René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"MCP SSE Server: Received request before initialization was complete","articleBody":"**Describe the bug**\n\nThe MCP server receives `Received request before initialization was complete` after a second deployment when a client is trying to list the different tools or even when it tries to run a tool on the server.\n\n**To Reproduce**\n\nSImple main.py which is one of your examples\n```python\nimport httpx\nfrom mcp.server.fastmcp import FastMCP\n\nmcp = FastMCP(\"My App\")\n\n\n@mcp.tool()\ndef calculate_bmi(weight_kg: float, height_m: float) -\u003e float:\n    \"\"\"Calculate BMI given weight in kg and height in meters\"\"\"\n    return weight_kg / (height_m**2)\n\n\n@mcp.tool()\nasync def fetch_weather(city: str) -\u003e str:\n    \"\"\"Fetch current weather for a city\"\"\"\n    async with httpx.AsyncClient() as client:\n        response = await client.get(f\"https://api.weather.com/{city}\")\n        return response.text\n\napp = mcp.sse_app()\n```\n\n* Start using uvicorn `FASTMCP_LOG_LEVEL=DEBUG uv run uvicorn main:app --port=8001 --host=0.0.0.0 --timeout-graceful-shutdown 5`\n\n* Run the inspector in SSE mode and listen on `http://localhost:8001/see`. You will be able to list tools.\n* But now, reload you server to simulate a new deployment using the same command: `FASTMCP_LOG_LEVEL=DEBUG uv run uvicorn main:app --port=8001 --host=0.0.0.0 --timeout-graceful-shutdown 5`\n* Go to the inspector and try to list the tools, it will timeout and you'll get the following server logs\n\n```bash\nFASTMCP_LOG_LEVEL=DEBUG uv run uvicorn main:app --port=8001 --host=0.0.0.0 --timeout-graceful-shutdown 5                                       \n[04/03/25 23:48:40] DEBUG    SseServerTransport initialized with endpoint: /messages/                                                                                                 sse.py:79\nINFO:     Started server process [89042]\nINFO:     Waiting for application startup.\nINFO:     Application startup complete.\nINFO:     Uvicorn running on http://0.0.0.0:8001 (Press CTRL+C to quit)\n[04/03/25 23:48:41] DEBUG    Setting up SSE connection                                                                                                                                    sse.py:87\n                    DEBUG    Created new session with ID: f55c6f64-be08-464c-bbab-203223adb44c                                                                                           sse.py:100\n                    DEBUG    Starting SSE response task                                                                                                                                  sse.py:127\n                    DEBUG    Yielding read and write streams                                                                                                                             sse.py:130\nINFO:     127.0.0.1:62930 - \"GET /sse HTTP/1.1\" 200 OK\n                    DEBUG    Starting SSE writer                                                                                                                                         sse.py:107\n                    DEBUG    Sent endpoint event: /messages/?session_id=f55c6f64be08464cbbab203223adb44c                                                                                 sse.py:110\n                    DEBUG    chunk: b'event: endpoint\\r\\ndata: /messages/?session_id=f55c6f64be08464cbbab203223adb44c\\r\\n\\r\\n'                                                           sse.py:156\n[04/03/25 23:48:45] DEBUG    Handling POST message                                                                                                                                       sse.py:136\n                    DEBUG    Parsed session ID: f55c6f64-be08-464c-bbab-203223adb44c                                                                                                     sse.py:147\nwriter:  MemoryObjectSendStream(_state=MemoryObjectStreamState(max_buffer_size=0, buffer=deque([]), open_send_channels=1, open_receive_channels=1, waiting_receivers=OrderedDict({\u003canyio._backends._asyncio.Event object at 0x103ff6f00\u003e: MemoryObjectItemReceiver(task_info=AsyncIOTaskInfo(id=4386289792, name='mcp.shared.session.BaseSession._receive_loop'), item=None)}), waiting_senders=OrderedDict()), _closed=False)\n                    DEBUG    Received JSON: b'{\"jsonrpc\":\"2.0\",\"id\":3,\"method\":\"tools/list\",\"params\":{}}'                                                                                sse.py:161\n                    DEBUG    Validated client message: root=JSONRPCRequest(method='tools/list', params={}, jsonrpc='2.0', id=3)                                                          sse.py:165\n                    DEBUG    Sending message to writer: root=JSONRPCRequest(method='tools/list', params={}, jsonrpc='2.0', id=3)                                                         sse.py:173\nINFO:     127.0.0.1:62936 - \"POST /messages/?session_id=f55c6f64be08464cbbab203223adb44c HTTP/1.1\" 202 Accepted\n                    DEBUG    Got event: http.disconnect. Stop streaming.                                                                                                                 