Allow arbitrary types in model creation by updating config in create_model

Author: coolbeevip

src/mcp/server/fastmcp/utilities/func_metadata.py

@@ -56,6 +56,14 @@ def model_dump_one_level(self) -> dict[str, Any]:
     )
 
 
+class OutputModelBase(BaseModel):
+    """A model representing the output of a function."""
+
+    model_config = ConfigDict(
+        arbitrary_types_allowed=True,
+    )
+
+
 class FuncMetadata(BaseModel):
     arg_model: Annotated[type[ArgModelBase], WithJsonSchema(None)]
     output_schema: dict[str, Any] | None = None
@@ -407,9 +415,7 @@ def _create_wrapped_model(func_name: str, annotation: Any, field_info: FieldInfo
     if annotation is None:
         annotation = type(None)
 
-    return create_model(
-        model_name, result=(annotation, field_info), __base__=BaseModel, __config__={"arbitrary_types_allowed": True}
-    )
+    return create_model(model_name, result=(annotation, field_info), __base__= OutputModelBase)
 
 
 def _create_dict_model(func_name: str, dict_annotation: Any) -> type[BaseModel]: