Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

No logging in _handle_stateful_request when session ID is unknown/expired

Description

The else branch in StreamableHTTPSessionManager._handle_stateful_request() has no logging when it rejects a request with an unknown session ID.

The other two branches both log:

• logger.debug("Session already exists for session ID ...")
• logger.info("Created new transport for session ...")

The else branch just returns 404 (or 400 on previous version) silently.

https://github.com/modelcontextprotocol/python-sdk/blob/main/src/mcp/server/streamable_http_manager.py

else:
    # Unknown or expired session ID - return 404 per MCP spec
    error_response = JSONRPCError(
        jsonrpc="2.0",
        id=None,
        error=ErrorData(code=INVALID_REQUEST, message="Session not found"),
    )
    response = Response(
        content=error_response.model_dump_json(by_alias=True, exclude_unset=True),
        status_code=HTTPStatus.NOT_FOUND,
        media_type="application/json",
    )
    await response(scope, receive, send)

We hit this in production after a server restart. A client not to be named was sending a stale mcp-session-id on every request and we had no idea why connections were failing. Wrapped the ASGI app in middleware to log rejected requests before we could see it was the same expired session ID every time.

A logger.warning here would have saved us a lot of time:

else:
    logger.warning(
        "Rejected request with unknown session ID: %s",
        session_id,
    )
    ...

Related issues

• StreamableHTTPSessionManager returns 400 instead of 404 for unknown session ID #1727 / fix: return HTTP 404 for unknown session IDs instead of 400 #1808 changed this branch from 400 to 404 but didn't add logging
• Clarify JSON-RPC error code for 'session not found' responses #1821 error code clarification for this response

Thanks for all your hard work!

Example Code

Python & MCP Python SDK

- MCP SDK: 1.24.0 (also confirmed on older versions back when this returned 400)
- Python: 3.13