Drop RootModel from JSONRPCMessage

Author: Kludex

src/mcp/client/sse.py

@@ -73,9 +73,7 @@ async def sse_client(
                     event_source.response.raise_for_status()
                     logger.debug("SSE connection established")
 
-                    async def sse_reader(
-                        task_status: TaskStatus[str] = anyio.TASK_STATUS_IGNORED,
-                    ):
+                    async def sse_reader(task_status: TaskStatus[str] = anyio.TASK_STATUS_IGNORED):
                         try:
                             async for sse in event_source.aiter_sse():  # pragma: no branch
                                 logger.debug(f"Received SSE event: {sse.event}")
@@ -108,7 +106,7 @@ async def sse_reader(
                                         if not sse.data:
                                             continue
                                         try:
-                                            message = types.JSONRPCMessage.model_validate_json(  # noqa: E501
+                                            message = types.jsonrpc_message_adapter.validate_json(
                                                 sse.data, by_name=False
                                             )
                                             logger.debug(f"Received server message: {message}")

src/mcp/client/stdio/__init__.py

@@ -150,7 +150,7 @@ async def stdout_reader():
 
                     for line in lines:
                         try:
-                            message = types.JSONRPCMessage.model_validate_json(line, by_name=False)
+                            message = types.jsonrpc_message_adapter.validate_json(line, by_name=False)
                         except Exception as exc:  # pragma: no cover
                             logger.exception("Failed to parse JSONRPC message from server")
                             await read_stream_writer.send(exc)

src/mcp/client/streamable_http.py

@@ -25,6 +25,7 @@
     JSONRPCRequest,
     JSONRPCResponse,
     RequestId,
+    jsonrpc_message_adapter,
 )
 
 logger = logging.getLogger(__name__)
@@ -95,11 +96,11 @@ def _prepare_headers(self) -> dict[str, str]:
 
     def _is_initialization_request(self, message: JSONRPCMessage) -> bool:
         """Check if the message is an initialization request."""
-        return isinstance(message.root, JSONRPCRequest) and message.root.method == "initialize"
+        return isinstance(message, JSONRPCRequest) and message.method == "initialize"
 
     def _is_initialized_notification(self, message: JSONRPCMessage) -> bool:
         """Check if the message is an initialized notification."""
-        return isinstance(message.root, JSONRPCNotification) and message.root.method == "notifications/initialized"
+        return isinstance(message, JSONRPCNotification) and message.method == "notifications/initialized"
 
     def _maybe_extract_session_id_from_response(self, response: httpx.Response) -> None:
         """Extract and store session ID from response headers."""
@@ -110,15 +111,15 @@ def _maybe_extract_session_id_from_response(self, response: httpx.Response) -> N
 
     def _maybe_extract_protocol_version_from_message(self, message: JSONRPCMessage) -> None:
         """Extract protocol version from initialization response message."""
-        if isinstance(message.root, JSONRPCResponse) and message.root.result:  # pragma: no branch
+        if isinstance(message, JSONRPCResponse) and message.result:  # pragma: no branch
             try:
                 # Parse the result as InitializeResult for type safety
-                init_result = InitializeResult.model_validate(message.root.result, by_name=False)
+                init_result = InitializeResult.model_validate(message.result, by_name=False)
                 self.protocol_version = str(init_result.protocol_version)
                 logger.info(f"Negotiated protocol version: {self.protocol_version}")
             except Exception:  # pragma: no cover
                 logger.warning("Failed to parse initialization response as InitializeResult", exc_info=True)
-                logger.warning(f"Raw result: {message.root.result}")
+                logger.warning(f"Raw result: {message.result}")
 
     async def _handle_sse_event(
         self,
@@ -137,16 +138,16 @@ async def _handle_sse_event(
                     await resumption_callback(sse.id)
                 return False
             try:
-                message = JSONRPCMessage.model_validate_json(sse.data, by_name=False)
+                message = jsonrpc_message_adapter.validate_json(sse.data, by_name=False)
                 logger.debug(f"SSE message: {message}")
 
