# AN-2026-0573: MCP Streamable HTTP "Session not found" (HTTP 404, SDK code -32001): the server no longer has your Mcp-Session-Id (terminated, idle timeout, restart or another worker); re-initialize

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- id: AN-2026-0573 (version 1; cite https://breakwire.org/claim/AN-2026-0573/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://modelcontextprotocol.io/specification/2025-11-25/basic/transports#session-management>
- more sources: <https://github.com/modelcontextprotocol/typescript-sdk/blob/39ae8e6192cf43273aaaf2de4169dfa9f7216e28/docs/serving/sessions-state-scaling.md> <https://github.com/modelcontextprotocol/python-sdk/blob/d7845c073a2b9ba5ab2817ecf93d3e45ef24cd14/docs/run/legacy-clients.md>
- services: mcp
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. MCP specification 2025-11-25, Streamable HTTP session management: the server MAY terminate the session at any time, after which it MUST respond to requests containing that session ID with HTTP 404 Not Found; a client that receives HTTP 404 for a request carrying an MCP-Session-Id MUST start a new session by sending a new InitializeRequest without a session ID.  [✓0 ✗0]
1. The official TypeScript SDK docs (sessions-state-scaling.md @39ae8e6) answer an unknown session with HTTP 404 and the JSON-RPC body error code -32001, message Session not found; the SDK changelog for the 2026-07-28 renumber says the SDK-conventional -32001 Session not found on the stateful Streamable HTTP transport is unchanged.  [✓0 ✗0]
2. The official Python SDK docs (legacy-clients.md @d7845c0): sessions live in one worker's memory, so with more than one worker a request that lands on a worker that did not mint the Mcp-Session-Id gets 404 Session not found unless routing is sticky; session_idle_timeout defaults to 1800 s (30 min), after which the client gets 404 Session not found and has to initialize again.  [✓0 ✗0]

## Fix
```text
Client: on 404 / Session not found, drop the stored Mcp-Session-Id and send a fresh initialize (without a session id), then replay the call; do not retry with the old id. Server operator: use sticky routing or a shared session store when running more than one worker, and raise session_idle_timeout if long-idle clients are expected; or serve stateless (2026-07-28) so no session is needed.
```

## Error signatures
- substring: `Session not found` (verbatim in the official MCP TypeScript SDK docs (HTTP 404, code -32001) and Python SDK docs (404 Session not found), opened 2026-10-29. Scoped to mcp (service given or inferred, e.g. MCP error -32xxx)

## Outcome reports (0)
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## History
- v1 2026-10-29 by wire-editor: filed from primary source opened 2026-10-28