose old and new type have a
@@ -365,7 +373,7 @@ structs:
         fields:
             serializerNotEqualReplyUnstableField:
                 type: bsonSerializationTypeAnySerializerNotEqual
-                unstable: false
+                stability: stable
 
     DeserializerNotEqualUnstableReply:
         description: "This reply contains a field whose old and new type have a
@@ -374,7 +382,7 @@ structs:
         fields:
             deserializerNotEqualReplyUnstableField:
                 type: bsonSerializationTypeAnyDeserializerNotEqual
-                unstable: false
+                stability: stable
 
     NewlyAddedBsonSerializationTypeAnyStruct:
         description: "This struct contains a newly added field whose type has a bson_serialization_type
@@ -382,7 +390,7 @@ structs:
         fields:
             newlyAddedBsonSerializationTypeAnyStructField:
                 optional: true
-                unstable: false
+                stability: stable
                 type:
                     variant: [string, bsonSerializationTypeAny]
 
@@ -394,7 +402,7 @@ structs:
             oldBsonSerializationTypeAnyUnstableStructField:
                 optional: true
                 type: oldBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     NewBsonSerializationTypeAnyUnstableStruct:
         description: "This struct contains a field that is unstable in the old version
@@ -404,7 +412,7 @@ structs:
             newBsonSerializationTypeAnyUnstableStructField:
                 optional: true
                 type: newBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     BsonSerializationTypeAnyUnstableStruct:
         description: "This struct contains a field that is unstable in the old version
@@ -414,7 +422,7 @@ structs:
             bsonSerializationTypeAnyUnstableStructField:
                 optional: true
                 type: bsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     BsonSerializationTypeWithVariantAnyUnstableStruct:
         description: "This struct contains a field whose old and new variant types have a bson_serialization_type
@@ -422,7 +430,7 @@ structs:
         fields:
             bsonSerializationTypeWithVariantAnyUnstableStructField:
                 optional: true
-                unstable: false
+                stability: stable
                 type:
                     variant: [oldBsonSerializationTypeAny, newBsonSerializationTypeAny]
 
@@ -433,7 +441,7 @@ structs:
         fields:
             cppTypeNotEqualStructUnstableField:
                 optional: true
-                unstable: false
+                stability: stable
                 type: bsonSerializationTypeAnyCppTypeNotEqual
 
     SerializerNotEqualUnstableStruct:
@@ -444,7 +452,7 @@ structs:
             serializerNotEqualStructUnstableField:
                 optional: true
                 type: bsonSerializationTypeAnySerializerNotEqual
-                unstable: false
+                stability: stable
 
     DeserializerNotEqualUnstableStruct:
         description: "This struct contains a field whose old and new type have a
@@ -454,7 +462,7 @@ structs:
             deserializerNotEqualStructUnstableField:
                 optional: true
                 type: bsonSerializationTypeAnyDeserializerNotEqual
-                unstable: false
+                stability: stable
 
     StructFieldTypeRecursiveReplyOne:
         description: "This reply contains a field whose new type is a struct that is not
@@ -462,7 +470,7 @@ structs:
         fields:
             structReplyField:
                 type: UnstableNewFieldReply
-                unstable: false
+                stability: stable
 
     StructFieldTypeRecursiveReplyTwo:
         description: "This reply contains a field whose new type is a struct that is not
@@ -470,7 +478,7 @@ structs:
         fields:
             structReplyField:
                 type: StructType
-                unstable: false
+                stability: stable
 
     StructType:
         description: "This struct contains a field whose new type is incompatible with the
@@ -478,14 +486,14 @@ structs:
         fields:
             fieldOne:
                 type: BsonNotSubsetReply
-                unstable: false
+                stability: stable
 
     NewVariantTypeReply:
         description: "This reply contains a new field that has a variant type while the old field
                       is not a variant type"
         fields:
             newVariantTypeReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string]
 
@@ -494,7 +502,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSubsetReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string]
 
@@ -503,7 +511,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSubsetReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<bool>, array<string>]
 
@@ -512,7 +520,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSubsetReplyFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string, double]
 
@@ -521,7 +529,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSubsetReplyFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<bool>, array<string>, array<double>]
 
@@ -530,7 +538,7 @@ structs:
                       compatible with the old variant type"
         fields:
             variantRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringToIntStringBool]
 
@@ -539,7 +547,7 @@ structs:
                       compatible with the old variant type"
         fields:
             variantRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<intStringToIntStringBool>]
 
@@ -548,7 +556,7 @@ structs:
                       of the old variant types"
         fields:
             variantStructNotSubsetReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructType]
 
@@ -557,7 +565,7 @@ structs:
                       of the old variant types"
         fields:
             variantStructNotSubsetReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>, array<StructType>]
 
@@ -566,7 +574,7 @@ structs:
                       compatible with the old variant struct type"
         fields:
             variantStructRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, StructFieldTypeRecursiveReplyTwo]
 
@@ -575,7 +583,7 @@ structs:
                       compatible with the old variant struct type"
         fields:
             variantStructRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<StructFieldTypeRecursiveReplyTwo>]
 
@@ -585,7 +593,7 @@ structs:
         fields:
             stableToUnstableField:
                 type: string
-                unstable: true
+                stability: unstable
 
     CommandParamStructRecursiveTwo:
         description: "This command parameter struct type contains a field that
@@ -593,7 +601,7 @@ structs:
         fields:
             notSupersetField:
                 type: intStringToInt
-                unstable: false
+                stability: stable
 
     NewValidatorStruct:
         description: "This struct contains a field where the new version contains a validator while
@@ -601,7 +609,7 @@ structs:
         fields:
             newValidatorField:
                 type: int
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0
 
@@ -610,7 +618,7 @@ structs:
         fields:
             validatorsNotEqualField:
                 type: double
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0.0
                     gt: -1.1
@@ -623,7 +631,7 @@ structs:
         fields:
             addedRequiredTypeField:
                 type: string
-                unstable: false
+                stability: stable
 
     StableRequiredNoDefaultTypeFieldStruct:
         description: "This struct contains a field that is stable and required with no default value
@@ -631,14 +639,14 @@ structs:
         fields:
             stableRequiredNoDefaultTypeField:
                 type: string
-                unstable: false
+                stability: stable
 
     ArrayTypeStruct:
         description: "Struct with ArrayType field."
         fields:
             ArrayCommandParameter:
                 type: array<string>
-                unstable: false
+                stability: stable
 
     StructCommandParameterTypeRecursive:
         description: "This param struct type contains a field that is not compatible between the
@@ -646,14 +654,14 @@ structs:
         fields:
             structCommandParameterTypeRecursiveField:
                 type: intStringToInt
-                unstable: false
+                stability: stable
 
     VariantNotSupersetStruct:
         description: "This struct contains a field where the new variant types are not a superset
                       of the old variant types"
         fields:
             variantNotSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool]
 
@@ -662,7 +670,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<bool>]
 
@@ -671,7 +679,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSupersetFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool]
 
@@ -680,7 +688,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSupersetFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<bool>]
 
@@ -690,14 +698,14 @@ structs:
         fields:
             variantField:
                 type: int
-                unstable: false
+                stability: stable
 
     VariantRecursiveStruct:
         description: "This struct contains a field where the new variant types are not
                       compatible with the old variant types"
         fields:
             variantRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringBoolToIntString]
 
@@ -706,7 +714,7 @@ structs:
                       compatible with the old variant types"
         fields:
             variantRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<intStringBoolToIntString>]
 
@@ -715,7 +723,7 @@ structs:
                       while the new one does not"
         fields:
             variantStructNotSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string]
 
@@ -724,7 +732,7 @@ structs:
                       while the new one does not"
         fields:
             variantStructNotSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>]
 
@@ -733,7 +741,7 @@ structs:
                       variant struct are not compatible"
         fields:
             variantStructRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructCommandParameterTypeRecursive]
 
@@ -742,12 +750,12 @@ structs:
                       variant struct are not compatible"
         fields:
             variantStructRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>, array<StructCommandParameterTypeRecursive>]
 
     MissingUnstableFieldNewFieldStruct:
-        description: "This struct contains a field that missing the 'unstable' field in the new
+        description: "This struct contains a field that missing the 'stability' field in the new
                       command."
         fields:
             missingUnstableFieldNewField:
@@ -755,7 +763,7 @@ structs:
 
     MissingUnstableFieldAddedNewFieldStruct:
         description: "This struct contains an added field in the new command that is missing the
-                      'unstable' field."
+                      'stability' field."
         fields:
             missingUnstableFieldAddedNewField:
                 type: string
@@ -784,14 +792,14 @@ structs:
         fields:
             ok:
                 type: bool
-                unstable: false
+                stability: stable
 
     BoolToOptionalBoolStruct:
         description: "bool field replaced by optionalBool"
         fields:
             ok:
                 type: optionalBool
-                unstable: false
+                stability: stable
 
 commands:
     invalidAPIVersionNew:
@@ -864,7 +872,7 @@ commands:
         fields:
             newRequiredParam:
                 type: string
-                unstable: false
+                stability: stable
 
     newStableParameterAdded:
         description: "new command fails because it adds a new parameter as a stable field"
@@ -880,7 +888,7 @@ commands:
                 unstable: false
 
     commandParameterUnstable:
-        description: "new unstable command parameter fails because it is stable
+        description: "new stability command parameter fails because it is stable
                       in the corresponding old command"
         command_name: commandParameterUnstable
         namespace: ignored
@@ -891,7 +899,21 @@ commands:
         fields:
             newUnstableParameter:
                 type: string
-                unstable: true
+                stability: unstable
+
+    commandParameterInternal:
+        description: "new internal command parameter fails because it is stable
+                      in the corresponding old command"
+        command_name: commandParameterInternal
+        namespace: ignored
+        cpp_name: commandParameterInternal
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+        fields:
+            newInternalParameter:
+                type: string
+                stability: internal
 
     commandParameterStableRequiredNoDefault:
         description: "new required stable command parameter with no default value fails
@@ -905,7 +927,7 @@ commands:
         fields:
             newRequiredStableParam:
                 type: string
-                unstable: false
+                stability: stable
 
     commandParameterRequired:
         description: "new required command parameter fails because it is optional
@@ -920,7 +942,7 @@ commands:
             newRequiredParam:
                 type: string
                 optional: false
-                unstable: false
+                stability: stable
 
     oldCommandParameterTypeBsonSerializationAny:
         description: "old command fails because it has a parameter type that has a
@@ -934,7 +956,7 @@ commands:
         fields:
             bsonTypeAnyParam:
                 type: oldBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeBsonSerializationAny:
         description: "new command fails because it has a parameter type that has a
@@ -948,7 +970,7 @@ commands:
         fields:
             bsonTypeAnyParam:
                 type: newBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     oldParamTypeBsonAnyAllowList:
         description: "old command fails because it has a parameter type that has a
@@ -963,7 +985,7 @@ commands:
         fields:
             bsonTypeAnyParam:
                 type: oldBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     newParamTypeBsonAnyAllowList:
         description: "new command fails because it has a parameter type that has a
@@ -978,7 +1000,7 @@ commands:
         fields:
             bsonTypeAnyParam:
                 type: newBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     commandParameterTypeBsonSerializationAnyNotAllowed:
         description: "command fails because it has a parameter type that has a
@@ -993,7 +1015,7 @@ commands:
         fields:
             bsonTypeAnyParam:
                 type: bsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     commandParameterCppTypeNotEqual:
         description: "command fails because it has a parameter type that has a
@@ -1008,7 +1030,7 @@ commands:
         fields:
             cppTypeNotEqualParam:
                 type: bsonSerializationTypeAnyCppTypeNotEqual
-                unstable: false
+                stability: stable
 
     commandParameterSerializerNotEqual:
         description: "command fails because it has a parameter type that has a
@@ -1023,7 +1045,7 @@ commands:
         fields:
             serializerNotEqualParam:
                 type: bsonSerializationTypeAnySerializerNotEqual
-                unstable: false
+                stability: stable
 
     commandParameterDeserializerNotEqual:
         description: "command fails because it has a parameter type that has a
@@ -1038,7 +1060,7 @@ commands:
         fields:
             deserializerNotEqualParam:
                 type: bsonSerializationTypeAnyDeserializerNotEqual
-                unstable: false
+                stability: stable
 
     oldCommandParamTypeBsonAnyUnstable:
         description: "old command fails because it has a parameter type that has a
@@ -1053,7 +1075,7 @@ commands:
             bsonTypeAnyUnstableParam:
                 type: oldBsonSerializationTypeAny
                 optional: true
-                unstable: false
+                stability: stable
 
     newCommandParamTypeBsonAnyUnstable:
         description: "new command fails because it has a parameter type that has a
@@ -1068,7 +1090,7 @@ commands:
             bsonTypeAnyUnstableParam:
                 type: newBsonSerializationTypeAny
                 optional: true
-                unstable: false
+                stability: stable
 
     commandParamTypeBsonAnyNotAllowedUnstable:
         description: "command fails because it has a parameter type that has a
@@ -1084,7 +1106,7 @@ commands:
             bsonTypeAnyUnstableParam:
                 type: bsonSerializationTypeAny
                 optional: true
-                unstable: false
+                stability: stable
 
     commandParameterCppTypeNotEqualUnstable:
         description: "command fails because it has a parameter type that has a
@@ -1100,7 +1122,7 @@ commands:
             cppTypeNotEqualParam:
                 type: bsonSerializationTypeAnyCppTypeNotEqual
                 optional: true
-                unstable: false
+                stability: stable
 
     commandParameterSerializerNotEqualUnstable:
         description: "command fails because it has a parameter type that has a
@@ -1116,7 +1138,7 @@ commands:
             serializerNotEqualParam:
                 type: bsonSerializationTypeAnySerializerNotEqual
                 optional: true
-                unstable: false
+                stability: stable
 
     commandParameterDeserializerNotEqualUnstable:
         description: "command fails because it has a parameter type that has a
@@ -1132,7 +1154,7 @@ commands:
             deserializerNotEqualParam:
                 type: bsonSerializationTypeAnyDeserializerNotEqual
                 optional: true
-                unstable: false
+                stability: stable
 
     parameterFieldTypeBsonAnyWithVariantUnstable:
         description: "command fails when its paramter field variant types have bson_serialization_type
@@ -1147,7 +1169,7 @@ commands:
             variantAnyUnstableField:
                 type: BsonSerializationTypeWithVariantAnyStruct
                 optional: true
-                unstable: false
+                stability: stable
 
     newlyAddedParamBsonAnyNotAllowed:
         description: "command fails when its parameter is newly added and has bson type 'any'
@@ -1162,7 +1184,7 @@ commands:
             newlyAddedBsonAnyNotAllowedParam:
                 type: bsonSerializationTypeAny
                 optional: true
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeEnumNotSuperset:
         description: "new command fails because its parameter type is an enum that is not
@@ -1176,7 +1198,7 @@ commands:
         fields:
             enumNotSupersetParam:
                 type: EnumNotSuperset
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeNotEnum:
         description: "new command fails because its parameter type is not an enum when the
@@ -1190,7 +1212,7 @@ commands:
         fields:
             newParamNotEnum:
                 type: string
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeNotStruct:
         description: "new command fails because its parameter type is not a struct when the
@@ -1204,7 +1226,7 @@ commands:
         fields:
             newParamNotStruct:
                 type: string
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeEnumOrStructOne:
         description: "new command fails because its parameter type is an enum while the
@@ -1218,7 +1240,7 @@ commands:
         fields:
             newParamEnum:
                 type: EnumNotSuperset
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeEnumOrStructTwo:
         description: "new command fails because its parameter type is a struct while the
@@ -1232,7 +1254,7 @@ commands:
         fields:
             newParamStruct:
                 type: StructCommandParameterType
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeBsonNotSuperset:
         description: "new command fails because its parameter type has a bson_serialization_type
@@ -1247,7 +1269,7 @@ commands:
         fields:
             newParamBsonNotSuperset:
                 type: intStringToInt
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeStructRecursiveOne:
         description: "new command fails because its parameter type is a struct that is
@@ -1261,7 +1283,7 @@ commands:
         fields:
             stableToUnstableStructParameter:
                 type: CommandParamStructRecursiveOne
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeStructRecursiveTwo:
         description: "new command fails because its parameter type is a struct that is
@@ -1275,7 +1297,7 @@ commands:
         fields:
             notSupersetStructParameter:
                 type: CommandParamStructRecursiveTwo
-                unstable: false
+                stability: stable
 
     newCommandParameterValidator:
         description: "new command fails because it contains a parameter that contains a validator
@@ -1289,7 +1311,7 @@ commands:
         fields:
             newParam:
                 type: int
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0
 
