Legacy GET /mcp with an unknown or expired Mcp-Session-Id opens a new session instead of 404
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Assessment
- Difficulty
- 2/5
- Estimated time
- 1-3 hours
- Newbie friendliness
- 78/100
- Issue type
- Bug
- Clarity
- Clearly specified
- Activity status
- Active
- Tech stack
- typescript
Research direction
Read the legacy SSE GET handler in src/routes/mcp.ts and compare its session handling with the related POST behavior in #304. Check the behavior for an unknown Mcp-Session-Id, including when Last-Event-ID is present, and for a GET without a session ID. Done means unknown IDs return 404 and GET does not create a session.
Written by the indexing model from the issue text.
Description
With enableSSE, a GET /mcp whose Mcp-Session-Id names a session the store does not have returns 200 with a new Mcp-Session-Id. This happens even when a Last-Event-ID is sent. The client believes it resumed, but nothing is replayed, and the session it negotiated is gone.
Spec (2025-11-25 Streamable HTTP, Session Management): once a session has ended, requests carrying its ID MUST get 404, and the client then starts a new session. A GET with no session ID also creates an uninitialized session.
Expected: 404 for an unknown session ID on GET, and no session created by GET.
Code: src/routes/mcp.ts, legacy SSE GET handler.
Related: #304 (the same behaviour for POST).
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- Dominant language
- TypeScript
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- Avg merge
- 1d 5h
- Merged PRs (30d)
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