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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"ClientDisconnect during _handle_post_request crashes stateless session with ClosedResourceError","articleBody":"### Initial Checks\n\n- [x] I confirm that I'm using the latest version of MCP Python SDK\n- [x] I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue\n\n### Description\n\n  When a client disconnects while a stateless streamable-HTTP server is reading the request body, `_handle_post_request` catches the\n  `ClientDisconnect` but the error handler in `_handle_message` (lowlevel/server.py:694) then tries to `send_log_message()` back to the\n  client. Since the session was already terminated and the write stream closed, this raises `ClosedResourceError`, which is unhandled and\n  crashes the stateless session with an `ExceptionGroup`.\n\n  This is a **different code path** from what PR #1384 fixed. That PR addressed `ClosedResourceError` in the message router loop. This bug\n  is in the error recovery path: catch exception → try to log it to client → write stream already closed → crash.\n\n  ## Versions\n\n  - **mcp**: 1.26.0 (also reproduced on 1.25.0; believed to affect \u003e= 1.12.0)\n  - **Python**: 3.14.2 (also reproducible on 3.12+)\n  - **starlette**: 0.48.0\n  - **uvicorn**: 0.34.3\n\n  ## Steps to reproduce\n  Run the attached repro script to reproduce the problem.\n\n  ## Expected behavior\n\n  The server should log a warning about the client disconnect and cleanly discard the failed request, without crashing the stateless session.\n\n  ## Root cause\n\n  In `lowlevel/server.py`, `_handle_message` has a catch-all exception handler (line ~690) that calls `session.send_log_message()` to notify\n   the client about the error. When the error *is* a client disconnect, the write stream is already closed, so `send_log_message` →\n  `send_notification` → `_write_stream.send()` raises `ClosedResourceError`. This is unhandled in the TaskGroup and crashes the session.\n\n  A possible fix would be to catch `ClosedResourceError` (and/or `BrokenResourceError`) in the error handler at `_handle_message`, since\n  failing to notify a disconnected client is expected and harmless.\n\n  ## Related issues\n\n  - #1190 — closed, partially fixed by PR #1384 (message router path only)\n  - #1219 — closed as duplicate of #1190\n  - #1658 — closed as duplicate of #1190\n\n  None of these cover the `_handle_post_request` → `_handle_message` → `send_log_message` path.\n\n\n\n### Example Code\n\n```Python\n\"\"\"Minimal reproduction: MCP SDK crashes with ClosedResourceError on client disconnect.\"\"\"\n\nimport asyncio\nimport contextlib\nimport logging\nimport time\n\nimport httpx\nimport mcp.types as types\nimport uvicorn\nfrom mcp.server.lowlevel.server import Server\nfrom mcp.server.streamable_http_manager import StreamableHTTPSessionManager\nfrom starlette.applications import Starlette\nfrom starlette.routing import Mount\n\nlogging.basicConfig(level=logging.INFO, format=\"%(levelname)s %(name)s: %(message)s\")\n\nmcp_server = Server(name=\"repro-server\", version=\"0.1.0\")\n\n\n@mcp_server.list_tools()\nasync def list_tools() -\u003e list[types.Tool]:\n    return [\n        types.Tool(\n            name=\"hello\",\n            description=\"A trivial tool.\",\n            inputSchema={\"type\": \"object\", \"properties\": {}},\n        )\n    ]\n\n\n@mcp_server.call_tool()\nasync def call_tool(name: str, arguments: dict) -\u003e list[types.TextContent]:\n    return [types.TextContent(type=\"text\", text=\"hello\")]\n\n\nsession_manager = StreamableHTTPSessionManager(app=mcp_server, stateless=True)\n\n\n@contextlib.asynccontextmanager\nasync def lifespan(app: Starlette):\n    async with session_manager.run():\n        yield\n\n\napp = Starlette(\n    routes=[Mount(\"/\", app=session_manager.handle_request)],\n    lifespan=lifespan,\n)\n\n\nasync def run_client() -\u003e None:\n    await asyncio.sleep(1)\n    url = \"http://127.0.0.1:19876/\"\n\n    # Step 1: Normal MCP initialize\n    async with httpx.AsyncClient() as client:\n        await client.post(url, json={\n            \"jsonrpc\": \"2.0\", \"id\": 1, \"method\": \"initialize\",\n            \"params\": {\n                \"protocolVersion\": \"2025-03-26\",\n                \"capabilities\": {},\n                \"clientInfo\": {\"name\": \"repro-client\", \"version\": \"0.1.0\"},\n            },\n        }, headers={\"Content-Type\": \"application/json\", \"Accept\": \"application/json, text/event-stream\"})\n\n        await client.post(url, json={\n            \"jsonrpc\": \"2.0\", \"method\": \"notifications/initialized\",\n        }, headers={\"Content-Type\": \"application/json\", \"Accept\": \"application/json, text/event-stream\"})\n\n    # Step 2: Send truncated body, then disconnect\n    _, writer = await asyncio.open_connection(\"127.0.0.1\", 19876)\n    writer.write((\n        \"POST / HTTP/1.1\\r\\nHost: 127.0.0.1:19876\\r\\n\"\n        \"Content-Type: application/json\\r\\nAccept: application/json, text/event-stream\\r\\n\"\n        \"Content-Length: 10000\\r\\n\\r\\n\"\n        '{\"jsonrpc\":\"2.0\",\"id\":2,\"method\":\"tools/call\"'\n    ).encode())\n    await writer.drain()\n    await asyncio.sleep(0.5)\n    writer.close()\n    await writer.wait_closed()\n    await asyncio.sleep(3)\n\n\nasync def main() -\u003e None:\n    config = uvicorn.Config(app, host=\"127.0.0.1\", port=19876, log_level=\"warning\")\n    server = uvicorn.Server(config)\n    server_task = asyncio.create_task(server.serve())\n    try:\n        await run_client()\n    finally:\n        server.should_exit = True\n        await server_task\n\n\nif __name__ == \"__main__\":\n    start = time.monotonic()\n    asyncio.run(main())\n    print(f\"Done in {time.monotonic() - start:.1f}s — check ERROR logs above.\")\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\nmcp 1.26.0 (also reproduced on 1.25.0; believed to affect \u003e= 1.12.0)\nPython 3.14.2 (also reproducible on 3.12+)\nstarlette 0.48.0\nuvicorn 0.34.3\n```","author":{"url":"https://github.com/einarfd","@type":"Person","name":"einarfd"},"datePublished":"2026-02-14T13:11:18.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":3},"url":"https://github.com/2064/python-sdk/issues/2064"}