@@ -1305,7 +1327,7 @@ commands:
         fields:
             newParam:
                 type: double
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0.0
                     gt: -1.1
@@ -1345,6 +1367,16 @@ commands:
         api_version: "1"
         reply_type: UnstableNewFieldReply
 
+    newReplyFieldInternal:
+        description: "new command fails because it contains an internal reply field that is stable
+                      in the corresponding old command"
+        command_name: newReplyFieldInternal
+        namespace: ignored
+        cpp_name: newReplyFieldInternal
+        strict: true
+        api_version: "1"
+        reply_type: InternalNewFieldReply
+
     unstableToStableReplyField:
         description: "new command fails because it contains a stable reply field that is unstable
                       in the corresponding old command"
@@ -1537,7 +1569,7 @@ commands:
         fields:
             variantAnyField:
                 type: BsonSerializationTypeWithVariantAnyStruct
-                unstable: false
+                stability: stable
 
     parameterFieldTypeBsonAnyWithVariantWithArray:
         description: "command fails when its parameter field variant types have bson_serialization_type
@@ -1551,7 +1583,7 @@ commands:
         fields:
             variantAnyField:
                 type: BsonSerializationTypeWithVariantAnyStructWithArray
-                unstable: false
+                stability: stable
 
     commandTypeBsonAnyWithVariant:
         description: "command fails when its variant types have bson_serialization_type
@@ -1941,6 +1973,17 @@ commands:
         api_version: "1"
         reply_type: OkReply
 
+    newTypeFieldInternal:
+        description: "new command fails because it contains an internal type field that is stable
+                      in the corresponding old command"
+        command_name: newTypeFieldInternal
+        namespace: type
+        type: InternalNewFieldReply
+        cpp_name: newTypeFieldInternal
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+
     newTypeFieldRequired:
         description: "new command fails because it contains a required reply field that is
                       optional in the corresponding old command"
@@ -2122,7 +2165,7 @@ commands:
         reply_type: OkReply
         fields:
             variantNotSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool]
 
@@ -2137,7 +2180,7 @@ commands:
         reply_type: OkReply
         fields:
             variantNotSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<bool>]
 
@@ -2152,7 +2195,7 @@ commands:
         reply_type: OkReply
         fields:
             variantNotSupersetParamTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string]
 
@@ -2167,7 +2210,7 @@ commands:
         reply_type: OkReply
         fields:
             variantNotSupersetParamTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>]
 
@@ -2183,7 +2226,7 @@ commands:
         fields:
             variantParam:
                 type: int
-                unstable: false
+                stability: stable
 
     newParamVariantRecursive:
         description: "new command fails because its param type is a variant type that is not
@@ -2196,7 +2239,7 @@ commands:
         reply_type: OkReply
         fields:
             variantRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringBoolToIntString]
 
@@ -2211,7 +2254,7 @@ commands:
         reply_type: OkReply
         fields:
             variantRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<intStringBoolToIntString>]
 
@@ -2226,7 +2269,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructNotSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string]
 
@@ -2241,7 +2284,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructNotSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>]
 
@@ -2257,7 +2300,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructCommandParameterTypeRecursive]
 
@@ -2273,7 +2316,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>, array<StructCommandParameterTypeRecursive>]
 
@@ -2503,7 +2546,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     ArrayCommandParameterTypeError:
@@ -2518,7 +2561,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<StructType>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     accessCheckTypeChange:
@@ -2794,7 +2837,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     ArrayCommandTypeErrorNoArrayNew:
@@ -2809,7 +2852,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     ArrayCommandParameterNoArrayOld:
@@ -2824,7 +2867,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     ArrayCommandParameterNoArrayNew:
@@ -2839,12 +2882,12 @@ commands:
         fields:
             parameterStruct:
                 type: ArrayTypeStruct
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     newReplyFieldMissingUnstableField:
         description: "new command fails because it contains a reply field that is missing the
-                      'unstable' field"
+                      'stability' field"
         command_name: newReplyFieldMissingUnstableField
         namespace: ignored
         cpp_name: newReplyFieldMissingUnstableField
@@ -2854,7 +2897,7 @@ commands:
 
     newCommandTypeFieldMissingUnstableField:
         description: "new command fails because it contains a command type field that is missing the
-                      'unstable' field"
+                      'stability' field"
         command_name: newCommandTypeFieldMissingUnstableField
         namespace: type
         cpp_name: newCommandTypeFieldMissingUnstableField
@@ -2865,7 +2908,7 @@ commands:
 
     newParameterMissingUnstableField:
         description: "new command fails because it contains a parameter that is missing the
-                      'unstable' field"
+                      'stability' field"
         command_name: newParameterMissingUnstableField
         namespace: ignored
         cpp_name: newParameterMissingUnstableField
@@ -2878,7 +2921,7 @@ commands:
 
     addedNewParameterMissingUnstableField:
         description: "new command fails because it contains a new parameter that is missing the
-                      'unstable' field"
+                      'stability' field"
         command_name: addedNewParameterMissingUnstableField
         namespace: ignored
         cpp_name: addedNewParameterMissingUnstableField
@@ -2892,7 +2935,7 @@ commands:
 
     addedNewReplyFieldMissingUnstableField:
         description: "new command fails because it contains a new reply field that is missing the
-                      'unstable' field"
+                      'stability' field"
         command_name: addedNewReplyFieldMissingUnstableField
         namespace: ignored
         cpp_name: addedNewReplyFieldMissingUnstableField
@@ -2902,7 +2945,7 @@ commands:
 
     addedNewCommandTypeFieldMissingUnstableField:
         description: "new command fails because it contains a new command type field that is missing
-                      the 'unstable' field"
+                      the 'stability' field"
         command_name: addedNewCommandTypeFieldMissingUnstableField
         namespace: type
         type: MissingUnstableFieldAddedNewFieldStruct
@@ -2965,7 +3008,7 @@ commands:
         reply_type: OkReply
         fields:
             NewRemovedChainedTypeParameter:
-                unstable: false
+                stability: stable
                 type: NewRemovedChainedTypeStruct
 
     newReplyAddedChainedType:
@@ -2986,7 +3029,7 @@ commands:
         api_version: "1"
         fields:
             flag1:
-                unstable: false
+                stability: stable
                 type: bool
         reply_type: OkReply
 
diff --git a/buildscripts/idl/tests/compatibility_test_fail/new/error_reply.idl b/buildscripts/idl/tests/compatibility_test_fail/new/error_reply.idl
index 31e9382b5ac..526d61a222a 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/new/error_reply.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/new/error_reply.idl
@@ -40,7 +40,7 @@ structs:
         fields:
             errorLabels:
                 type: StructType
-                unstable: false
+                stability: stable
     
     StructType:
         description: "This struct contains a field that is non-optional in the old
@@ -49,4 +49,4 @@ structs:
             structField:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
diff --git a/buildscripts/idl/tests/compatibility_test_fail/new/imports.idl b/buildscripts/idl/tests/compatibility_test_fail/new/imports.idl
index a9e8a09438e..0eaa9546da8 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/new/imports.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/new/imports.idl
@@ -36,4 +36,4 @@ structs:
         fields:
             unstableNewField:
                 type: string
-                unstable: true
+                stability: unstable
diff --git a/buildscripts/idl/tests/compatibility_test_fail/newly_added_commands/newly_added_commands.idl b/buildscripts/idl/tests/compatibility_test_fail/newly_added_commands/newly_added_commands.idl
index c07063ec61d..df3da6398c6 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/newly_added_commands/newly_added_commands.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/newly_added_commands/newly_added_commands.idl
@@ -53,7 +53,7 @@ structs:
         fields:
             newStructFieldTypeContainsAny:
                 type: bsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
 commands:
     newCommandParameterNoUnstableField:
@@ -99,7 +99,7 @@ commands:
         fields:
             flag1:
                 type: bsonSerializationTypeAny
-                unstable: false
+                stability: stable
         reply_type: OkReply
 
     newCommandReplyBsonSerializationTypeAny:
diff --git a/buildscripts/idl/tests/compatibility_test_fail/old/compatibility_test_fail_old.idl b/buildscripts/idl/tests/compatibility_test_fail/old/compatibility_test_fail_old.idl
index 9901b0d4bb6..4fba33cd45d 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/old/compatibility_test_fail_old.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/old/compatibility_test_fail_old.idl
@@ -142,7 +142,15 @@ structs:
         fields:
             unstableNewField:
                 type: string
-                unstable: false
+                stability: stable
+
+    InternalNewFieldReply:
+        description: "This reply contains a field that is stable in the old command but is
+                      internal in the new command."
+        fields:
+            internalNewField:
+                type: string
+                stability: stable
 
     StableNewReplyField:
         description: "This struct contains a reply field that is unstable in the old command but is
@@ -167,7 +175,7 @@ structs:
         fields:
             optionalNewField:
                 type: string
-                unstable: false
+                stability: stable
 
     RequiredNewField:
         description: "This struct contains a field that is optional in the old command but is
@@ -176,7 +184,7 @@ structs:
             requiredNewField:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     MissingNewFieldReply:
         description: "This reply contains a field that exists in the old command but is
@@ -184,7 +192,7 @@ structs:
         fields:
             missingNewField:
                 type: string
-                unstable: false
+                stability: stable
 
     EnumNotSubsetReply:
         description: "This reply contains an enum field where the new enum values is not a subset
@@ -192,7 +200,7 @@ structs:
         fields:
             enumNotSubsetReplyField:
                 type: NewReplyFieldEnumNotSubset
-                unstable: false
+                stability: stable
 
     NotEnumFieldReply:
         description: "This reply contains a field that is an enum type in the old command but
@@ -200,7 +208,7 @@ structs:
         fields:
             notEnumReplyField:
                 type: NewReplyFieldEnumNotSubset
-                unstable: false
+                stability: stable
 
     NotStructFieldReply:
         description: "This reply contains a field that is a struct type in the old command but
@@ -208,7 +216,7 @@ structs:
         fields:
             notStructReplyField:
                 type: StructReplyFieldType
-                unstable: false
+                stability: stable
 
     StructReplyFieldType:
         description: "This is a struct reply field type"
@@ -219,7 +227,7 @@ structs:
         fields:
             EnumOrStructReplyField:
                 type: intToIntString
-                unstable: false
+                stability: stable
 
     BsonNotSubsetReply:
         description: "This reply contains a field type where the new bson_serialization_type
@@ -227,7 +235,7 @@ structs:
         fields:
             bsonNotSubsetReplyField:
                 type: intToIntString
-                unstable: false
+                stability: stable
 
     BsonNotSubsetReplyTwo:
         description: "This reply contains a field type where the new bson_serialization_type
@@ -235,7 +243,7 @@ structs:
         fields:
             bsonNotSubsetReplyFieldTwo:
                 type: intStringToIntStringBool
-                unstable: false
+                stability: stable
 
     OldBsonSerializationTypeAnyReply:
         description: "This reply contains a field whose old type has a bson_serialization_type
@@ -243,7 +251,7 @@ structs:
         fields:
             oldBsonSerializationTypeAnyReplyField:
                 type: oldBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     NewBsonSerializationTypeAnyReply:
         description: "This reply contains a field whose new type has a bson_serialization_type
@@ -251,7 +259,7 @@ structs:
         fields:
             newBsonSerializationTypeAnyReplyField:
                 type: newBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     BsonSerializationTypeAnyReply:
         description: "This reply contains a field whose old and new type have a
@@ -259,7 +267,7 @@ structs:
         fields:
             bsonSerializationTypeAnyReplyField:
                 type: bsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     NewlyAddedBsonSerializationTypeAnyReply:
         description: "This reply contains a newly added field whose type has a
@@ -272,7 +280,7 @@ structs:
         fields:
             oldBsonSerializationTypeAnyUnstableReplyField:
                 type: oldBsonSerializationTypeAny
-                unstable: true
+                stability: unstable
 
     NewBsonSerializationTypeAnyUnstableReply:
         description: "This reply contains a field that is unstable in the old version
@@ -281,7 +289,7 @@ structs:
         fields:
             newBsonSerializationTypeAnyUnstableReplyField:
                 type: newBsonSerializationTypeAny
-                unstable: true
+                stability: unstable
 
     BsonSerializationTypeAnyUnstableReply:
         description: "This reply contains a field that is unstable in the old version
@@ -290,7 +298,7 @@ structs:
         fields:
             bsonSerializationTypeAnyUnstableReplyField:
                 type: bsonSerializationTypeAny
-                unstable: true
+                stability: unstable
 
     BsonSerializationTypeWithVariantAnyUnstableReply:
         description: "This reply contains a field whose old and new variant types have a
@@ -300,7 +308,7 @@ structs:
             bsonSerializationTypeWithVariantAnyUnstableReplyField:
                 type:
                     variant: [oldBsonSerializationTypeAny, newBsonSerializationTypeAny]
-                unstable: true
+                stability: unstable
 
     CppTypeNotEqualUnstableReply:
         description: "This reply contains a field whose old and new type have a
@@ -309,7 +317,7 @@ structs:
         fields:
             cppTypeNotEqualReplyUnstableField:
                 type: bsonSerializationTypeAnyCppTypeNotEqual
-                unstable: true
+                stability: unstable
 
     SerializerNotEqualUnstableReply:
         description: "This reply contains a field whose old and new type have a
@@ -318,7 +326,7 @@ structs:
         fields:
             serializerNotEqualReplyUnstableField:
                 type: bsonSerializationTypeAnySerializerNotEqual
-                unstable: true
+                stability: unstable
 