                 # Extract protocol version from initialization response
                 if is_initialization:
                     self._maybe_extract_protocol_version_from_message(message)
 
                 # If this is a response and we have original_request_id, replace it
-                if original_request_id is not None and isinstance(message.root, JSONRPCResponse | JSONRPCError):
-                    message.root.id = original_request_id
+                if original_request_id is not None and isinstance(message, JSONRPCResponse | JSONRPCError):
+                    message.id = original_request_id
 
                 session_message = SessionMessage(message)
                 await read_stream_writer.send(session_message)
@@ -157,7 +158,7 @@ async def _handle_sse_event(
 
                 # If this is a response or error return True indicating completion
                 # Otherwise, return False to continue listening
-                return isinstance(message.root, JSONRPCResponse | JSONRPCError)
+                return isinstance(message, JSONRPCResponse | JSONRPCError)
 
             except Exception as exc:  # pragma: no cover
                 logger.exception("Error parsing SSE message")
@@ -222,8 +223,8 @@ async def _handle_resumption_request(self, ctx: RequestContext) -> None:
 
         # Extract original request ID to map responses
         original_request_id = None
-        if isinstance(ctx.session_message.message.root, JSONRPCRequest):  # pragma: no branch
-            original_request_id = ctx.session_message.message.root.id
+        if isinstance(ctx.session_message.message, JSONRPCRequest):  # pragma: no branch
+            original_request_id = ctx.session_message.message.id
 
         async with aconnect_sse(ctx.client, "GET", self.url, headers=headers) as event_source:
             event_source.response.raise_for_status()
@@ -257,20 +258,17 @@ async def _handle_post_request(self, ctx: RequestContext) -> None:
                 return
 
             if response.status_code == 404:  # pragma: no branch
-                if isinstance(message.root, JSONRPCRequest):
-                    await self._send_session_terminated_error(  # pragma: no cover
-                        ctx.read_stream_writer,  # pragma: no cover
-                        message.root.id,  # pragma: no cover
-                    )  # pragma: no cover
-                return  # pragma: no cover
+                if isinstance(message, JSONRPCRequest):  # pragma: no branch
+                    await self._send_session_terminated_error(ctx.read_stream_writer, message.id)
+                return
 
             response.raise_for_status()
             if is_initialization:
                 self._maybe_extract_session_id_from_response(response)
 
             # Per https://modelcontextprotocol.io/specification/2025-06-18/basic#notifications:
             # The server MUST NOT send a response to notifications.
-            if isinstance(message.root, JSONRPCRequest):
+            if isinstance(message, JSONRPCRequest):
                 content_type = response.headers.get("content-type", "").lower()
                 if content_type.startswith("application/json"):
                     await self._handle_json_response(response, ctx.read_stream_writer, is_initialization)
@@ -291,7 +289,7 @@ async def _handle_json_response(
         """Handle JSON response from the server."""
         try:
             content = await response.aread()
-            message = JSONRPCMessage.model_validate_json(content, by_name=False)
+            message = jsonrpc_message_adapter.validate_json(content, by_name=False)
 
             # Extract protocol version from initialization response
             if is_initialization:
@@ -365,8 +363,8 @@ async def _handle_reconnection(
 
         # Extract original request ID to map responses
         original_request_id = None
-        if isinstance(ctx.session_message.message.root, JSONRPCRequest):  # pragma: no branch
-            original_request_id = ctx.session_message.message.root.id
+        if isinstance(ctx.session_message.message, JSONRPCRequest):  # pragma: no branch
+            original_request_id = ctx.session_message.message.id
 
         try:
             async with aconnect_sse(ctx.client, "GET", self.url, headers=headers) as event_source:
@@ -463,7 +461,7 @@ async def handle_request_async():
                             await self._handle_post_request(ctx)
 