sse.py:177\nERROR:    Exception in ASGI application\n  + Exception Group Traceback (most recent call last):\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/uvicorn/protocols/http/h11_impl.py\", line 403, in run_asgi\n  |     result = await app(  # type: ignore[func-returns-value]\n  |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py\", line 60, in __call__\n  |     return await self.app(scope, receive, send)\n  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/applications.py\", line 112, in __call__\n  |     await self.middleware_stack(scope, receive, send)\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py\", line 187, in __call__\n  |     raise exc\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/middleware/errors.py\", line 165, in __call__\n  |     await self.app(scope, receive, _send)\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/middleware/exceptions.py\", line 62, in __call__\n  |     await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py\", line 53, in wrapped_app\n  |     raise exc\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py\", line 42, in wrapped_app\n  |     await app(scope, receive, sender)\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/routing.py\", line 714, in __call__\n  |     await self.middleware_stack(scope, receive, send)\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/routing.py\", line 734, in app\n  |     await route.handle(scope, receive, send)\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/routing.py\", line 288, in handle\n  |     await self.app(scope, receive, send)\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/routing.py\", line 76, in app\n  |     await wrap_app_handling_exceptions(app, request)(scope, receive, send)\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py\", line 53, in wrapped_app\n  |     raise exc\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/_exception_handler.py\", line 42, in wrapped_app\n  |     await app(scope, receive, sender)\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/starlette/routing.py\", line 73, in app\n  |     response = await f(request)\n  |                ^^^^^^^^^^^^^^^^\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/mcp/server/fastmcp/server.py\", line 485, in handle_sse\n  |     async with sse.connect_sse(\n  |   File \"/Users/xxxxx/.pyenv/versions/3.12.2/lib/python3.12/contextlib.py\", line 231, in __aexit__\n  |     await self.gen.athrow(value)\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/mcp/server/sse.py\", line 123, in connect_sse\n  |     async with anyio.create_task_group() as tg:\n  |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/anyio/_backends/_asyncio.py\", line 772, in __aexit__\n  |     raise BaseExceptionGroup(\n  | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)\n  +-+---------------- 1 ----------------\n    | Exception Group Traceback (most recent call last):\n    |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/mcp/server/sse.py\", line 131, in connect_sse\n    |     yield (read_stream, write_stream)\n    |   File \"/Users/xxxxx.venv/lib/python3.12/site-packages/mcp/server/fastmcp/server.py\", line 490, in handle_sse\n    |     await self._mcp_server.run(\n    |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/mcp/server/lowlevel/server.py\", line 483, in run\n    |     async with AsyncExitStack() as stack:\n    |   File \"/Users/xxxxx/.pyenv/versions/3.12.2/lib/python3.12/contextlib.py\", line 754, in __aexit__\n    |     raise exc_details[1]\n    |   File \"/Users/xxxxx/.pyenv/versions/3.12.2/lib/python3.12/contextlib.py\", line 737, in __aexit__\n    |     cb_suppress = await cb(*exc_details)\n    |                   ^^^^^^^^^^^^^^^^^^^^^^\n    |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/mcp/shared/session.py\", line 210, in __aexit__\n    |     return await self._task_group.