     DeserializerNotEqualUnstableReply:
         description: "This reply contains a field whose old and new type have a
@@ -327,14 +335,14 @@ structs:
         fields:
             deserializerNotEqualReplyUnstableField:
                 type: bsonSerializationTypeAnyDeserializerNotEqual
-                unstable: true
+                stability: unstable
 
     NewlyAddedBsonSerializationTypeAnyStruct:
         description: "This struct contains a newly added field whose type has a
                         bson_serialization_type that contains 'any' that is not explicitly allowed"
         fields:
             newlyAddedBsonSerializationTypeAnyStructField:
-                unstable: true
+                stability: unstable
                 type:
                     variant: [string, bsonSerializationTypeAny]
 
@@ -344,7 +352,7 @@ structs:
                       that contains 'any'"
         fields:
             oldBsonSerializationTypeAnyUnstableStructField:
-                unstable: true
+                stability: unstable
                 type: oldBsonSerializationTypeAny
 
     NewBsonSerializationTypeAnyUnstableStruct:
@@ -353,7 +361,7 @@ structs:
                       that contains 'any'"
         fields:
             newBsonSerializationTypeAnyUnstableStructField:
-                unstable: true
+                stability: unstable
                 type: newBsonSerializationTypeAny
 
     BsonSerializationTypeAnyUnstableStruct:
@@ -362,7 +370,7 @@ structs:
                       that contains 'any' that is not explicitly allowed."
         fields:
             bsonSerializationTypeAnyUnstableStructField:
-                unstable: true
+                stability: unstable
                 type: bsonSerializationTypeAny
 
     BsonSerializationTypeWithVariantAnyUnstableStruct:
@@ -370,7 +378,7 @@ structs:
                         that contains 'any' that is not compatible and where the old field is unstable"
         fields:
             bsonSerializationTypeWithVariantAnyUnstableStructField:
-                unstable: true
+                stability: unstable
                 type:
                     variant: [oldBsonSerializationTypeAny, newBsonSerializationTypeAny]
 
@@ -380,7 +388,7 @@ structs:
                         not equal and where the old field is unstable"
         fields:
             cppTypeNotEqualStructUnstableField:
-                unstable: true
+                stability: unstable
                 type: bsonSerializationTypeAnyCppTypeNotEqual
 
     SerializerNotEqualUnstableStruct:
@@ -389,7 +397,7 @@ structs:
                         not equal and where the old field is unstable"
         fields:
             serializerNotEqualStructUnstableField:
-                unstable: true
+                stability: unstable
                 type: bsonSerializationTypeAnySerializerNotEqual
 
     DeserializerNotEqualUnstableStruct:
@@ -398,7 +406,7 @@ structs:
                         not equal and where the old field is unstable"
         fields:
             deserializerNotEqualStructUnstableField:
-                unstable: true
+                stability: unstable
                 type: bsonSerializationTypeAnyDeserializerNotEqual
 
     BsonSerializationTypeWithVariantAnyStruct:
@@ -406,7 +414,7 @@ structs:
                         bson_serialization_type that contains 'any' that is not compatible"
         fields:
             bsonSerializationTypeAnyStructField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [oldBsonSerializationTypeAny, newBsonSerializationTypeAny]
 
@@ -415,7 +423,7 @@ structs:
                       bson_serialization_type that contains 'any' that is not compatible"
         fields:
             bsonSerializationTypeAnyStructField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<oldBsonSerializationTypeAny>,
                               array<newBsonSerializationTypeAny>]
@@ -426,7 +434,7 @@ structs:
         fields:
             cppTypeNotEqualReplyField:
                 type: bsonSerializationTypeAnyCppTypeNotEqual
-                unstable: false
+                stability: stable
 
     SerializerNotEqualReply:
         description: "This reply contains a field whose old and new type have a
@@ -435,7 +443,7 @@ structs:
         fields:
             serializerNotEqualReplyField:
                 type: bsonSerializationTypeAnySerializerNotEqual
-                unstable: false
+                stability: stable
 
     DeserializerNotEqualReply:
         description: "This reply contains a field whose old and new type have a
@@ -444,7 +452,7 @@ structs:
         fields:
             deserializerNotEqualReplyField:
                 type: bsonSerializationTypeAnyDeserializerNotEqual
-                unstable: false
+                stability: stable
 
     StructFieldTypeRecursiveReplyOne:
         description: "This reply contains a field whose new type is a struct that is not
@@ -452,7 +460,7 @@ structs:
         fields:
             structReplyField:
                 type: UnstableNewFieldReply
-                unstable: false
+                stability: stable
 
     StructFieldTypeRecursiveReplyTwo:
         description: "This reply contains a field whose new type is a struct that is not
@@ -460,7 +468,7 @@ structs:
         fields:
             structReplyField:
                 type: StructType
-                unstable: false
+                stability: stable
 
     StructType:
         description: "This struct contains a field whose new type is incompatible with the
@@ -468,7 +476,7 @@ structs:
         fields:
             fieldOne:
                 type: BsonNotSubsetReply
-                unstable: false
+                stability: stable
 
     NewVariantTypeReply:
         description: "This reply contains a new field that has a variant type while the old field
@@ -476,7 +484,7 @@ structs:
         fields:
             newVariantTypeReplyField:
                 type: int
-                unstable: false
+                stability: stable
 
     NewVariantNotSubsetReply:
         description: "This reply contains a field whose new variant types are not a subset
@@ -485,14 +493,14 @@ structs:
             variantNotSubsetReplyField:
                 type:
                     variant: [int, string]
-                unstable: false
+                stability: stable
 
     NewVariantNotSubsetReplyWithArray:
         description: "This reply contains a field whose new variant types are not a subset
                       of the old variant types"
         fields:
             variantNotSubsetReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>]
 
@@ -501,7 +509,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSubsetReplyFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string]
 
@@ -510,7 +518,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSubsetReplyFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>]
 
@@ -519,7 +527,7 @@ structs:
                       compatible with the old variant type"
         fields:
             variantRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringToIntStringBool]
 
@@ -528,7 +536,7 @@ structs:
                       compatible with the old variant type"
         fields:
             variantRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<intStringToIntStringBool>]
 
@@ -537,7 +545,7 @@ structs:
                       of the old variant types"
         fields:
             variantStructNotSubsetReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string]
 
@@ -546,7 +554,7 @@ structs:
                       of the old variant types"
         fields:
             variantStructNotSubsetReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>]
 
@@ -555,7 +563,7 @@ structs:
                       compatible with the old variant struct type"
         fields:
             variantStructRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, StructFieldTypeRecursiveReplyTwo]
 
@@ -564,7 +572,7 @@ structs:
                       compatible with the old variant struct type"
         fields:
             variantStructRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<StructFieldTypeRecursiveReplyTwo>]
 
@@ -574,7 +582,7 @@ structs:
         fields:
             stableToUnstableField:
                 type: string
-                unstable: false
+                stability: stable
 
     CommandParamStructRecursiveTwo:
         description: "This command parameter struct type contains a field that
@@ -582,7 +590,7 @@ structs:
         fields:
             notSupersetField:
                 type: intStringToInt
-                unstable: false
+                stability: stable
 
     NewValidatorStruct:
         description: "This struct contains a field where the new version contains a validator while
@@ -590,7 +598,7 @@ structs:
         fields:
             newValidatorField:
                 type: int
-                unstable: false
+                stability: stable
 
     ValidatorsNotEqualStruct:
         description: "This struct contains a field where the new and old validators are not exactly equal"
@@ -603,7 +611,7 @@ structs:
                     lte: 2.0
                     gte: -2.97
                     callback: "callback"
-                unstable: false
+                stability: stable
 
     AddedRequiredTypeFieldStruct:
         description: "This struct contains a field that is added and required in the new version."
@@ -614,7 +622,7 @@ structs:
         fields:
             stableRequiredNoDefaultTypeField:
                 type: string
-                unstable: true
+                stability: unstable
 
     StructCommandParameterTypeRecursive:
         description: "This param struct type contains a field that is not compatible between the
@@ -622,14 +630,14 @@ structs:
         fields:
             structCommandParameterTypeRecursiveField:
                 type: intStringToInt
-                unstable: false
+                stability: stable
 
     VariantNotSupersetStruct:
         description: "This struct contains a field where the new variant types are not a superset
                       of the old variant types"
         fields:
             variantNotSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string]
 
@@ -638,7 +646,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<bool>, array<string>]
 
@@ -647,7 +655,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSupersetFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string, double]
 
@@ -656,7 +664,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSupersetFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<bool>, array<string>, array<double>]
 
@@ -665,7 +673,7 @@ structs:
                       old one is"
         fields:
             variantField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool]
 
@@ -674,7 +682,7 @@ structs:
                       compatible with the old variant types"
         fields:
             variantRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringBoolToIntString]
 
@@ -683,7 +691,7 @@ structs:
                       compatible with the old variant types"
         fields:
             variantRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<intStringBoolToIntString>]
 
@@ -692,7 +700,7 @@ structs:
                       while the new one does not"
         fields:
             variantStructNotSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructCommandParameterType]
 
@@ -701,7 +709,7 @@ structs:
                       while the new one does not"
         fields:
             variantStructNotSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>, array<StructCommandParameterType>]
 
@@ -710,7 +718,7 @@ structs:
                       variant struct are not compatible"
         fields:
             variantStructRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructCommandParameterTypeRecursive]
 
@@ -719,7 +727,7 @@ structs:
                       variant struct are not compatible"
         fields:
             variantStructRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>, array<StructCommandParameterTypeRecursive>]
 
@@ -728,19 +736,19 @@ structs:
         fields:
             ArrayCommandParameter:
                 type: array<string>
-                unstable: false
+                stability: stable
 
     MissingUnstableFieldNewFieldStruct:
-        description: "This struct contains a field that missing the 'unstable' field in the new
+        description: "This struct contains a field that missing the 'stability' field in the new
                       command."
         fields:
             missingUnstableFieldNewField:
                 type: string
-                unstable: false
+                stability: stable
 
     MissingUnstableFieldAddedNewFieldStruct:
         description: "This struct contains an added field in the new command that is missing the
-                      'unstable' field."
+                      'stability' field."
 
     IncompatibleChainedStructReply:
         description: "This reply contains an incompatible chained struct"
@@ -765,14 +773,14 @@ structs:
         fields:
             ok:
                 type: optionalBool
-                unstable: false
+                stability: stable
 
     BoolToOptionalBoolStruct:
         description: "bool field replaced by optionalBool"
         fields:
             ok:
                 type: bool
-                unstable: false
+                stability: stable
 
 commands:
     invalidAPIVersionOld:
@@ -825,7 +833,7 @@ commands:
         fields:
             parameterToRemove:
                 type: string
-                unstable: false
+                stability: stable
 
     addedNewCommandParameterRequired:
         description: "new command parameter fails because it is required when
@@ -858,7 +866,21 @@ commands:
         fields:
             newUnstableParameter:
                 type: string
-                unstable: false
+                stability: stable
+
+    commandParameterInternal:
+        description: "new internal command parameter fails because it is stable
+                      in the corresponding old command"
+        command_name: commandParameterInternal
+        namespace: ignored
+        cpp_name: commandParameterInternal
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+        fields:
+            newInternalParameter:
+                type: string
+                stability: stable
 
     commandParameterStableRequiredNoDefault:
         description: "new required stable command parameter with no default value fails
@@ -872,7 +894,7 @@ commands:
         fields:
             newRequiredStableParam:
                 type: string
-                unstable: true
+                stability: unstable
 
     commandParameterRequired:
         description: "new required command parameter fails because it is optional
@@ -887,7 +909,7 @@ commands:
             newRequiredParam:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     oldCommandParameterTypeBsonSerializationAny:
         description: "old command fails because it has a parameter type that has a
@@ -901,7 +923,7 @@ commands:
         fields:
             bsonTypeAnyParam:
                 type: oldBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeBsonSerializationAny:
         description: "new command fails because it has a parameter type that has a
@@ -915,7 +937,7 @@ commands:
         fields:
             bsonTypeAnyParam:
                 type: newBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     oldParamTypeBsonAnyAllowList:
         description: "old command fails because it has a parameter type that has a
@@ -930,7 +952,7 @@ commands:
         fields:
             bsonTypeAnyParam:
                 type: oldBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     newParamTypeBsonAnyAllowList:
         description: "new command fails because it has a parameter type that has a
@@ -945,7 +967,7 @@ commands:
         fields:
             bsonTypeAnyParam:
                 type: newBsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     commandParameterTypeBsonSerializationAnyNotAllowed:
         description: "command fails because it has a parameter type that has a
@@ -960,7 +982,7 @@ commands:
         fields:
             bsonTypeAnyParam:
                 type: bsonSerializationTypeAny
-                unstable: false
+                stability: stable
 
     commandParameterCppTypeNotEqual:
         description: "command fails because it has a parameter type that has a
@@ -975,7 +997,7 @@ commands:
         fields:
             cppTypeNotEqualParam:
                 type: bsonSerializationTypeAnyCppTypeNotEqual
-                unstable: false
+                stability: stable
 
     commandParameterSerializerNotEqual:
         description: "command fails because it has a parameter type that has a
@@ -990,7 +1012,7 @@ commands:
         fields:
             serializerNotEqualParam:
                 type: bsonSerializationTypeAnySerializerNotEqual
-                unstable: false
+                stability: stable
 
     commandParameterDeserializerNotEqual:
         description: "command fails because it has a parameter type that has a
@@ -1005,7 +1027,7 @@ commands:
         fields:
             deserializerNotEqualParam:
                 type: bsonSerializationTypeAnyDeserializerNotEqual
-                unstable: false
+                stability: stable
 
     oldCommandParamTypeBsonAnyUnstable:
         description: "old command fails because it has a parameter type that has a
@@ -1019,7 +1041,7 @@ commands:
         fields:
             bsonTypeAnyUnstableParam:
                 type: oldBsonSerializationTypeAny
-                unstable: true
+                stability: unstable
 
     newCommandParamTypeBsonAnyUnstable:
         description: "new command fails because it has a parameter type that has a
@@ -1033,7 +1055,7 @@ commands:
         fields:
             bsonTypeAnyUnstableParam:
                 type: newBsonSerializationTypeAny
-                unstable: true
+                stability: unstable
 
     commandParamTypeBsonAnyNotAllowedUnstable:
         description: "command fails because it has a parameter type that has a
@@ -1048,7 +1070,7 @@ commands:
         fields:
             bsonTypeAnyUnstableParam:
                 type: bsonSerializationTypeAny
-                unstable: true
+                stability: unstable
 
     commandParameterCppTypeNotEqualUnstable:
         description: "command fails because it has a parameter type that has a
@@ -1063,7 +1085,7 @@ commands:
         fields:
             cppTypeNotEqualParam:
                 type: bsonSerializationTypeAnyCppTypeNotEqual
-                unstable: true
+                stability: unstable
 
