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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"SSE: ConnectionClosed exception after session disconnect","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\nIn our environment, the SSE connections sometimes get dropped:\n\n```\nDEBUG:sse_starlette.sse:Got event: http.disconnect. Stop streaming.\nDEBUG:root:Client session disconnected 081769ba-c683-471b-af22-1333409ab669\n```\n\nWhen this happens, on the next request for that session, we get a `ClosedResourceError` exception:\n\n```\nINFO: 100.100.0.154:49316 - \"POST /messages/?session_id=081769bac683471baf221333409ab669 HTTP/1.1\" 202 Accepted\nERROR:__main__:SSE post message error:\nTraceback (most recent call last):\nFile \"/app/mcp_renewables/server.py\", line 714, in handle_post_message_with_error_handling\nreturn await sse.handle_post_message(*args, **kwargs)\n^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^\nFile \"/app/.venv/lib/python3.12/site-packages/mcp/server/sse.py\", line 249, in handle_post_message\nawait writer. Send(session_message)\nFile \"/app/.venv/lib/python3.12/site-packages/anyio/streams/memory.py\", line 242, in send\nself.send_nowait(item)\nFile \"/app/.venv/lib/python3.12/site-packages/anyio/streams/memory.py\", line 211, in send_nowait\nraise ClosedResourceError\nanyio.ClosedResourceError\n```\n\nI believe the reason is that when the session disconnects, its file handles are not properly removed as explained in #703. I have forked the repo and applied the fix and it seems to solve the problem.\n\n\n\n### Example Code: the fix I applied\n\n```Python\ndiff --git a/src/mcp/server/sse.py b/src/mcp/server/sse.py\nindex b7ff332..29acecb 100644\n--- a/src/mcp/server/sse.py\n+++ b/src/mcp/server/sse.py\n@@ -179,7 +179,7 @@ class SseServerTransport:\n \n         async with anyio.create_task_group() as tg:\n \n-            async def response_wrapper(scope: Scope, receive: Receive, send: Send):\n+            async def response_wrapper(scope: Scope, receive: Receive, send: Send, transport: SseServerTransport):\n                 \"\"\"\n                 The EventSourceResponse returning signals a client close / disconnect.\n                 In this case we close our side of the streams to signal the client that\n@@ -190,10 +190,13 @@ class SseServerTransport:\n                 )\n                 await read_stream_writer.aclose()\n                 await write_stream_reader.aclose()\n+                await read_stream.aclose()\n+                await write_stream.aclose()\n+                transport._read_stream_writers.pop(session_id)\n                 logging.debug(f\"Client session disconnected {session_id}\")\n \n             logger.debug(\"Starting SSE response task\")\n-            tg.start_soon(response_wrapper, scope, receive, send)\n+            tg.start_soon(response_wrapper, scope, receive, send, self)\n \n             logger.debug(\"Yielding read and write streams\")\n             yield (read_stream, write_stream)\n```\n\n### Python \u0026 MCP Python SDK\n\n```Text\n1.12.2\n```\n\n### References\n\nRelated PRs:\n\n- #612 \n- #703 ","author":{"url":"https://github.com/tomconte","@type":"Person","name":"tomconte"},"datePublished":"2025-08-01T08:10:02.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/1227/python-sdk/issues/1227"}