Proper handle extra data in MCP objects

Author: Kludex

pyproject.toml

@@ -26,8 +26,7 @@ dependencies = [
     "anyio>=4.5",
     "httpx>=0.27.1",
     "httpx-sse>=0.4",
-    "pydantic>=2.12.0; python_version >= '3.14'",
-    "pydantic>=2.11.0; python_version < '3.14'",
+    "pydantic>=2.12.0",
     "starlette>=0.48.0; python_version >= '3.14'",
     "starlette>=0.27; python_version < '3.14'",
     "python-multipart>=0.0.9",

src/mcp/types/_types.py

@@ -32,8 +32,7 @@
 class MCPModel(BaseModel):
     """Base class for all MCP protocol types. Allows extra fields for forward compatibility."""
 
-    # TODO(Marcelo): The extra="allow" should be only on specific types e.g. `Meta`, not on the base class.
-    model_config = ConfigDict(extra="allow", alias_generator=to_camel, populate_by_name=True)
+    model_config = ConfigDict(alias_generator=to_camel, populate_by_name=True)
 
 
 Meta: TypeAlias = dict[str, Any]
@@ -472,10 +471,13 @@ class GetTaskPayloadRequest(Request[GetTaskPayloadRequestParams, Literal["tasks/
 
 class GetTaskPayloadResult(Result):
     """The response to a tasks/result request.
+
     The structure matches the result type of the original request.
     For example, a tools/call task would return the CallToolResult structure.
     """
 
+    model_config = ConfigDict(extra="allow")
+
 
 class CancelTaskRequestParams(RequestParams):
     task_id: str