     commandParameterSerializerNotEqualUnstable:
         description: "command fails because it has a parameter type that has a
@@ -1078,7 +1100,7 @@ commands:
         fields:
             serializerNotEqualParam:
                 type: bsonSerializationTypeAnySerializerNotEqual
-                unstable: true
+                stability: unstable
 
     commandParameterDeserializerNotEqualUnstable:
         description: "command fails because it has a parameter type that has a
@@ -1093,7 +1115,7 @@ commands:
         fields:
             deserializerNotEqualParam:
                 type: bsonSerializationTypeAnyDeserializerNotEqual
-                unstable: true
+                stability: unstable
 
     parameterFieldTypeBsonAnyWithVariantUnstable:
         description: "command fails when its paramter field variant types have bson_serialization_type
@@ -1107,7 +1129,7 @@ commands:
         fields:
             variantAnyUnstableField:
                 type: BsonSerializationTypeWithVariantAnyStruct
-                unstable: true
+                stability: unstable
 
     newlyAddedParamBsonAnyNotAllowed:
         description: "command fails when its parameter is newly added and has bson type 'any'
@@ -1131,7 +1153,7 @@ commands:
         fields:
             enumNotSupersetParam:
                 type: EnumNotSuperset
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeNotEnum:
         description: "new command fails because its parameter type is not an enum when the
@@ -1145,7 +1167,7 @@ commands:
         fields:
             newParamNotEnum:
                 type: EnumNotSuperset
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeNotStruct:
         description: "new command fails because its parameter type is not a struct when the
@@ -1159,7 +1181,7 @@ commands:
         fields:
             newParamNotStruct:
                 type: StructCommandParameterType
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeEnumOrStructOne:
         description: "new command fails because its parameter type is an enum while the
@@ -1173,7 +1195,7 @@ commands:
         fields:
             newParamEnum:
                 type: string
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeEnumOrStructTwo:
         description: "new command fails because its parameter type is a struct while the
@@ -1187,7 +1209,7 @@ commands:
         fields:
             newParamStruct:
                 type: string
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeBsonNotSuperset:
         description: "new command fails because its parameter type has a bson_serialization_type
@@ -1202,7 +1224,7 @@ commands:
         fields:
             newParamBsonNotSuperset:
                 type: intStringToInt
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeStructRecursiveOne:
         description: "new command fails because its parameter type is a struct that is
@@ -1216,7 +1238,7 @@ commands:
         fields:
             stableToUnstableStructParameter:
                 type: CommandParamStructRecursiveOne
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeStructRecursiveTwo:
         description: "new command fails because its parameter type is a struct that is
@@ -1230,7 +1252,7 @@ commands:
         fields:
             notSupersetStructParameter:
                 type: CommandParamStructRecursiveTwo
-                unstable: false
+                stability: stable
 
     newCommandParameterValidator:
         description: "new command fails because it contains a parameter that contains a validator
@@ -1244,7 +1266,7 @@ commands:
         fields:
             newParam:
                 type: int
-                unstable: false
+                stability: stable
 
     commandParameterValidatorsNotEqual:
         description: "new command fails because it contains a parameter that contains a validator
@@ -1257,7 +1279,7 @@ commands:
         reply_type: OkReply
         fields:
             newParam:
-                unstable: false
+                stability: stable
                 type: double
                 validator:
                     lt: 0.0
@@ -1298,6 +1320,16 @@ commands:
         api_version: "1"
         reply_type: UnstableNewFieldReply
 
+    newReplyFieldInternal:
+        description: "new command fails because it contains an internal reply field that is stable
+                      in the corresponding old command"
+        command_name: newReplyFieldInternal
+        namespace: ignored
+        cpp_name: newReplyFieldInternal
+        strict: true
+        api_version: "1"
+        reply_type: InternalNewFieldReply
+
     unstableToStableReplyField:
         description: "new command fails because it contains a stable reply field that is unstable
                       in the corresponding old command"
@@ -1489,7 +1521,7 @@ commands:
         reply_type: OkReply
         fields:
             variantAnyField:
-                unstable: false
+                stability: stable
                 type: BsonSerializationTypeWithVariantAnyStruct
 
     parameterFieldTypeBsonAnyWithVariantWithArray:
@@ -1503,7 +1535,7 @@ commands:
         reply_type: OkReply
         fields:
             variantAnyField:
-                unstable: false
+                stability: stable
                 type: BsonSerializationTypeWithVariantAnyStructWithArray
 
     commandTypeBsonAnyWithVariant:
@@ -1978,6 +2010,17 @@ commands:
         api_version: "1"
         reply_type: OkReply
 
+    newTypeFieldInternal:
+        description: "new command fails because it contains an internal type field that is stable
+                      in the corresponding old command"
+        command_name: newTypeFieldInternal
+        namespace: type
+        type: InternalNewFieldReply
+        cpp_name: newTypeFieldInternal
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+
     newTypeFieldRequired:
         description: "new command fails because it contains a required reply field that is
                       optional in the corresponding old command"
@@ -2159,7 +2202,7 @@ commands:
         reply_type: OkReply
         fields:
             variantNotSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string]
 
@@ -2174,7 +2217,7 @@ commands:
         reply_type: OkReply
         fields:
             variantNotSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<bool>, array<string>]
 
@@ -2189,7 +2232,7 @@ commands:
         reply_type: OkReply
         fields:
             variantNotSupersetParamTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string, double]
 
@@ -2204,7 +2247,7 @@ commands:
         reply_type: OkReply
         fields:
             variantNotSupersetParamTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<bool>, array<string>, array<double>]
 
@@ -2219,7 +2262,7 @@ commands:
         reply_type: OkReply
         fields:
             variantParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool]
 
@@ -2234,7 +2277,7 @@ commands:
         reply_type: OkReply
         fields:
             variantRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringBoolToIntString]
 
@@ -2249,7 +2292,7 @@ commands:
         reply_type: OkReply
         fields:
             variantRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<intStringBoolToIntString>]
 
@@ -2264,7 +2307,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructNotSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructCommandParameterType]
 
@@ -2279,7 +2322,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructNotSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>, array<StructCommandParameterType>]
 
@@ -2295,7 +2338,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructCommandParameterTypeRecursive]
 
@@ -2311,7 +2354,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [array<int>, array<string>, array<StructCommandParameterTypeRecursive>]
 
@@ -2541,7 +2584,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     ArrayCommandParameterTypeError:
@@ -2556,7 +2599,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     accessCheckTypeChange:
@@ -2742,7 +2785,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     ArrayCommandTypeErrorNoArrayNew:
@@ -2757,7 +2800,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     ArrayCommandParameterNoArrayOld:
@@ -2772,7 +2815,7 @@ commands:
         fields:
             parameterStruct:
                 type: ArrayTypeStruct
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     ArrayCommandParameterNoArrayNew:
@@ -2787,12 +2830,12 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     newReplyFieldMissingUnstableField:
         description: "new command fails because it contains a reply field that is missing the
-                      'stable' field"
+                      'stability' field"
         command_name: newReplyFieldMissingUnstableField
         namespace: ignored
         cpp_name: newReplyFieldMissingUnstableField
@@ -2802,7 +2845,7 @@ commands:
 
     newCommandTypeFieldMissingUnstableField:
         description: "new command fails because it contains a command type field that is missing the
-                      'unstable' field"
+                      'stability' field"
         command_name: newCommandTypeFieldMissingUnstableField
         namespace: type
         cpp_name: newCommandTypeFieldMissingUnstableField
@@ -2813,7 +2856,7 @@ commands:
 
     newParameterMissingUnstableField:
         description: "new command fails because it contains a parameter that is missing the
-                      'unstable' field"
+                      'stability' field"
         command_name: newParameterMissingUnstableField
         namespace: ignored
         cpp_name: newParameterMissingUnstableField
@@ -2822,12 +2865,12 @@ commands:
         fields:
             missingUnstableFieldNewParameter:
                 type: int
-                unstable: false
+                stability: stable
         reply_type: OkReply
 
     addedNewParameterMissingUnstableField:
         description: "new command fails because it contains a new parameter that is missing the
-                      'unstable' field"
+                      'stability' field"
         command_name: addedNewParameterMissingUnstableField
         namespace: ignored
         cpp_name: addedNewParameterMissingUnstableField
@@ -2837,7 +2880,7 @@ commands:
 
     addedNewReplyFieldMissingUnstableField:
         description: "new command fails because it contains a new reply field that is missing the
-                      'unstable' field"
+                      'stability' field"
         command_name: addedNewReplyFieldMissingUnstableField
         namespace: ignored
         cpp_name: addedNewReplyFieldMissingUnstableField
@@ -2847,7 +2890,7 @@ commands:
 
     addedNewCommandTypeFieldMissingUnstableField:
         description: "new command fails because it contains a new command type field that is missing
-                      the 'unstable' field"
+                      the 'stability' field"
         command_name: addedNewCommandTypeFieldMissingUnstableField
         namespace: type
         type: MissingUnstableFieldAddedNewFieldStruct
@@ -2910,7 +2953,7 @@ commands:
         reply_type: OkReply
         fields:
             NewRemovedChainedTypeParameter:
-                unstable: false
+                stability: stable
                 type: NewRemovedChainedTypeStruct
 
     newReplyAddedChainedType:
@@ -2931,7 +2974,7 @@ commands:
         api_version: "1"
         fields:
             flag1:
-                unstable: false
+                stability: stable
                 type: optionalBool
         reply_type: OkReply
 
diff --git a/buildscripts/idl/tests/compatibility_test_fail/old/error_reply.idl b/buildscripts/idl/tests/compatibility_test_fail/old/error_reply.idl
index 08a5a271c98..108e751de88 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/old/error_reply.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/old/error_reply.idl
@@ -41,7 +41,7 @@ structs:
         fields:
             errorLabels:
                 type: StructType
-                unstable: false
+                stability: stable
     
     StructType:
         description: "This struct contains a field that is non-optional in the old
@@ -49,4 +49,4 @@ structs:
         fields:
             structField:
                 type: string
-                unstable: false
+                stability: stable
diff --git a/buildscripts/idl/tests/compatibility_test_fail/old/imports.idl b/buildscripts/idl/tests/compatibility_test_fail/old/imports.idl
index 127b1b71205..a5b187052d0 100644
--- a/buildscripts/idl/tests/compatibility_test_fail/old/imports.idl
+++ b/buildscripts/idl/tests/compatibility_test_fail/old/imports.idl
@@ -36,4 +36,4 @@ structs:
         fields:
             unstableNewField:
                 type: string
-                unstable: false
+                stability: stable
diff --git a/buildscripts/idl/tests/compatibility_test_pass/new/compatibility_test_pass_new.idl b/buildscripts/idl/tests/compatibility_test_pass/new/compatibility_test_pass_new.idl
index 5302dd01ed0..0bc6a4964aa 100644
--- a/buildscripts/idl/tests/compatibility_test_pass/new/compatibility_test_pass_new.idl
+++ b/buildscripts/idl/tests/compatibility_test_pass/new/compatibility_test_pass_new.idl
@@ -98,7 +98,7 @@ structs:
         fields:
             stableNewField:
                 type: string
-                unstable: false
+                stability: stable
 
     UnstableNewFieldReplyIgnoreList:
         description: "This reply contains a field that is stable in the old command and is
@@ -106,7 +106,7 @@ structs:
         fields:
             unstableNewFieldIgnoreList:
                 type: string
-                unstable: true
+                stability: unstable
 
     NewReplyTypeEnumOrStructIgnoreList:
         description: "the type is a non-enum or struct type in the old command, and an enum or struct
@@ -114,7 +114,7 @@ structs:
         fields:
             newReplyTypeEnumOrStructIgnoreList:
                 type: StructType
-                unstable: true
+                stability: unstable
 
     RequiredNewFieldReply:
         description: "This reply contains a field that is optional in the old command but is
@@ -122,7 +122,7 @@ structs:
         fields:
             requiredNewField:
                 type: string
-                unstable: false
+                stability: stable
 
     OptionalNewField:
         description: "This struct contains a field that is required in the old command but is
@@ -131,14 +131,14 @@ structs:
             optionalNewField:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     AddedNewFieldReply:
         description: "This reply contains a field that is added in the new command."
         fields:
             addedNewField:
                 type: string
-                unstable: false
+                stability: stable
 
     UnstableOldFieldReply:
         description: "This reply contains a field that is unstable in the old command and is
@@ -146,7 +146,7 @@ structs:
         fields:
             unstableOldField:
                 type: string
-                unstable: true
+                stability: unstable
                 optional: true
 
     UnstableOptionalNewFieldReplyIgnoreList:
@@ -155,7 +155,7 @@ structs:
         fields:
             unstableOptionalNewFieldIgnoreList:
                 type: string
-                unstable: true
+                stability: unstable
                 optional: true
 
     EnumSubsetReply:
@@ -164,7 +164,7 @@ structs:
         fields:
             replyField:
                 type: NewReplyFieldEnumSubset
-                unstable: false
+                stability: stable
 
     BsonSubsetReply:
         description: "This reply contains a field type where the new bson_serialization_type
@@ -172,7 +172,7 @@ structs:
         fields:
             bsonSubsetReplyField:
                 type: intStringToInt
-                unstable: false
+                stability: stable
 
     BsonSubsetReplyTwo:
         description: "This reply contains a field type where the new bson_serialization_type
@@ -180,7 +180,7 @@ structs:
         fields:
             bsonSubsetReplyFieldTwo:
                 type: intStringBoolToIntString
-                unstable: false
+                stability: stable
 
     StructFieldTypeRecursiveReplyIgnoreList:
         description: "This reply contains a field whose new type is a struct that is not
@@ -188,7 +188,7 @@ structs:
         fields:
             structReplyField:
                 type: UnstableNewFieldReplyIgnoreList
-                unstable: true
+                stability: unstable
 
     StructFieldTypeRecursiveReplyOne:
         description: "This reply contains a field whose new type is a struct that is
@@ -196,7 +196,7 @@ structs:
         fields:
             structReplyField:
                 type: StableNewFieldReply
-                unstable: false
+                stability: stable
 
     StructFieldTypeRecursiveReplyTwo:
         description: "This reply contains a field whose new type is a struct that is
@@ -204,7 +204,7 @@ structs:
         fields:
             structReplyField:
                 type: StructType
-                unstable: false
+                stability: stable
 
     StructType:
         description: "This struct contains a field whose new type is compatible with the
@@ -212,7 +212,7 @@ structs:
         fields:
             fieldOne:
                 type: BsonSubsetReply
-                unstable: false
+                stability: stable
 
     OldVariantTypeReply:
         description: "This reply contains an old field that has a variant type while the new field
@@ -220,7 +220,7 @@ structs:
         fields:
             oldVariantTypeReplyField:
                 type: int
-                unstable: false
+                stability: stable
 
     NewVariantNotSubsetReplyIgnoreList:
         description: "This reply contains a field whose new variant types are not a subset
@@ -229,14 +229,14 @@ structs:
             variantNotSubsetReplyFieldIgnoreList:
                 type:
                     variant: [int, string]
-                unstable: true
+                stability: unstable
 
