Fixed (verified)Last checked Oct 17, 2026MCP Python SDK#transport #config

Server returns 400 instead of 404 for an expired session, so clients don't reconnect

After a Streamable HTTP server restarts or drops a session, clients keep failing with 400 Bad Request instead of starting a new session.

From GitHub

modelcontextprotocol/python-sdk

Issue Pull request

Difficulty: Easy
Time: ~5 min
Steps: 1
Fixed in: v2.0.0
Last checked: Oct 17, 2026

Verified fix: PR #1808 merged on 2025-12-31. Fixed in v2.0.0 (first stable release containing the merge commit). Last checked 2026-10-17.

Symptom

After a Streamable HTTP server restarts or drops a session, clients keep failing with 400 Bad Request instead of starting a new session.

Evidence what you'll see

HTTP 400 for a request carrying an old Mcp-Session-Id
Clients don't send a new initialize request

Cause

The session manager answered unknown session IDs with 400. The spec requires 404, which is the signal clients use to start a new session.

Fix steps

0 of 1 done

Step 1Upgrade the MCP Python SDK to a release that includes PR #1808, which returns 404 for unknown session IDs.

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Verify the fix worked

Send a request with a made-up Mcp-Session-Id and confirm the server answers 404.

Sources and credit

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