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claude-code - 💡(How to fix) Fix Auto-reconnect MCP streamable-HTTP servers on `Session not found`

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When an MCP server using streamable HTTP transport evicts a session (idle timeout, restart, deploy), the client keeps using its stale session ID and every tool call fails with:

Streamable HTTP error: Error POSTing to endpoint:
{"error":{"message":"Session not found","code":-32600},...}

/mcp reports the server as "Connected" because that status reflects the last successful handshake, not live session validity. Users must manually reconnect via /mcp to recover.

Error Message

Streamable HTTP error: Error POSTing to endpoint: {"error":{"message":"Session not found","code":-32600},...} On receiving JSON-RPC error code -32600 with message "Session not found" surfacing the error to the user. 4. Observe Session not found error; /mcp still shows "Connected".

Root Cause

/mcp reports the server as "Connected" because that status reflects the last successful handshake, not live session validity. Users must manually reconnect via /mcp to recover.

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Summary

When an MCP server using streamable HTTP transport evicts a session (idle timeout, restart, deploy), the client keeps using its stale session ID and every tool call fails with:

Streamable HTTP error: Error POSTing to endpoint:
{"error":{"message":"Session not found","code":-32600},...}

/mcp reports the server as "Connected" because that status reflects the last successful handshake, not live session validity. Users must manually reconnect via /mcp to recover.

Proposed behavior

On receiving JSON-RPC error code -32600 with message "Session not found" (or equivalent session-eviction signal), Claude Code should transparently re-establish the session and retry the failed call once, rather than surfacing the error to the user.

Repro

  1. Connect to any streamable-HTTP MCP server (e.g. Fastmail).
  2. Leave the session idle long enough for the server to evict it (or wait for a server-side restart).
  3. Invoke any tool from that server.
  4. Observe Session not found error; /mcp still shows "Connected".
  5. Manually /mcp → reconnect → tool works again.

Environment

Claude Code (CLI), darwin.

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#generation error#database connection#vector store#embedding generation#cache error

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