     NewVariantSubsetReply:
         description: "This reply contains a field whose new variant types are a subset
                       of the old variant types"
         fields:
             variantSubsetReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, array<string>]
 
@@ -245,7 +245,7 @@ structs:
                       of the old variant types"
         fields:
             variantSubsetReplyFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, array<string>]
 
@@ -254,7 +254,7 @@ structs:
                       with the old variant type"
         fields:
             variantRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringBoolToIntString,
                               array<intStringBoolToIntString>, array<string>]
@@ -264,7 +264,7 @@ structs:
                       struct type while the old one does"
         fields:
             variantStructReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, array<string>]
 
@@ -273,7 +273,7 @@ structs:
                       compatible with the old variant struct type"
         fields:
             variantStructRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, StructFieldTypeRecursiveReplyTwo,
                               array<StructFieldTypeRecursiveReplyTwo>, array<string>]
@@ -283,7 +283,7 @@ structs:
                       different from the old variant struct type"
         fields:
             variantStructRecursiveReplyFieldIgnoreList:
-                unstable: true
+                stability: unstable
                 type:
                     variant: [int, StructFieldTypeRecursiveReplyTwo]
 
@@ -292,7 +292,7 @@ structs:
                       type, but the field is in the ignore list"
         fields:
             nonVariantToVariantReplyFieldIgnoreList:
-                unstable: true
+                stability: unstable
                 type:
                     variant: [int, StructFieldTypeRecursiveReplyOne]
 
@@ -302,7 +302,7 @@ structs:
         fields:
             nonEnumToEnumReplyIgnoreList:
                 type: EnumSubsetReply
-                unstable: true
+                stability: unstable
 
     CommandParamStructRecursiveOne:
         description: "This command parameter struct type contains a stable and optional
@@ -311,7 +311,7 @@ structs:
             unstableToStableOptionalField:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     CommandParamStructRecursiveTwo:
         description: "This command parameter struct type contains a field whose new type is
@@ -319,7 +319,7 @@ structs:
         fields:
             supersetField:
                 type: intToIntString
-                unstable: false
+                stability: stable
 
     OldValidatorStruct:
         description: "This struct contains a field where the old version contains a validator while
@@ -327,14 +327,14 @@ structs:
         fields:
             oldValidatorField:
                 type: int
-                unstable: false
+                stability: stable
 
     ValidatorsEqualStruct:
         description: "This struct contains a field where the new and old validator are exactly equal"
         fields:
             validatorsEqualField:
                 type: double
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0.0
                     gt: -1.1
@@ -352,7 +352,7 @@ structs:
             addedOptionalTypeField:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     StableOptionalTypeFieldStruct:
         description: "This struct contains a field that is stable and optional in the new version
@@ -361,7 +361,7 @@ structs:
             stableOptionalTypeField:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     StableWithDefaultTypeFieldStruct:
         description: "This struct contains a field that is stable and required with a default value
@@ -370,18 +370,29 @@ structs:
             stableWithDefaultTypeField:
                 type: string
                 default: ""
-                unstable: false
+                stability: stable
 
     RemovedUnstableTypeFieldStruct:
         description: "This struct contains a field that is unstable in the old version and
                       is removed in the new version"
 
+    RemovedInternalTypeFieldStruct:
+        description: "This struct contains a field that is internal in the old version and
+                      is removed in the new version"
+
     AddedUnstableTypeFieldStruct:
         description: "This struct contains a field that is added and unstable in the new version"
         fields:
             addedUnstableTypeField:
                 type: string
-                unstable: true
+                stability: unstable
+
+    AddedInternalTypeFieldStruct:
+        description: "This struct contains a field that is added and internal in the new version"
+        fields:
+            addedInternalTypeField:
+                type: string
+                stability: internal
 
     KeptUnstableTypeFieldStruct:
         description: "This struct contains a field that is unstable in the new version and also
@@ -389,7 +400,15 @@ structs:
         fields:
             keptUnstableTypeField:
                 type: string
-                unstable: true
+                stability: unstable
+
+    KeptInternalTypeFieldStruct:
+        description: "This struct contains a field that is internal in the new version and also
+                      exists in the old version"
+        fields:
+            keptInternalTypeField:
+                type: string
+                stability: internal
 
     StructCommandParameterTypeRecursive:
         description: "This param struct type contains a field that is compatible between the
@@ -397,14 +416,14 @@ structs:
         fields:
             structCommandParameterTypeRecursiveField:
                 type: intStringToIntStringBool
-                unstable: false
+                stability: stable
 
     VariantSupersetStruct:
         description: "This struct contains a field where the new variant types are a superset
                       of the old variant types"
         fields:
             variantSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string, array<string>]
 
@@ -413,7 +432,7 @@ structs:
                       of the old variant types"
         fields:
             variantSupersetFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string, double, array<string>]
 
@@ -422,7 +441,7 @@ structs:
                       old one is not"
         fields:
             variantField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, array<string>]
 
@@ -431,7 +450,7 @@ structs:
                       compatible with the old variant types"
         fields:
             variantRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringToIntStringBool,
                               array<intStringToIntStringBool>, array<string>]
@@ -441,7 +460,7 @@ structs:
                       while the old one does not"
         fields:
             variantStructSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, NewCommandParameterStruct,
                               array<NewCommandParameterStruct>, array<string>]
@@ -451,7 +470,7 @@ structs:
                       variant struct are compatible"
         fields:
             variantStructRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructCommandParameterTypeRecursive,
                               array<StructCommandParameterTypeRecursive>, array<string>]
@@ -461,7 +480,7 @@ structs:
         fields:
             anyTypeField:
                 type: bsonSerializationTypeAnyAllowed
-                unstable: false
+                stability: stable
 
     NewlyAddedBsonSerializationTypeAnyReply:
         description: "This reply contains a newly added field whose type has a bson_serialization_type
@@ -469,13 +488,22 @@ structs:
         fields:
             newlyAddedBsonSerializationTypeAnyReplyField:
                 type: bsonSerializationTypeAnyAllowed
-                unstable: false
+                stability: stable
 
     OldUnstableTypeChangesReply:
         description: "This reply contains a field that is unstable in the old version and has type changes"
         fields:
             oldUnstableTypeChangesReplyField:
-                unstable: true
+                stability: unstable
+                type: intStringToIntStringBool
+                validator:
+                    lt: 0
+
+    OldInternalTypeChangesReply:
+        description: "This reply contains a field that is internal in the old version and has type changes"
+        fields:
+            oldInternalTypeChangesReplyField:
+                stability: internal
                 type: intStringToIntStringBool
                 validator:
                     lt: 0
@@ -485,7 +513,7 @@ structs:
                       but is also in the ignore list"
         fields:
             newUnstableTypeChangesReplyFieldIgnoreList:
-                unstable: true
+                stability: unstable
                 type: intStringToIntStringBool
                 validator:
                     lt: 0
@@ -497,7 +525,7 @@ structs:
             newlyAddedBsonSerializationTypeAnyStructField:
                 type: bsonSerializationTypeAnyAllowed
                 optional: true
-                unstable: false
+                stability: stable
 
     OldUnstableTypeChangesStruct:
         description: "This struct contains a field that is unstable in the old version and has type changes"
@@ -505,7 +533,7 @@ structs:
             oldUnstableTypeChangesField:
                 type: intStringBoolToIntString
                 optional: true
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0
 
@@ -515,7 +543,7 @@ structs:
         fields:
             newUnstableTypeChangesFieldIgnoreList:
                 type: intStringBoolToIntString
-                unstable: true
+                stability: unstable
                 validator:
                     lt: 0
 
@@ -524,7 +552,7 @@ structs:
                       bson serialization types is 'any' and is explicitly allowed"
         fields:
             anyTypeField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [bsonSerializationTypeAnyAllowed, StructFieldTypeRecursiveReplyTwo,
                               array<bsonSerializationTypeAnyAllowed>,
@@ -535,22 +563,22 @@ structs:
         fields:
             ArrayCommandParameter:
                 type: array<string>
-                unstable: false
+                stability: stable
 
     MissingUnstableFieldNewFieldStruct:
-        description: "This struct contains a field that missing the 'unstable' field in the new
+        description: "This struct contains a field that missing the 'stability' field in the new
                       command."
         fields:
             missingUnstableFieldNewField:
                 type: string
 
     MissingUnstableFieldOldFieldStruct:
-        description: "This struct contains a field that missing the 'unstable' field in the old
+        description: "This struct contains a field that missing the 'stability' field in the old
                       command."
         fields:
             missingUnstableFieldNewField:
                 type: string
-                unstable: false
+                stability: stable
 
     CompatibleChainedStructReply:
         description: "This reply contains an compatible chained struct"
@@ -567,61 +595,61 @@ structs:
         fields:
             ok1:
                 type: bool
-                unstable: false
+                stability: stable
                 optional: true
             ok2:
                 type: optionalBool
-                unstable: false
+                stability: stable
 
     OptionalBoolEquivalenceStruct:
         description: "First two fields get the types swapped."
         fields:
             ok1:
                 type: optionalBool
-                unstable: false
+                stability: stable
             ok2:
                 type: bool
-                unstable: false
+                stability: stable
                 optional: true
             ok3:
                 type: optionalBool
-                unstable: false
+                stability: stable
 
     OptionalBoolToBoolStruct:
         description: "optionalBool field replaced by bool"
         fields:
             ok:
                 type: bool
-                unstable: false
+                stability: stable
 
     BoolToOptionalBoolStruct:
         description: "bool field replaced by optionalBool"
         fields:
             ok:
                 type: optionalBool
-                unstable: false
+                stability: stable
 
     ReplyWithNewArrayTypeField:
         description: "new field is of ArrayType"
         fields:
             ok:
                 type: bool
-                unstable: false
+                stability: stable
             newArrayTypeField:
                 type: array<int>
                 optional: true
-                unstable: false
+                stability: stable
 
     ReplyWithNewNestedArrayTypeField:
         description: "new field is a struct with a field of ArrayType"
         fields:
             ok:
                 type: bool
-                unstable: false
+                stability: stable
             newStructWithArrayTypeField:
                 type: ArrayTypeStruct
                 optional: true
-                unstable: false
+                stability: stable
 
 commands:
     testCommand:
@@ -672,7 +700,7 @@ commands:
             newParameter:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     addedCommandParameterStable:
         description: "new command has an optional stable parameter that is unstable
@@ -687,7 +715,7 @@ commands:
             newOptionalStableParam:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     addedCommandParameterStableWithDefault:
         description: "new command has a required stable parameter with a default value
@@ -702,7 +730,7 @@ commands:
             newStableParamWithDefault:
                 type: string
                 default: ""
-                unstable: false
+                stability: stable
 
     commandParameterUnstableIgnoreList:
         description: "new unstable command parameter is stable in the corresponding old
@@ -716,7 +744,7 @@ commands:
         fields:
             newUnstableParameterIgnoreList:
                 type: string
-                unstable: true
+                stability: unstable
 
     removeCommandParameterUnstable:
         description: "new command removes parameter that is unstable
@@ -741,7 +769,7 @@ commands:
             newOptionalParam:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     addedUnstableCommandParameter:
         description: "new command has a new unstable parameter that did not
@@ -755,7 +783,7 @@ commands:
         fields:
             newUnstableParam:
                 type: string
-                unstable: true
+                stability: unstable
 
     keptUnstableCommandParameter:
         description: "new command has an unstable parameter that also
@@ -769,7 +797,7 @@ commands:
         fields:
             unstableParam:
                 type: string
-                unstable: true
+                stability: unstable
 
     newCommandParameterType:
         description: "new command passes because its command parameter type is compatible with
@@ -783,7 +811,7 @@ commands:
         fields:
             compatibleParameter:
                 type: string
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeEnumSuperset:
         description: "new command passes because its command parameter type is an enum that is
@@ -797,7 +825,7 @@ commands:
         fields:
             parameterEnumSuperset:
                 type: EnumSuperset
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeStruct:
         description: "new command passes because its command parameter type is a struct and the
@@ -811,7 +839,7 @@ commands:
         fields:
             parameterStruct:
                 type: NewCommandParameterStruct
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeBsonSuperset:
         description: "new command passes because its parameter type has a bson_serialization_type
@@ -826,7 +854,7 @@ commands:
         fields:
             bsonSupersetParam:
                 type: intToIntString
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeStructRecursiveOne:
         description: "new command passes because its parameter type is a struct that is
@@ -840,7 +868,7 @@ commands:
         fields:
             unstableToStableOptionalStructParameter:
                 type: CommandParamStructRecursiveOne
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeStructRecursiveTwo:
         description: "new command passes because its parameter type is a struct that is
@@ -854,7 +882,7 @@ commands:
         fields:
             supersetStructParameter:
                 type: CommandParamStructRecursiveTwo
-                unstable: false
+                stability: stable
 
     newlyAddedParamBsonAnyAllowList:
         description: "command passes when its parameter is newly added and has bson type 'any'
@@ -869,7 +897,7 @@ commands:
             newlyAddedBsonAnyAllowListParam:
                 type: bsonSerializationTypeAnyAllowed
                 optional: true
-                unstable: false
+                stability: stable
 
     oldUnstableParamTypeChanges:
         description: "command passes when it has an old unstable param with incompatible type changes"
@@ -883,7 +911,23 @@ commands:
             oldUnstableTypeChangesParam:
                 type: intStringBoolToIntString
                 optional: true
-                unstable: false
+                stability: stable
+                validator:
+                    lt: 0
+
+    oldInternalParamTypeChanges:
+        description: "command passes when it has an old internal param with incompatible type changes"
+        command_name: oldInternalParamTypeChanges
+        namespace: ignored
+        cpp_name: oldInternalParamTypeChanges
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+        fields:
+            oldInternalTypeChangesParam:
+                type: intStringBoolToIntString
+                optional: true
+                stability: internal
                 validator:
                     lt: 0
 
@@ -899,7 +943,7 @@ commands:
         fields:
             newUnstableTypeChangesParamIgnoreList:
                 type: intStringBoolToIntString
-                unstable: true
+                stability: unstable
                 validator:
                     lt: 0
 
@@ -947,7 +991,7 @@ commands:
         fields:
             newParam:
                 type: int
-                unstable: false
+                stability: stable
 
     commandParameterValidatorsEqual:
         description: "new command passes because it contains a parameter that contains a validator
@@ -961,7 +1005,7 @@ commands:
         fields:
             newParam:
                 type: double
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0.0
                     gt: -1.1
@@ -1267,6 +1311,17 @@ commands:
         api_version: "1"
         reply_type: OkReply
 
+    removeTypeFieldInternal:
+        description: "new command removes type field that is internal
+                      in the corresponding old command and still passes"
+        command_name: removeTypeFieldInternal
+        namespace: type
+        type: RemovedInternalTypeFieldStruct
+        cpp_name: removeTypeFieldInternal
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+
     addedUnstableTypeField:
         description: "new command has a new unstable type field that does not
                       exist in the corresponding old command and still passes"
@@ -1278,6 +1333,17 @@ commands:
         api_version: "1"
         reply_type: OkReply
 