__aexit__(exc_type, exc_val, exc_tb)\n    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\n    |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/anyio/_backends/_asyncio.py\", line 772, in __aexit__\n    |     raise BaseExceptionGroup(\n    | ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)\n    +-+---------------- 1 ----------------\n      | Traceback (most recent call last):\n      |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/mcp/shared/session.py\", line 324, in _receive_loop\n      |     await self._received_request(responder)\n      |   File \"/Users/xxxxx/.venv/lib/python3.12/site-packages/mcp/server/session.py\", line 163, in _received_request\n      |     raise RuntimeError(\n      | RuntimeError: Received request before initialization was complete\n      +------------------------------------\n[04/03/25 23:48:48] DEBUG    Setting up SSE connection                                                                                                                                    sse.py:87\n                    DEBUG    Created new session with ID: 1541ccf4-7bf9-407b-b2a2-94552715a527                                                                                           sse.py:100\n                    DEBUG    Starting SSE response task                                                                                                                                  sse.py:127\n                    DEBUG    Yielding read and write streams                                                                                                                             sse.py:130\nINFO:     127.0.0.1:62938 - \"GET /sse HTTP/1.1\" 200 OK\n                    DEBUG    Starting SSE writer                                                                                                                                         sse.py:107\n                    DEBUG    Sent endpoint event: /messages/?session_id=1541ccf47bf9407bb2a294552715a527                                                                                 sse.py:110\n                    DEBUG    chunk: b'event: endpoint\\r\\ndata: /messages/?session_id=1541ccf47bf9407bb2a294552715a527\\r\\n\\r\\n'                                                           sse.py:156\n[04/03/25 23:48:55] DEBUG    Handling POST message                                                                                                                                       sse.py:136\n                    DEBUG    Parsed session ID: 1541ccf4-7bf9-407b-b2a2-94552715a527                                                                                                     sse.py:147\nwriter:  MemoryObjectSendStream(_state=MemoryObjectStreamState(max_buffer_size=0, buffer=deque([]), open_send_channels=1, open_receive_channels=1, waiting_receivers=OrderedDict({\u003canyio._backends._asyncio.Event object at 0x105916510\u003e: MemoryObjectItemReceiver(task_info=AsyncIOTaskInfo(id=4386290368, name='mcp.shared.session.BaseSession._receive_loop'), item=None)}), waiting_senders=OrderedDict()), _closed=False)\n                    DEBUG    Received JSON: b'{\"jsonrpc\":\"2.0\",\"method\":\"notifications/cancelled\",\"params\":{\"requestId\":3,\"reason\":\"Request timed out\"}}'                                sse.py:161\n                    DEBUG    Validated client message: root=JSONRPCNotification(method='notifications/cancelled', params={'requestId': 3, 'reason': 'Request timed out'}, jsonrpc='2.0') sse.py:165\n                    DEBUG    Sending message to writer: root=JSONRPCNotification(method='notifications/cancelled', params={'requestId': 3, 'reason': 'Request timed out'},               sse.py:173\n                             jsonrpc='2.0')                                                                                                                                                        \nINFO:     127.0.0.1:62945 - \"POST /messages/?session_id=1541ccf47bf9407bb2a294552715a527 HTTP/1.1\" 202 Accepted\n[04/03/25 23:49:03] DEBUG    ping: b': ping - 2025-04-03 21:49:03.579244+00:00\\r\\n\\r\\n'\n\n```\n\n**Expected behavior**\n\nSomehow, a POST message is accepted while it should not. Is it correct to accept such request?\nShouldn't we have a 404 here instead of a 200 after the second reload? 🤔  https://github.com/modelcontextprotocol/python-sdk/blob/c2ca8e03e046908935d089a2ceed4e80b0c29a24/src/mcp/server/sse.py#L153-L157\n\nEDIT: I actually tested to comment this line and it works correctly now! \n\nhttps://github.com/modelcontextprotocol/python-sdk/blob/c2ca8e03e046908935d089a2ceed4e80b0c29a24/src/mcp/server/session.py#L162-L165\n\nTested with fast-agent, LibreChat and Inspector. Even after a server reload, call tools are still working.\nIf I understand correctly the [global mechanism ](https://spec.modelcontextprotocol.io/specification/2024-11-05/architecture/#capability-negotiation) It should not be possible to have, at this point (after a server reload), a session-id corresponding to a SSE transport that should not have been initialized. So this comment actually is a patch on something that should not happen 😬 \n\n\n**Desktop:**\n - OS: MacOS 15.3\n - Browser chrome latest\n - Python 3.12\n ```\n\"mcp[cli]\u003e=1.6.0\",\n\"starlette\u003e=0.46.1\",\n\"uvicorn\u003e=0.34.0\",\n```","author":{"url":"https://github.com/folkvir","@type":"Person","name":"folkvir"},"datePublished":"2025-04-03T22:11:23.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":25},"url":"https://github.com/423/python-sdk/issues/423"}