                     # If this is a request, start a new task to handle it
-                    if isinstance(message.root, JSONRPCRequest):
+                    if isinstance(message, JSONRPCRequest):
                         tg.start_soon(handle_request_async)
                     else:
                         await handle_request_async()

src/mcp/client/websocket.py

@@ -51,7 +51,7 @@ async def ws_reader():
             async with read_stream_writer:
                 async for raw_text in ws:
                     try:
-                        message = types.JSONRPCMessage.model_validate_json(raw_text, by_name=False)
+                        message = types.jsonrpc_message_adapter.validate_json(raw_text, by_name=False)
                         session_message = SessionMessage(message)
                         await read_stream_writer.send(session_message)
                     except ValidationError as exc:  # pragma: no cover

src/mcp/server/sse.py

@@ -227,7 +227,7 @@ async def handle_post_message(self, scope: Scope, receive: Receive, send: Send)
         logger.debug(f"Received JSON: {body}")
 
         try:
-            message = types.JSONRPCMessage.model_validate_json(body, by_name=False)
+            message = types.jsonrpc_message_adapter.validate_json(body, by_name=False)
             logger.debug(f"Validated client message: {message}")
         except ValidationError as err:
             logger.exception("Failed to parse message")

src/mcp/server/stdio.py

@@ -60,7 +60,7 @@ async def stdin_reader():
             async with read_stream_writer:
                 async for line in stdin:
                     try:
-                        message = types.JSONRPCMessage.model_validate_json(line, by_name=False)
+                        message = types.jsonrpc_message_adapter.validate_json(line, by_name=False)
                     except Exception as exc:  # pragma: no cover
                         await read_stream_writer.send(exc)
                         continue

src/mcp/server/streamable_http.py

@@ -42,6 +42,7 @@
     JSONRPCRequest,
     JSONRPCResponse,
     RequestId,
+    jsonrpc_message_adapter,
 )
 
 logger = logging.getLogger(__name__)
@@ -301,10 +302,7 @@ def _create_error_response(
         error_response = JSONRPCError(
             jsonrpc="2.0",
             id="server-error",  # We don't have a request ID for general errors
-            error=ErrorData(
-                code=error_code,
-                message=error_message,
-            ),
+            error=ErrorData(code=error_code, message=error_message),
         )
 
         return Response(
@@ -455,14 +453,15 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
             body = await request.body()
 
             try:
+                # TODO(Marcelo): Replace `json.loads` with `pydantic_core.from_json`.
                 raw_message = json.loads(body)
             except json.JSONDecodeError as e:
                 response = self._create_error_response(f"Parse error: {str(e)}", HTTPStatus.BAD_REQUEST, PARSE_ERROR)
                 await response(scope, receive, send)
                 return
 
             try:  # pragma: no cover
-                message = JSONRPCMessage.model_validate(raw_message, by_name=False)
+                message = jsonrpc_message_adapter.validate_python(raw_message, by_name=False)
             except ValidationError as e:  # pragma: no cover
                 response = self._create_error_response(
                     f"Validation error: {str(e)}",
@@ -473,9 +472,7 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
                 return
 
             # Check if this is an initialization request
-            is_initialization_request = (
-                isinstance(message.root, JSONRPCRequest) and message.root.method == "initialize"
-            )  # pragma: no cover
+            is_initialization_request = isinstance(message, JSONRPCRequest) and message.method == "initialize"
 
             if is_initialization_request:  # pragma: no cover
                 # Check if the server already has an established session
@@ -495,7 +492,7 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
                 return
 
             # For notifications and responses only, return 202 Accepted
-            if not isinstance(message.root, JSONRPCRequest):  # pragma: no cover
+            if not isinstance(message, JSONRPCRequest):  # pragma: no cover
                 # Create response object and send it
                 response = self._create_json_response(
                     None,
@@ -514,13 +511,13 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
             # For initialize requests, get from request params.
             # For other requests, get from header (already validated).
             protocol_version = (
-                str(message.root.params.get("protocolVersion", DEFAULT_NEGOTIATED_VERSION))
-                if is_initialization_request and message.root.params
+                str(message.params.get("protocolVersion", DEFAULT_NEGOTIATED_VERSION))
+                if is_initialization_request and message.params
                 else request.headers.get(MCP_PROTOCOL_VERSION_HEADER, DEFAULT_NEGOTIATED_VERSION)
             )
 