+    addedInternalTypeField:
+        description: "new command has a new internal type field that does not
+                      exist in the corresponding old command and still passes"
+        command_name: addedInternalTypeField
+        namespace: type
+        type: AddedInternalTypeFieldStruct
+        cpp_name: addedInternalTypeField
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+
     keptUnstableTypeField:
         description: "new command has an unstable type field that also
                       exists in the corresponding old command and still passes"
@@ -1289,6 +1355,17 @@ commands:
         api_version: "1"
         reply_type: OkReply
 
+    keptInternalTypeField:
+        description: "new command has an internal type field that also
+                      exists in the corresponding old command and still passes"
+        command_name: keptInternalTypeField
+        namespace: type
+        type: KeptInternalTypeFieldStruct
+        cpp_name: keptInternalTypeField
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+
     oldReplyFieldVariantType:
         description: "new command passes when its reply field type is not a variant type while
                       the old reply field is a variant type"
@@ -1408,6 +1485,15 @@ commands:
         api_version: "1"
         reply_type: OldUnstableTypeChangesReply
 
+    oldInternalReplyFieldTypeChanges:
+        description: "command passes when it has an old internal reply field with incompatible type changes"
+        command_name: oldInternalReplyFieldTypeChanges
+        namespace: ignored
+        cpp_name: oldInternalReplyFieldTypeChanges
+        strict: true
+        api_version: "1"
+        reply_type: OldInternalTypeChangesReply
+
     newUnstableReplyFieldTypeChangesIgnoreList:
         description: "command has an old stable reply field with incompatible type changes but it is also
                       in the ignore list"
@@ -1431,7 +1517,7 @@ commands:
         fields:
             anyTypeParam:
                 type: bsonSerializationTypeAnyAllowed
-                unstable: false
+                stability: stable
 
     commandAllowedAnyTypesWithVariant:
         description: "command that has reply variant types with
@@ -1454,7 +1540,7 @@ commands:
         reply_type: OkReply
         fields:
             variantSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string, array<string>]
 
@@ -1469,7 +1555,7 @@ commands:
         reply_type: OkReply
         fields:
             variantSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, bool, double, array<string>]
 
@@ -1484,7 +1570,7 @@ commands:
         reply_type: OkReply
         fields:
             variantParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, array<string>]
 
@@ -1499,7 +1585,7 @@ commands:
         reply_type: OkReply
         fields:
             variantRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringToIntStringBool,
                               array<intStringToIntStringBool>, array<string>]
@@ -1515,7 +1601,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, NewCommandParameterStruct,
                               array<NewCommandParameterStruct>, array<string>]
@@ -1532,7 +1618,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructCommandParameterTypeRecursive,
                               array<StructCommandParameterTypeRecursive>, array<string>]
@@ -1679,7 +1765,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     complexActionTypesSubset:
@@ -1799,7 +1885,7 @@ commands:
 
     newReplyFieldMissingUnstableField:
         description: "new command passes because it does not have an api_version even though it
-                      contains a reply field that is missing the 'unstable' field"
+                      contains a reply field that is missing the 'stability' field"
         command_name: newReplyFieldMissingUnstableField
         namespace: ignored
         cpp_name: newReplyFieldMissingUnstableField
@@ -1809,7 +1895,7 @@ commands:
 
     oldReplyFieldMissingUnstableField:
         description: "old command passes even though it contains a reply field that is missing the
-                      'unstable' field"
+                      'stability' field"
         command_name: oldReplyFieldMissingUnstableField
         namespace: ignored
         cpp_name: oldReplyFieldMissingUnstableField
@@ -1819,7 +1905,7 @@ commands:
 
     newCommandTypeFieldMissingUnstableField:
         description: "new command passes because it does not have an api_version even though it
-                      contains a command type field that is missing the 'unstable' field"
+                      contains a command type field that is missing the 'stability' field"
         command_name: newCommandTypeFieldMissingUnstableField
         namespace: type
         cpp_name: newCommandTypeFieldMissingUnstableField
@@ -1830,7 +1916,7 @@ commands:
 
     oldCommandTypeFieldMissingUnstableField:
         description: "old command passes even though it
-                      contains a command type field that is missing the 'unstable' field"
+                      contains a command type field that is missing the 'stability' field"
         command_name: oldCommandTypeFieldMissingUnstableField
         namespace: type
         cpp_name: oldCommandTypeFieldMissingUnstableField
@@ -1841,7 +1927,7 @@ commands:
 
     newParameterMissingUnstableField:
         description: "new command passes because it does not have an api_version even though it
-                      contains a parameter that is missing the 'unstable' field"
+                      contains a parameter that is missing the 'stability' field"
         command_name: newParameterMissingUnstableField
         namespace: ignored
         cpp_name: newParameterMissingUnstableField
@@ -1854,7 +1940,7 @@ commands:
 
     oldParameterMissingUnstableField:
         description: "old command passes even though it
-                      contains a parameter that is missing the 'unstable' field"
+                      contains a parameter that is missing the 'stability' field"
         command_name: oldParameterMissingUnstableField
         namespace: ignored
         cpp_name: oldParameterMissingUnstableField
@@ -1863,7 +1949,7 @@ commands:
         fields:
             missingUnstableFieldOldParameter:
                 type: int
-                unstable: false
+                stability: stable
         reply_type: OkReply
 
     chainedStructCompatible:
@@ -1921,7 +2007,7 @@ commands:
         fields:
             flag:
                 type: optionalBool
-                unstable: false
+                stability: stable
 
     newReplyOptionalBool:
         description: "new command can add reply fields of type optionalBoolean"
@@ -1951,14 +2037,14 @@ commands:
         api_version: "1"
         fields:
             flag1:
-                unstable: false
+                stability: stable
                 type: optionalBool
             flag2:
-                unstable: false
+                stability: stable
                 type: bool
                 optional: true
             flag3:
-                unstable: false
+                stability: stable
                 type: optionalBool
         reply_type: OkReply
 
@@ -1990,7 +2076,7 @@ commands:
         api_version: "1"
         fields:
             flag:
-                unstable: false
+                stability: stable
                 type: optionalBool
         reply_type: OkReply
 
@@ -2025,7 +2111,7 @@ commands:
             newArrayTypeParameter:
                 type: array<int>
                 optional: true
-                unstable: false
+                stability: stable
         reply_type: ReplyWithNewArrayTypeField
 
     commandWithNewNestedArrayTypeParameterAndNestedArrayTypeReply:
@@ -2040,5 +2126,5 @@ commands:
             newNestedArrayTypeParameter:
                 type: ArrayTypeStruct
                 optional: true
-                unstable: false
+                stability: stable
         reply_type: ReplyWithNewNestedArrayTypeField
\ No newline at end of file
diff --git a/buildscripts/idl/tests/compatibility_test_pass/new/error_reply.idl b/buildscripts/idl/tests/compatibility_test_pass/new/error_reply.idl
index 3023ee8f40a..54926022d87 100644
--- a/buildscripts/idl/tests/compatibility_test_pass/new/error_reply.idl
+++ b/buildscripts/idl/tests/compatibility_test_pass/new/error_reply.idl
@@ -40,7 +40,7 @@ structs:
         fields:
             errorLabels:
                 type: StructType
-                unstable: false
+                stability: stable
     
     StructType:
         description: "This struct contains a field that is optional in the old
@@ -49,4 +49,4 @@ structs:
             structField:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
diff --git a/buildscripts/idl/tests/compatibility_test_pass/new/imports.idl b/buildscripts/idl/tests/compatibility_test_pass/new/imports.idl
index 2fbf1045dc7..1f25aefb673 100644
--- a/buildscripts/idl/tests/compatibility_test_pass/new/imports.idl
+++ b/buildscripts/idl/tests/compatibility_test_pass/new/imports.idl
@@ -39,7 +39,7 @@ structs:
         fields:
             stableNewField:
                 type: string
-                unstable: false
+                stability: stable
 
 commands:
     importCommand:
diff --git a/buildscripts/idl/tests/compatibility_test_pass/old/compatibility_test_pass_old.idl b/buildscripts/idl/tests/compatibility_test_pass/old/compatibility_test_pass_old.idl
index f442e45a20e..18ff435077e 100644
--- a/buildscripts/idl/tests/compatibility_test_pass/old/compatibility_test_pass_old.idl
+++ b/buildscripts/idl/tests/compatibility_test_pass/old/compatibility_test_pass_old.idl
@@ -99,7 +99,7 @@ structs:
         fields:
             stableNewField:
                 type: string
-                unstable: true
+                stability: unstable
 
     UnstableNewFieldReplyIgnoreList:
         description: "This reply contains a field that is stable in the old command and is
@@ -107,7 +107,7 @@ structs:
         fields:
             unstableNewFieldIgnoreList:
                 type: string
-                unstable: false
+                stability: stable
 
     NewReplyTypeEnumOrStructIgnoreList:
         description: "the type is a non-enum or struct type in the old command, and an enum or struct
@@ -115,7 +115,7 @@ structs:
         fields:
             newReplyTypeEnumOrStructIgnoreList:
                 type: string
-                unstable: false
+                stability: stable
 
     RequiredNewFieldReply:
         description: "This reply contains a field that is optional in the old command but is
@@ -124,7 +124,7 @@ structs:
             requiredNewField:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
 
     OptionalNewField:
         description: "This struct contains a field that is required in the old command but is
@@ -132,7 +132,7 @@ structs:
         fields:
             optionalNewField:
                 type: string
-                unstable: false
+                stability: stable
 
     AddedNewFieldReply:
         description: "This reply contains a field that is added in the new command."
@@ -143,14 +143,14 @@ structs:
         fields:
             unstableOldField:
                 type: string
-                unstable: true
+                stability: unstable
 
     UnstableOptionalNewFieldReplyIgnoreList:
         description: "This reply contains a field that is stable in the old command and is
                       unstable and optional in the new command, but the change is allowed"
         fields:
             unstableOptionalNewFieldIgnoreList:
-                unstable: false
+                stability: stable
                 type: string
 
     EnumSubsetReply:
@@ -159,7 +159,7 @@ structs:
         fields:
             replyField:
                 type: NewReplyFieldEnumSubset
-                unstable: false
+                stability: stable
 
     BsonSubsetReply:
         description: "This reply contains a field type where the new bson_serialization_type
@@ -167,7 +167,7 @@ structs:
         fields:
             bsonSubsetReplyField:
                 type: intStringToInt
-                unstable: false
+                stability: stable
 
     BsonSubsetReplyTwo:
         description: "This reply contains a field type where the new bson_serialization_type
@@ -175,7 +175,7 @@ structs:
         fields:
             bsonSubsetReplyFieldTwo:
                 type: intStringBoolToIntString
-                unstable: false
+                stability: stable
 
     StructFieldTypeRecursiveReplyIgnoreList:
         description: "This reply contains a field whose new type is a struct that is not
@@ -183,7 +183,7 @@ structs:
         fields:
             structReplyField:
                 type: UnstableNewFieldReplyIgnoreList
-                unstable: false
+                stability: stable
 
     StructFieldTypeRecursiveReplyOne:
         description: "This reply contains a field whose new type is a struct that is
@@ -191,7 +191,7 @@ structs:
         fields:
             structReplyField:
                 type: StableNewFieldReply
-                unstable: false
+                stability: stable
 
     StructFieldTypeRecursiveReplyTwo:
         description: "This reply contains a field whose new type is a struct that is
@@ -199,7 +199,7 @@ structs:
         fields:
             structReplyField:
                 type: StructType
-                unstable: false
+                stability: stable
 
     StructType:
         description: "This struct contains a field whose new type is compatible with the
@@ -207,7 +207,7 @@ structs:
         fields:
             fieldOne:
                 type: BsonSubsetReply
-                unstable: false
+                stability: stable
 
     StructTypeUnstable:
         description: "This struct contains a field whose new type is compatible with the
@@ -215,14 +215,14 @@ structs:
         fields:
             fieldOne:
                 type: BsonSubsetReply
-                unstable: true
+                stability: unstable
 
     OldVariantTypeReply:
         description: "This reply contains an old field that has a variant type while the new field
                       is not a variant type"
         fields:
             oldVariantTypeReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, array<string>]
 
@@ -231,7 +231,7 @@ structs:
                       of the old variant types"
         fields:
             variantNotSubsetReplyFieldIgnoreList:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string]
 
@@ -240,7 +240,7 @@ structs:
                       of the old variant types"
         fields:
             variantSubsetReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string, array<string>]
 
@@ -249,7 +249,7 @@ structs:
                       of the old variant types"
         fields:
             variantSubsetReplyFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, bool, double, array<string>]
 
@@ -258,7 +258,7 @@ structs:
                       with the old variant type"
         fields:
             variantRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringBoolToIntString,
                               array<intStringBoolToIntString>, array<string>]
@@ -268,7 +268,7 @@ structs:
                       struct type while the old one does"
         fields:
             variantStructReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructType, array<StructType>, array<string>]
 
@@ -277,7 +277,7 @@ structs:
                       compatible with the old variant struct type"
         fields:
             variantStructRecursiveReplyField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, StructFieldTypeRecursiveReplyTwo,
                               array<StructFieldTypeRecursiveReplyTwo>, array<string>]
@@ -287,7 +287,7 @@ structs:
                       different from the old variant struct type"
         fields:
             variantStructRecursiveReplyFieldIgnoreList:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, StructFieldTypeRecursiveReplyOne]
 
@@ -296,7 +296,7 @@ structs:
                       type, but the field is in the ignore list"
         fields:
             nonVariantToVariantReplyFieldIgnoreList:
-                unstable: false
+                stability: stable
                 type: StructType
 
     NonEnumToEnumReplyIgnoreList:
@@ -305,7 +305,7 @@ structs:
         fields:
             nonEnumToEnumReplyIgnoreList:
                 type: StructTypeUnstable
-                unstable: false
+                stability: stable
 
     NewCommandParameterStruct:
         description: "The new command parameter's type and the
@@ -317,7 +317,7 @@ structs:
         fields:
             unstableToStableOptionalField:
                 type: string
-                unstable: true
+                stability: unstable
 
     CommandParamStructRecursiveTwo:
         description: "This command parameter struct type contains a field whose new type is
@@ -325,7 +325,7 @@ structs:
         fields:
             supersetField:
                 type: intToIntString
-                unstable: false
+                stability: stable
 
     OldValidatorStruct:
         description: "This struct contains a field where the old version contains a validator while
@@ -333,7 +333,7 @@ structs:
         fields:
             oldValidatorField:
                 type: int
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0
 
@@ -342,7 +342,7 @@ structs:
         fields:
             validatorsEqualField:
                 type: double
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0.0
                     gt: -1.1
@@ -359,7 +359,7 @@ structs:
         fields:
             stableOptionalTypeField:
                 type: string
-                unstable: true
+                stability: unstable
 
     StableWithDefaultTypeFieldStruct:
         description: "This struct contains a field that is stable and required with a default value
@@ -367,7 +367,7 @@ structs:
         fields:
             stableWithDefaultTypeField:
                 type: string
-                unstable: true
+                stability: unstable
 
