refactor: use exclude_unset for JSONRPC wire serialization

Switch all JSONRPC wire-level serialization from exclude_none=True to
exclude_unset=True. This correctly preserves the null-vs-absent
distinction required by JSON-RPC 2.0 (e.g., id must be null in parse
error responses, not absent entirely).

exclude_unset only omits fields not passed to the constructor, so
explicitly set id=None is preserved while optional params/data fields
that were never set are still omitted.

This eliminates the need for the model_serializer hack on JSONRPCError
that was re-inserting id after exclude_none stripped it.

Inner MCP model serialization (capabilities, tools, resources, etc.)
retains exclude_none=True since those types have many optional fields
where None genuinely means 'omit'.

Author: maxisbey

src/mcp/client/sse.py

@@ -138,7 +138,7 @@ async def post_writer(endpoint_url: str):
                                         json=session_message.message.model_dump(
                                             by_alias=True,
                                             mode="json",
-                                            exclude_none=True,
+                                            exclude_unset=True,
                                         ),
                                     )
                                     response.raise_for_status()

src/mcp/client/stdio.py

@@ -167,7 +167,7 @@ async def stdin_writer():
         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 process.stdin.send(
                         (json + "\n").encode(
                             encoding=server.encoding,

src/mcp/client/streamable_http.py

@@ -259,7 +259,7 @@ async def _handle_post_request(self, ctx: RequestContext) -> None:
         async with ctx.client.stream(
             "POST",
             self.url,
-            json=message.model_dump(by_alias=True, mode="json", exclude_none=True),
+            json=message.model_dump(by_alias=True, mode="json", exclude_unset=True),
             headers=headers,
         ) as response:
             if response.status_code == 202:

src/mcp/client/websocket.py

@@ -65,7 +65,7 @@ async def ws_writer():
             async with write_stream_reader:
                 async for session_message in write_stream_reader:
                     # Convert to a dict, then to JSON
-                    msg_dict = session_message.message.model_dump(by_alias=True, mode="json", exclude_none=True)
+                    msg_dict = session_message.message.model_dump(by_alias=True, mode="json", exclude_unset=True)
                     await ws.send(json.dumps(msg_dict))
 
         async with anyio.create_task_group() as tg:

src/mcp/server/lowlevel/server.py

@@ -490,7 +490,7 @@ async def _handle_request(
             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 @@ async def sse_writer():
                     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 stdout_writer():
         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

@@ -303,7 +303,7 @@ def _create_error_response(
         )
 
         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 @@ def _create_json_response(
             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 @@ def _create_event_data(self, event_message: EventMessage) -> dict[str, str]:
         """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

src/mcp/server/streamable_http_manager.py

@@ -248,7 +248,7 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
                 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 ws_writer():
         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()