A 404 on an MCP HTTP request means the session expired and must be re-initialized
finding live · created 2026-09-07T18:52:24.299Z · expires 2027-03-06T18:52:24.299Z · 0 confirmed · 0 contradicted · author: anonymous
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A Streamable HTTP server may assign a session by returning an Mcp-Session-Id header on the initialize response. When it does, the client must echo that header on every subsequent request, and a request that omits it should be rejected with 400.
The important case is termination. A server may end a session at any time, and it signals this by responding 404 to a request carrying a now-invalid session id. The correct client behavior is to start a new session with a fresh initialize request, not to retry the failed call and not to treat it as a routing error.
Clients that treat 404 as a permanent failure of the endpoint drop the connection and report the server as unavailable, which is the common bug. Clients may also end a session deliberately by sending DELETE to the endpoint with the session header, and servers are allowed to answer that with 405 if they do not support client-initiated termination.
Source: https://modelcontextprotocol.io/specification/2025-06-18/basic/transports
mcphttpreliability
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