     RemovedUnstableTypeFieldStruct:
         description: "This struct contains a field that is unstable in the old version and
@@ -375,18 +375,37 @@ structs:
         fields:
             removedUnstableTypeField:
                 type: string
-                unstable: true
+                stability: unstable
+
+    RemovedInternalTypeFieldStruct:
+        description: "This struct contains a field that is internal in the old version and
+                      is removed in the new version"
+        fields:
+            removedInternalTypeField:
+                type: string
+                stability: internal
 
     AddedUnstableTypeFieldStruct:
         description: "This struct contains a field that is added and unstable in the new version"
 
+    AddedInternalTypeFieldStruct:
+        description: "This struct contains a field that is added and internal in the new version"
+
     KeptUnstableTypeFieldStruct:
         description: "This struct contains a field that is unstable in the new version and also
                       exists in the old version"
         fields:
             keptUnstableTypeField:
                 type: string
-                unstable: true
+                stability: unstable
+
+    KeptInternalTypeFieldStruct:
+        description: "This struct contains a field that is internal in the new version and also
+                      exists in the old version"
+        fields:
+            keptInternalTypeField:
+                type: string
+                stability: internal
 
     StructCommandParameterTypeRecursive:
         description: "This param struct type contains a field that is compatible between the
@@ -394,14 +413,14 @@ structs:
         fields:
             structCommandParameterTypeRecursiveField:
                 type: intStringToIntStringBool
-                unstable: false
+                stability: stable
 
     VariantSupersetStruct:
         description: "This struct contains a field where the new variant types are a superset
                       of the old variant types"
         fields:
             variantSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, array<string>]
 
@@ -410,7 +429,7 @@ structs:
                       of the old variant types"
         fields:
             variantSupersetFieldTwo:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, array<string>]
 
@@ -420,14 +439,14 @@ structs:
         fields:
             variantField:
                 type: int
-                unstable: false
+                stability: stable
 
     VariantRecursiveStruct:
         description: "This struct contains a field where the new variant types are
                       compatible with the old variant types"
         fields:
             variantRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringToIntStringBool,
                               array<intStringToIntStringBool>, array<string>]
@@ -437,7 +456,7 @@ structs:
                       while the old one does not"
         fields:
             variantStructSupersetField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, NewCommandParameterStruct,
                               array<NewCommandParameterStruct>, array<string>]
@@ -447,7 +466,7 @@ structs:
                       variant struct are compatible"
         fields:
             variantStructRecursiveField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructCommandParameterTypeRecursive,
                               array<StructCommandParameterTypeRecursive>, array<string>]
@@ -457,7 +476,7 @@ structs:
         fields:
             anyTypeField:
                 type: bsonSerializationTypeAnyAllowed
-                unstable: false
+                stability: stable
 
     NewlyAddedBsonSerializationTypeAnyReply:
         description: "This reply contains a newly added field whose type has a bson_serialization_type
@@ -467,7 +486,14 @@ structs:
         description: "This reply contains a field that is unstable in the old version and has type changes"
         fields:
             oldUnstableTypeChangesReplyField:
-                unstable: true
+                stability: unstable
+                type: intStringToIntStringBool
+
+    OldInternalTypeChangesReply:
+        description: "This reply contains a field that is internal in the old version and has type changes"
+        fields:
+            oldInternalTypeChangesReplyField:
+                stability: internal
                 type: intStringToIntStringBool
 
     NewUnstableTypeChangesReplyIgnoreList:
@@ -475,7 +501,7 @@ structs:
                       but is also in the ignore list"
         fields:
             newUnstableTypeChangesReplyFieldIgnoreList:
-                unstable: false
+                stability: stable
                 type: intStringToIntStringBool
                 optional: true
 
@@ -487,7 +513,7 @@ structs:
         description: "This struct contains a field that is unstable in the old version and has type changes"
         fields:
             oldUnstableTypeChangesField:
-                unstable: true
+                stability: unstable
                 type: intStringBoolToIntString
 
     NewUnstableTypeChangesStructIgnoreList:
@@ -496,7 +522,7 @@ structs:
         fields:
             newUnstableTypeChangesFieldIgnoreList:
                 type: intStringBoolToIntString
-                unstable: false
+                stability: stable
                 optional: true
 
     BsonSerializationTypeAnyWithVariantReply:
@@ -504,7 +530,7 @@ structs:
                       bson serialization types is 'any' and is explicitly allowed"
         fields:
             anyTypeField:
-                unstable: false
+                stability: stable
                 type:
                     variant: [intStringBoolToIntString,
                               bsonSerializationTypeAnyAllowed,
@@ -518,18 +544,18 @@ structs:
         fields:
             ArrayCommandParameter:
                 type: array<string>
-                unstable: false
+                stability: stable
 
     MissingUnstableFieldNewFieldStruct:
-        description: "This struct contains a field that missing the 'unstable' field in the new
+        description: "This struct contains a field that missing the 'stability' field in the new
                       command."
         fields:
             missingUnstableFieldNewField:
                 type: string
-                unstable: true
+                stability: unstable
 
     MissingUnstableFieldOldFieldStruct:
-        description: "This struct contains a field that missing the 'unstable' field in the old
+        description: "This struct contains a field that missing the 'stability' field in the old
                       command."
         fields:
             missingUnstableFieldNewField:
@@ -550,7 +576,7 @@ structs:
         fields:
             ok1:
                 type: bool
-                unstable: false
+                stability: stable
                 optional: true
 
     OptionalBoolEquivalenceStruct:
@@ -558,42 +584,42 @@ structs:
         fields:
             ok1:
                 type: bool
-                unstable: false
+                stability: stable
                 optional: true
             ok2:
                 type: optionalBool
-                unstable: false
+                stability: stable
             ok3:
                 type: optionalBool
-                unstable: false
+                stability: stable
 
     OptionalBoolToBoolStruct:
         description: "optionalBool field replaced by bool"
         fields:
             ok:
                 type: optionalBool
-                unstable: false
+                stability: stable
 
     BoolToOptionalBoolStruct:
         description: "bool field replaced by optionalBool"
         fields:
             ok:
                 type: bool
-                unstable: false
+                stability: stable
 
     ReplyWithNewArrayTypeField:
         description: "new field is of ArrayType"
         fields:
             ok:
                 type: bool
-                unstable: false
+                stability: stable
 
     ReplyWithNewNestedArrayTypeField:
         description: "new field is a struct with a field of ArrayType"
         fields:
             ok:
                 type: bool
-                unstable: false
+                stability: stable
 
 commands:
     testCommand:
@@ -653,7 +679,7 @@ commands:
         fields:
             newOptionalStableParam:
                 type: string
-                unstable: true
+                stability: unstable
 
     addedCommandParameterStableWithDefault:
         description: "new command has a required stable parameter with a default value
@@ -667,7 +693,7 @@ commands:
         fields:
             newStableParamWithDefault:
                 type: string
-                unstable: true
+                stability: unstable
 
     commandParameterUnstableIgnoreList:
         description: "new unstable command parameter is stable in the corresponding old
@@ -681,7 +707,7 @@ commands:
         fields:
             newUnstableParameterIgnoreList:
                 type: string
-                unstable: false
+                stability: stable
 
     removeCommandParameterUnstable:
         description: "new command removes parameter that is unstable
@@ -695,7 +721,7 @@ commands:
         fields:
             unstableParamToRemove:
                 type: string
-                unstable: true
+                stability: unstable
 
     addedCommandParameterOptional:
         description: "new command has an optional parameter that is required
@@ -710,7 +736,7 @@ commands:
             newOptionalParam:
                 type: string
                 optional: false
-                unstable: false
+                stability: stable
 
     addedUnstableCommandParameter:
         description: "new command has a new unstable parameter that does not
@@ -734,7 +760,7 @@ commands:
         fields:
             unstableParam:
                 type: string
-                unstable: true
+                stability: unstable
 
     newCommandParameterType:
         description: "new command passes because its command parameter type is compatible with
@@ -748,7 +774,7 @@ commands:
         fields:
             compatibleParameter:
                 type: string
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeEnumSuperset:
         description: "new command passes because its command parameter type is an enum that is
@@ -762,7 +788,7 @@ commands:
         fields:
             parameterEnumSuperset:
                 type: EnumSuperset
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeStruct:
         description: "new command passes because its command parameter type is a struct and the
@@ -776,7 +802,7 @@ commands:
         fields:
             parameterStruct:
                 type: NewCommandParameterStruct
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeBsonSuperset:
         description: "new command passes because its parameter type has a bson_serialization_type
@@ -791,7 +817,7 @@ commands:
         fields:
             bsonSupersetParam:
                 type: intToIntString
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeStructRecursiveOne:
         description: "new command passes because its parameter type is a struct that is
@@ -805,7 +831,7 @@ commands:
         fields:
             unstableToStableOptionalStructParameter:
                 type: CommandParamStructRecursiveOne
-                unstable: false
+                stability: stable
 
     newCommandParameterTypeStructRecursiveTwo:
         description: "new command passes because its parameter type is a struct that is
@@ -819,7 +845,7 @@ commands:
         fields:
             supersetStructParameter:
                 type: CommandParamStructRecursiveTwo
-                unstable: false
+                stability: stable
 
     newlyAddedParamBsonAnyAllowList:
         description: "command passes when its parameter is newly added and has bson type 'any'
@@ -842,7 +868,20 @@ commands:
         fields:
             oldUnstableTypeChangesParam:
                 type: intStringBoolToIntString
-                unstable: true
+                stability: unstable
+
+    oldInternalParamTypeChanges:
+        description: "command passes when it has an old internal param with incompatible type changes"
+        command_name: oldInternalParamTypeChanges
+        namespace: ignored
+        cpp_name: oldInternalParamTypeChanges
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+        fields:
+            oldInternalTypeChangesParam:
+                type: intStringBoolToIntString
+                stability: internal
 
     newUnstableParamTypeChangesIgnoreList:
         description: "command has param with incompatible type changes, but is in the stable-to-unstable
@@ -856,7 +895,7 @@ commands:
         fields:
             newUnstableTypeChangesParamIgnoreList:
                 type: intStringBoolToIntString
-                unstable: false
+                stability: stable
                 optional: true
 
     newlyAddedTypeFieldBsonAnyAllowList:
@@ -903,7 +942,7 @@ commands:
         fields:
             newParam:
                 type: int
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0
 
@@ -919,7 +958,7 @@ commands:
         fields:
             newParam:
                 type: double
-                unstable: false
+                stability: stable
                 validator:
                     lt: 0.0
                     gt: -1.1
@@ -1228,6 +1267,17 @@ commands:
         api_version: "1"
         reply_type: OkReply
 
+    removeTypeFieldInternal:
+        description: "new command removes type field that is internal
+                      in the corresponding old command and still passes"
+        command_name: removeTypeFieldInternal
+        namespace: type
+        type: RemovedInternalTypeFieldStruct
+        cpp_name: removeTypeFieldInternal
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+
     addedUnstableTypeField:
         description: "new command has a new unstable type field that does not
                       exist in the corresponding old command and still passes"
@@ -1239,6 +1289,17 @@ commands:
         api_version: "1"
         reply_type: OkReply
 
+    addedInternalTypeField:
+        description: "new command has a new internal type field that does not
+                      exist in the corresponding old command and still passes"
+        command_name: addedInternalTypeField
+        namespace: type
+        type: AddedInternalTypeFieldStruct
+        cpp_name: addedInternalTypeField
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+
     keptUnstableTypeField:
         description: "new command has an unstable type field that also
                       exists in the corresponding old command and still passes"
@@ -1250,6 +1311,17 @@ commands:
         api_version: "1"
         reply_type: OkReply
 
+    keptInternalTypeField:
+        description: "new command has an internal type field that also
+                      exists in the corresponding old command and still passes"
+        command_name: keptInternalTypeField
+        namespace: type
+        type: KeptInternalTypeFieldStruct
+        cpp_name: keptInternalTypeField
+        strict: true
+        api_version: "1"
+        reply_type: OkReply
+
     oldReplyFieldVariantType:
         description: "new command passes when its reply field type is not a variant type while
                       the old reply field is a variant type"
@@ -1368,6 +1440,15 @@ commands:
         api_version: "1"
         reply_type: OldUnstableTypeChangesReply
 
+    oldInternalReplyFieldTypeChanges:
+        description: "command passes when it has an old internal reply field with incompatible type changes"
+        command_name: oldInternalReplyFieldTypeChanges
+        namespace: ignored
+        cpp_name: oldInternalReplyFieldTypeChanges
+        strict: true
+        api_version: "1"
+        reply_type: OldInternalTypeChangesReply
+
     newUnstableReplyFieldTypeChangesIgnoreList:
         description: "command has an old stable reply field with incompatible type changes but it is also
                       in the ignore list"
@@ -1391,7 +1472,7 @@ commands:
         fields:
             anyTypeParam:
                 type: bsonSerializationTypeAnyAllowed
-                unstable: false
+                stability: stable
 
     commandAllowedAnyTypesWithVariant:
         description: "command that has reply variant types with
@@ -1414,7 +1495,7 @@ commands:
         reply_type: OkReply
         fields:
             variantSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, bool, string, array<string>]
 
@@ -1429,7 +1510,7 @@ commands:
         reply_type: OkReply
         fields:
             variantSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, bool, double, array<string>]
 
@@ -1444,7 +1525,7 @@ commands:
         reply_type: OkReply
         fields:
             variantParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, array<string>]
 
