fix: allow null id in JSONRPCError per JSON-RPC 2.0 spec (#2056)

src/mcp/server/lowlevel/server.py

@@ -490,7 +490,7 @@ class Server(Generic[LifespanResultT]):
             except Exception as err:
                 if raise_exceptions:  # pragma: no cover
                     raise err
-                response = types.ErrorData(code=0, message=str(err), data=None)
+                response = types.ErrorData(code=0, message=str(err))
 
             await message.respond(response)
         else:  # pragma: no cover

src/mcp/server/sse.py

@@ -170,7 +170,7 @@ class SseServerTransport:
                     await sse_stream_writer.send(
                         {
                             "event": "message",
-                            "data": session_message.message.model_dump_json(by_alias=True, exclude_none=True),
+                            "data": session_message.message.model_dump_json(by_alias=True, exclude_unset=True),
                         }
                     )
 

src/mcp/server/stdio.py

@@ -71,7 +71,7 @@ async def stdio_server(stdin: anyio.AsyncFile[str] | None = None, stdout: anyio.
         try:
             async with write_stream_reader:
                 async for session_message in write_stream_reader:
-                    json = session_message.message.model_dump_json(by_alias=True, exclude_none=True)
+                    json = session_message.message.model_dump_json(by_alias=True, exclude_unset=True)
                     await stdout.write(json + "\n")
                     await stdout.flush()
         except anyio.ClosedResourceError:  # pragma: no cover

src/mcp/server/streamable_http.py

@@ -298,12 +298,12 @@ class StreamableHTTPServerTransport:
         # Return a properly formatted JSON error response
         error_response = JSONRPCError(
             jsonrpc="2.0",
-            id="server-error",  # We don't have a request ID for general errors
+            id=None,
             error=ErrorData(code=error_code, message=error_message),
         )
 
         return Response(
-            error_response.model_dump_json(by_alias=True, exclude_none=True),
+            error_response.model_dump_json(by_alias=True, exclude_unset=True),
             status_code=status_code,
             headers=response_headers,
         )
@@ -323,7 +323,7 @@ class StreamableHTTPServerTransport:
             response_headers[MCP_SESSION_ID_HEADER] = self.mcp_session_id
 
         return Response(
-            response_message.model_dump_json(by_alias=True, exclude_none=True) if response_message else None,
+            response_message.model_dump_json(by_alias=True, exclude_unset=True) if response_message else None,
             status_code=status_code,
             headers=response_headers,
         )
@@ -336,7 +336,7 @@ class StreamableHTTPServerTransport:
         """Create event data dictionary from an EventMessage."""
         event_data = {
             "event": "message",
-            "data": event_message.message.model_dump_json(by_alias=True, exclude_none=True),
+            "data": event_message.message.model_dump_json(by_alias=True, exclude_unset=True),
         }
 
         # If an event ID was provided, include it
@@ -975,12 +975,11 @@ class StreamableHTTPServerTransport:
                         # Determine which request stream(s) should receive this message
                         message = session_message.message
                         target_request_id = None
-                        # Check if this is a response
-                        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
+                        # Check if this is a response with a known request id.
+                        # Null-id errors (e.g., parse errors) fall through to
+                        # the GET stream since they can't be correlated.
+                        if isinstance(message, JSONRPCResponse | JSONRPCError) and message.id is not None:
+                            target_request_id = str(message.id)
                         # Extract related_request_id from meta if it exists
                         elif (  # pragma: no cover
                             session_message.metadata is not None

src/mcp/server/streamable_http_manager.py

@@ -244,11 +244,11 @@ class StreamableHTTPSessionManager:
             # See: https://github.com/modelcontextprotocol/python-sdk/issues/1821
             error_response = JSONRPCError(
                 jsonrpc="2.0",
-                id="server-error",
+                id=None,
                 error=ErrorData(code=INVALID_REQUEST, message="Session not found"),
             )
             response = Response(
-                content=error_response.model_dump_json(by_alias=True, exclude_none=True),
+                content=error_response.model_dump_json(by_alias=True, exclude_unset=True),
                 status_code=HTTPStatus.NOT_FOUND,
                 media_type="application/json",
             )

src/mcp/server/websocket.py

@@ -47,7 +47,7 @@ async def websocket_server(scope: Scope, receive: Receive, send: Send):
         try:
             async with write_stream_reader:
                 async for session_message in write_stream_reader:
-                    obj = session_message.message.model_dump_json(by_alias=True, exclude_none=True)
+                    obj = session_message.message.model_dump_json(by_alias=True, exclude_unset=True)
                     await websocket.send_text(obj)
         except anyio.ClosedResourceError:
             await websocket.close()