Fix/log unknown session

[dpdanpittman]

Add logger.warning() call in _handle_stateful_request() when a request is rejected due to an unknown or expired session ID. The other two branches (existing session, new session) both log, but this error path was silent.

Motivation and Context

As described in #2204, the else branch in StreamableHTTPSessionManager._handle_stateful_request() returns a 404 response without any logging. In production, when a client sends a stale mcp-session-id after a server restart, there is no way to observe these rejections without wrapping the ASGI app in custom middleware. A logger.warning here makes the error immediately visible in server logs.

How Has This Been Tested?

Extended the existing test_unknown_session_id_returns_404 test to capture log output via caplog and assert the warning message contains the rejected session ID. All 11 tests in tests/server/test_streamable_http_manager.py pass. Lint passes.

Breaking Changes

None. This only adds a log message at WARNING level. Existing code is unaffected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

Single-line fix — adds logger.warning("Rejected request with unknown or expired session ID: %s", request_mcp_session_id) before the existing error response. Consistent with the logging pattern used throughout the rest of the method.

[maxisbey]

Thanks for the contribution — this was addressed in #2212 which was opened first. For good first issue tickets, it's worth checking the linked PRs on the issue before opening a new one.

[dpdanpittman]

Ahh, no worries. I did look, but must have missed it. I'll keep looking for more!

…

On Fri, Mar 6, 2026, 9:51 AM Max Isbey ***@***.***> wrote: *maxisbey* left a comment (modelcontextprotocol/python-sdk#2223) <#2223 (comment)> Thanks for the contribution — this was addressed in #2212 <#2212> which was opened first. For good first issue tickets, it's worth checking the linked PRs on the issue before opening a new one. — Reply to this email directly, view it on GitHub <#2223 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ACBIMSURXBXHM5DFKO6MMPL4PL6ZHAVCNFSM6AAAAACWI4GHXKVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHM2DAMJSHA2TMOJQGI> . You are receiving this because you authored the thread.Message ID: ***@***.***>