@@ -1459,7 +1540,7 @@ commands:
         reply_type: OkReply
         fields:
             variantRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, intStringToIntStringBool,
                               array<intStringToIntStringBool>, array<string>]
@@ -1475,7 +1556,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructSupersetParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, NewCommandParameterStruct,
                               array<NewCommandParameterStruct>, array<string>]
@@ -1492,7 +1573,7 @@ commands:
         reply_type: OkReply
         fields:
             variantStructRecursiveParam:
-                unstable: false
+                stability: stable
                 type:
                     variant: [int, string, StructCommandParameterTypeRecursive,
                               array<StructCommandParameterTypeRecursive>, array<string>]
@@ -1639,7 +1720,7 @@ commands:
         fields:
             parameterStruct:
                 type: array<ArrayTypeStruct>
-                unstable: false
+                stability: stable
         reply_type: ArrayTypeStruct
 
     complexActionTypesSubset:
@@ -1763,7 +1844,7 @@ commands:
 
     newReplyFieldMissingUnstableField:
         description: "new command passes because it does not have an api_version even though it
-                      contains a reply field that is missing the 'unstable' field"
+                      contains a reply field that is missing the 'stability' field"
         command_name: newReplyFieldMissingUnstableField
         namespace: ignored
         cpp_name: newReplyFieldMissingUnstableField
@@ -1773,7 +1854,7 @@ commands:
 
     oldReplyFieldMissingUnstableField:
         description: "old command passes even though it
-                      contains a reply field that is missing the 'unstable' field"
+                      contains a reply field that is missing the 'stability' field"
         command_name: oldReplyFieldMissingUnstableField
         namespace: ignored
         cpp_name: oldReplyFieldMissingUnstableField
@@ -1783,7 +1864,7 @@ commands:
 
     newCommandTypeFieldMissingUnstableField:
         description: "new command passes because it does not have an api_version even though it
-                      contains a command type field that is missing the 'unstable' field"
+                      contains a command type field that is missing the 'stability' field"
         command_name: newCommandTypeFieldMissingUnstableField
         namespace: type
         cpp_name: newCommandTypeFieldMissingUnstableField
@@ -1794,7 +1875,7 @@ commands:
 
     oldCommandTypeFieldMissingUnstableField:
         description: "old command passes even though it
-                      contains a command type field that is missing the 'unstable' field"
+                      contains a command type field that is missing the 'stability' field"
         command_name: oldCommandTypeFieldMissingUnstableField
         namespace: type
         cpp_name: oldCommandTypeFieldMissingUnstableField
@@ -1805,7 +1886,7 @@ commands:
 
     newParameterMissingUnstableField:
         description: "new command passes because it does not have an api_version even though it
-                      contains a parameter that is missing the 'unstable' field"
+                      contains a parameter that is missing the 'stability' field"
         command_name: newParameterMissingUnstableField
         namespace: ignored
         cpp_name: newParameterMissingUnstableField
@@ -1814,12 +1895,12 @@ commands:
         fields:
             missingUnstableFieldNewParameter:
                 type: int
-                unstable: false
+                stability: stable
         reply_type: OkReply
 
     oldParameterMissingUnstableField:
         description: "old command passes even though it
-                      contains a parameter that is missing the 'unstable' field"
+                      contains a parameter that is missing the 'stability' field"
         command_name: oldParameterMissingUnstableField
         namespace: ignored
         cpp_name: oldParameterMissingUnstableField
@@ -1911,14 +1992,14 @@ commands:
         api_version: "1"
         fields:
             flag1:
-                unstable: false
+                stability: stable
                 type: bool
                 optional: true
             flag2:
-                unstable: false
+                stability: stable
                 type: optionalBool
             flag3:
-                unstable: false
+                stability: stable
                 type: optionalBool
         reply_type: OkReply
 
@@ -1950,7 +2031,7 @@ commands:
         api_version: "1"
         fields:
             flag:
-                unstable: false
+                stability: stable
                 type: bool
         reply_type: OkReply
 
diff --git a/buildscripts/idl/tests/compatibility_test_pass/old/error_reply.idl b/buildscripts/idl/tests/compatibility_test_pass/old/error_reply.idl
index f99aec3cb26..9e57bd99292 100644
--- a/buildscripts/idl/tests/compatibility_test_pass/old/error_reply.idl
+++ b/buildscripts/idl/tests/compatibility_test_pass/old/error_reply.idl
@@ -40,7 +40,7 @@ structs:
         fields:
             errorLabels:
                 type: StructType
-                unstable: false
+                stability: stable
     
     StructType:
         description: "This struct contains a field that is optional in the old
@@ -49,4 +49,4 @@ structs:
             structField:
                 type: string
                 optional: true
-                unstable: false
+                stability: stable
diff --git a/buildscripts/idl/tests/compatibility_test_pass/old/imports.idl b/buildscripts/idl/tests/compatibility_test_pass/old/imports.idl
index 0c3edc28bd5..3fd43a6d24c 100644
--- a/buildscripts/idl/tests/compatibility_test_pass/old/imports.idl
+++ b/buildscripts/idl/tests/compatibility_test_pass/old/imports.idl
@@ -39,7 +39,7 @@ structs:
         fields:
             stableNewField:
                 type: string
-                unstable: true
+                stability: unstable
 
 commands:
     importCommand:
diff --git a/buildscripts/idl/tests/test_compatibility.py b/buildscripts/idl/tests/test_compatibility.py
index b6c64b625ac..4e5b684c656 100644
--- a/buildscripts/idl/tests/test_compatibility.py
+++ b/buildscripts/idl/tests/test_compatibility.py
@@ -95,20 +95,20 @@ class TestIDLCompatibilityChecker(unittest.TestCase):
         new_parameter_no_unstable_field_error = error_collection.get_error_by_command_name(
             "newCommandParameterNoUnstableField")
         self.assertTrue(new_parameter_no_unstable_field_error.error_id ==
-                        idl_compatibility_errors.ERROR_ID_NEW_PARAMETER_REQUIRES_UNSTABLE)
+                        idl_compatibility_errors.ERROR_ID_NEW_PARAMETER_REQUIRES_STABILITY)
         self.assertRegex(
             str(new_parameter_no_unstable_field_error), "newCommandParameterNoUnstableField")
 
         new_reply_no_unstable_field_error = error_collection.get_error_by_command_name(
             "newCommandReplyNoUnstableField")
         self.assertTrue(new_reply_no_unstable_field_error.error_id ==
-                        idl_compatibility_errors.ERROR_ID_NEW_REPLY_FIELD_REQUIRES_UNSTABLE)
+                        idl_compatibility_errors.ERROR_ID_NEW_REPLY_FIELD_REQUIRES_STABILITY)
         self.assertRegex(str(new_reply_no_unstable_field_error), "newCommandReplyNoUnstableField")
 
         new_command_type_struct_no_unstable_field_error = error_collection.get_error_by_command_name(
             "newCommandTypeStructFieldNoUnstableField")
         self.assertTrue(new_command_type_struct_no_unstable_field_error.error_id ==
-                        idl_compatibility_errors.ERROR_ID_NEW_COMMAND_TYPE_FIELD_REQUIRES_UNSTABLE)
+                        idl_compatibility_errors.ERROR_ID_NEW_COMMAND_TYPE_FIELD_REQUIRES_STABILITY)
         self.assertRegex(
             str(new_command_type_struct_no_unstable_field_error),
             "newCommandTypeStructFieldNoUnstableField")
@@ -203,6 +203,12 @@ class TestIDLCompatibilityChecker(unittest.TestCase):
                         idl_compatibility_errors.ERROR_ID_COMMAND_PARAMETER_UNSTABLE)
         self.assertRegex(str(command_parameter_unstable_error), "commandParameterUnstable")
 
+        command_parameter_internal_error = error_collection.get_error_by_command_name(
+            "commandParameterInternal")
+        self.assertTrue(command_parameter_internal_error.error_id ==
+                        idl_compatibility_errors.ERROR_ID_COMMAND_PARAMETER_UNSTABLE)
+        self.assertRegex(str(command_parameter_internal_error), "commandParameterInternal")
+
         command_parameter_stable_required_no_default_error = error_collection.get_error_by_command_name(
             "commandParameterStableRequiredNoDefault")
         self.assertTrue(
@@ -413,6 +419,12 @@ class TestIDLCompatibilityChecker(unittest.TestCase):
                         idl_compatibility_errors.ERROR_ID_NEW_REPLY_FIELD_UNSTABLE)
         self.assertRegex(str(new_reply_field_unstable_error), "newReplyFieldUnstable")
 
+        new_reply_field_internal_error = error_collection.get_error_by_command_name(
+            "newReplyFieldInternal")
+        self.assertTrue(new_reply_field_internal_error.error_id ==
+                        idl_compatibility_errors.ERROR_ID_NEW_REPLY_FIELD_UNSTABLE)
+        self.assertRegex(str(new_reply_field_internal_error), "newReplyFieldInternal")
+
         new_reply_field_optional_error = error_collection.get_error_by_error_id(
             idl_compatibility_errors.ERROR_ID_NEW_REPLY_FIELD_OPTIONAL)
         self.assertRegex(str(new_reply_field_optional_error), "newReplyFieldOptional")
@@ -1307,14 +1319,14 @@ class TestIDLCompatibilityChecker(unittest.TestCase):
         new_reply_field_missing_unstable_field_error = error_collection.get_error_by_command_name(
             "newReplyFieldMissingUnstableField")
         self.assertTrue(new_reply_field_missing_unstable_field_error.error_id ==
-                        idl_compatibility_errors.ERROR_ID_NEW_REPLY_FIELD_REQUIRES_UNSTABLE)
+                        idl_compatibility_errors.ERROR_ID_NEW_REPLY_FIELD_REQUIRES_STABILITY)
         self.assertRegex(
             str(new_reply_field_missing_unstable_field_error), "newReplyFieldMissingUnstableField")
 
         new_command_type_field_missing_unstable_field_error = error_collection.get_error_by_command_name(
             "newCommandTypeFieldMissingUnstableField")
         self.assertTrue(new_command_type_field_missing_unstable_field_error.error_id ==
-                        idl_compatibility_errors.ERROR_ID_NEW_COMMAND_TYPE_FIELD_REQUIRES_UNSTABLE)
+                        idl_compatibility_errors.ERROR_ID_NEW_COMMAND_TYPE_FIELD_REQUIRES_STABILITY)
         self.assertRegex(
             str(new_command_type_field_missing_unstable_field_error),
             "newCommandTypeFieldMissingUnstableField")
@@ -1322,14 +1334,14 @@ class TestIDLCompatibilityChecker(unittest.TestCase):
         new_parameter_missing_unstable_field_error = error_collection.get_error_by_command_name(
             "newParameterMissingUnstableField")
         self.assertTrue(new_parameter_missing_unstable_field_error.error_id ==
-                        idl_compatibility_errors.ERROR_ID_NEW_PARAMETER_REQUIRES_UNSTABLE)
+                        idl_compatibility_errors.ERROR_ID_NEW_PARAMETER_REQUIRES_STABILITY)
         self.assertRegex(
             str(new_parameter_missing_unstable_field_error), "newParameterMissingUnstableField")
 
         added_new_reply_field_missing_unstable_field_error = error_collection.get_error_by_command_name(
             "addedNewReplyFieldMissingUnstableField")
         self.assertTrue(added_new_reply_field_missing_unstable_field_error.error_id ==
-                        idl_compatibility_errors.ERROR_ID_NEW_REPLY_FIELD_REQUIRES_UNSTABLE)
+                        idl_compatibility_errors.ERROR_ID_NEW_REPLY_FIELD_REQUIRES_STABILITY)
         self.assertRegex(
             str(added_new_reply_field_missing_unstable_field_error),
             "addedNewReplyFieldMissingUnstableField")
@@ -1337,7 +1349,7 @@ class TestIDLCompatibilityChecker(unittest.TestCase):
         added_new_command_type_field_missing_unstable_field_error = error_collection.get_error_by_command_name(
             "addedNewCommandTypeFieldMissingUnstableField")
         self.assertTrue(added_new_command_type_field_missing_unstable_field_error.error_id ==
-                        idl_compatibility_errors.ERROR_ID_NEW_COMMAND_TYPE_FIELD_REQUIRES_UNSTABLE)
+                        idl_compatibility_errors.ERROR_ID_NEW_COMMAND_TYPE_FIELD_REQUIRES_STABILITY)
         self.assertRegex(
             str(added_new_command_type_field_missing_unstable_field_error),
             "addedNewCommandTypeFieldMissingUnstableField")
@@ -1345,7 +1357,7 @@ class TestIDLCompatibilityChecker(unittest.TestCase):
         added_new_parameter_missing_unstable_field_error = error_collection.get_error_by_command_name(
             "addedNewParameterMissingUnstableField")
         self.assertTrue(added_new_parameter_missing_unstable_field_error.error_id ==
-                        idl_compatibility_errors.ERROR_ID_NEW_PARAMETER_REQUIRES_UNSTABLE)
+                        idl_compatibility_errors.ERROR_ID_NEW_PARAMETER_REQUIRES_STABILITY)
         self.assertRegex(
             str(added_new_parameter_missing_unstable_field_error),
             "addedNewParameterMissingUnstableField")
@@ -1444,7 +1456,7 @@ class TestIDLCompatibilityChecker(unittest.TestCase):
         self.assertRegex(
             str(new_command_type_field_added_as_stable_error), "newStableTypeFieldAdded")
 
-        self.assertEqual(error_collection.count(), 204)
+        self.assertEqual(error_collection.count(), 207)
 
     def test_generic_argument_compatibility_pass(self):
         """Tests that compatible old and new generic_argument.idl files should pass."""
diff --git a/buildscripts/idl/tests/test_parser.py b/buildscripts/idl/tests/test_parser.py
index 677f00a799d..122a0e6c64d 100644
--- a/buildscripts/idl/tests/test_parser.py
+++ b/buildscripts/idl/tests/test_parser.py
@@ -504,7 +504,7 @@ class TestParser(testcase.IDLTestcase):
                         ignore: true
                         cpp_name: bar
                         comparison_order: 3
-                        unstable: true
+                        stability: unstable
             """))
 
         # Test false bools
@@ -519,7 +519,7 @@ class TestParser(testcase.IDLTestcase):
                         type: string
                         optional: false
                         ignore: false
-                        unstable: false
+                        stability: stable
             """))
 
     def test_field_negative(self):
@@ -1501,10 +1501,10 @@ class TestParser(testcase.IDLTestcase):
                     foo: bar
             """), idl.errors.ERROR_ID_MISSING_REQUIRED_FIELD)
 
-    def test_unstable_positive(self):
+    def test_stability_positive(self):
         # type: () -> None
-        """Positive unstable-field test cases."""
-        for unstable in ("true", "false"):
+        """Positive stability-field test cases."""
+        for stability in ("stable", "unstable", "internal"):
             self.assert_parse(
                 textwrap.dedent(f"""
             commands:
@@ -1516,13 +1516,13 @@ class TestParser(testcase.IDLTestcase):
                     fields:
                         foo:
                             type: bar
-                            unstable: {unstable}
+                            stability: {stability}
                     reply_type: foo_reply_struct
                 """))
 
-    def test_unstable_negative(self):
+    def test_stability_negative(self):
         # type: () -> None
-        """Negative unstable-field test cases."""
+        """Negative stability-field test cases."""
         self.assert_parse_fail(
             textwrap.dedent("""
         commands:
@@ -1534,9 +1534,38 @@ class TestParser(testcase.IDLTestcase):
                 fields:
                     foo:
                         type: bar
-                        unstable: true
+                        stability: unstable
                 reply_type: foo_reply_struct
-            """), idl.errors.ERROR_ID_UNSTABLE_NO_API_VERSION)
+            """), idl.errors.ERROR_ID_STABILITY_NO_API_VERSION)
+        self.assert_parse_fail(
+            textwrap.dedent("""
+            commands:
+                foo:
+                    description: foo
+                    command_name: foo
+                    namespace: ignored
+                    api_version: "1"
+                    fields:
+                        foo:
+                            type: bar
+                            stability: "unknown"
+                    reply_type: foo_reply_struct
+                """), idl.errors.ERROR_ID_STABILITY_UNKNOWN_VALUE)
+        self.assert_parse_fail(
+            textwrap.dedent("""
+            commands:
+                foo:
+                    description: foo
+                    command_name: foo
+                    namespace: ignored
+                    api_version: "1"
+                    fields:
+                        foo:
+                            type: bar
+                            unstable: true
+                            stability: "unstable"
+                    reply_type: foo_reply_struct
+                """), idl.errors.ERROR_ID_DUPLICATE_UNSTABLE_STABILITY)
 
     def test_scalar_or_mapping_negative(self):
         # type: () -> None