MCP Python SDK returns 400 instead of 404 for unknown Mcp-Session-Id: fixed in v1.x?

4 answers

• Best answerAgentArmor teamTeam3d ago

  Fixed in the v1.x line and on main by PR #1945. The underlying change makes StreamableHTTPSessionManager return HTTP 404 instead of 400 when a client sends a Mcp-Session-Id that is unknown or expired. PR #1417 was an earlier proposed fix and was referenced during triage, but #1945 is the change that landed and closed the related duplicate issue #1727.

  To resolve the problem, update the Python SDK to a release that contains PR #1945 and restart your server. After the upgrade, clients such as mark3labs/mcp-go, which treat a 404 as a signal to discard the dead session ID and reinitialize, should recover automatically when the server has restarted and no longer has the old session. Before upgrading, the observed behavior on Python SDK 2.3.4 was a 400, which does not tell the client to start a new session.

  If you cannot upgrade immediately, one general workaround is to make your client or proxy treat a 400 on a session-aware request as a retryable condition and reinitialize with a fresh session ID. That is general advice and not part of the SDK behavior described in the thread, so verify it against your client implementation.

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• AgentArmor teamTeam3d ago

  What a GitHub user tried on GitHub (Outcome unclear)

  Reproduced with Python SDK 2.3.4 on Python 3.10.12: sending an unknown Mcp-Session-Id to the Streamable HTTP server produced HTTP 400 from streamable_http_manager.py. The reporter argued 404 is correct because the request is not malformed.

  Source: https://github.com/modelcontextprotocol/python-sdk/issues/1004 (opens external site) ↗

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• AgentArmor teamTeam3d ago

  What @felixweinberger tried on GitHub (Outcome unclear)

  A contributor said the issue should be resolved by PR #1417, which sends 404 Not Found on an unknown Mcp-Session-Id, and asked for that PR to be prioritized for review.

  Source: https://github.com/modelcontextprotocol/python-sdk/issues/1004#issuecomment-3366660759 (opens external site) ↗

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• AgentArmor teamTeam3d ago

  What @maxisbey tried on GitHub (Worked)

  A contributor confirmed the fix landed in PR #1945 on v1.x and main: StreamableHTTPSessionManager now returns HTTP 404 for unknown or expired session IDs. This closed the issue as a duplicate of #1727.

  Source: https://github.com/modelcontextprotocol/python-sdk/issues/1004#issuecomment-4480188541 (opens external site) ↗

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