             # Extract the request ID outside the try block for proper scope
-            request_id = str(message.root.id)  # pragma: no cover
+            request_id = str(message.id)  # pragma: no cover
             # Register this stream for the request ID
             self._request_streams[request_id] = anyio.create_memory_object_stream[EventMessage](0)  # pragma: no cover
             request_stream_reader = self._request_streams[request_id][1]  # pragma: no cover
@@ -538,12 +535,12 @@ async def _handle_post_request(self, scope: Scope, request: Request, receive: Re
                     # Use similar approach to SSE writer for consistency
                     async for event_message in request_stream_reader:
                         # If it's a response, this is what we're waiting for
-                        if isinstance(event_message.message.root, JSONRPCResponse | JSONRPCError):
+                        if isinstance(event_message.message, JSONRPCResponse | JSONRPCError):
                             response_message = event_message.message
                             break
                         # For notifications and request, keep waiting
                         else:
-                            logger.debug(f"received: {event_message.message.root.method}")
+                            logger.debug(f"received: {event_message.message.method}")
 
                     # At this point we should have a response
                     if response_message:
@@ -589,10 +586,7 @@ async def sse_writer():
                                 await sse_stream_writer.send(event_data)
 
                                 # If response, remove from pending streams and close
-                                if isinstance(
-                                    event_message.message.root,
-                                    JSONRPCResponse | JSONRPCError,
-                                ):
+                                if isinstance(event_message.message, JSONRPCResponse | JSONRPCError):
                                     break
                     except anyio.ClosedResourceError:
                         # Expected when close_sse_stream() is called
@@ -984,8 +978,8 @@ async def message_router():  # pragma: no cover
                         message = session_message.message
                         target_request_id = None
                         # Check if this is a response
-                        if isinstance(message.root, JSONRPCResponse | JSONRPCError):
-                            response_id = str(message.root.id)
+                        if isinstance(message, JSONRPCResponse | JSONRPCError):
+                            response_id = str(message.id)
                             # If this response is for an existing request stream,
                             # send it there
                             target_request_id = response_id
@@ -1022,7 +1016,7 @@ async def message_router():  # pragma: no cover
                                 self._request_streams.pop(request_stream_id, None)
                         else:
                             logger.debug(
-                                f"""Request stream {request_stream_id} not found 
+                                f"""Request stream {request_stream_id} not found
                                 for message. Still processing message as the client
                                 might reconnect and replay."""
                             )

src/mcp/server/websocket.py

@@ -36,7 +36,7 @@ async def ws_reader():
             async with read_stream_writer:
                 async for msg in websocket.iter_text():
                     try:
-                        client_message = types.JSONRPCMessage.model_validate_json(msg, by_name=False)
+                        client_message = types.jsonrpc_message_adapter.validate_json(msg, by_name=False)
                     except ValidationError as exc:
                         await read_stream_writer.send(exc)
                         continue

src/mcp/types.py

@@ -4,7 +4,7 @@
 from datetime import datetime
 from typing import Annotated, Any, Final, Generic, Literal, TypeAlias, TypeVar
 
-from pydantic import BaseModel, ConfigDict, Field, FileUrl, RootModel
+from pydantic import BaseModel, ConfigDict, Field, FileUrl, RootModel, TypeAdapter
 from pydantic.alias_generators import to_camel
 
 LATEST_PROTOCOL_VERSION = "2025-11-25"
@@ -198,6 +198,7 @@ class JSONRPCError(MCPModel):
 
 
 JSONRPCMessage = JSONRPCRequest | JSONRPCNotification | JSONRPCResponse | JSONRPCError
+jsonrpc_message_adapter = TypeAdapter[JSONRPCMessage](JSONRPCMessage)
 
 
 class EmptyResult(Result):