Consume server-side session-expired envelope to auto-reinitialize mid-session

[miguelglopes]

Context

The remote MCP server now rewrites rmcp's plaintext 404 (Not Found: Session not found) into a structured JSON-RPC error envelope when a request carries an mcp-session-id header that the server has dropped (keep-alive timeout, replica restart, OOM, drain rehash). Source: OutSystems/mcp-server-remote#65.

Today the TypeScript MCP SDK that this plugin / Power uses does not inspect 404 bodies, so users still see Streamable HTTP error: ... Not Found: Session not found and must /mcp reconnect manually. The server change is a producer-side preposition — this issue tracks the consumer side that makes it actually recoverable.

Wire shape

HTTP/1.1 404 Not Found
Content-Type: application/json

{
  "jsonrpc": "2.0",
  "error": {
    "code": -32003,
    "message": "Session expired; reinitialize",
    "data": {
      "reason": "session_expired",
      "resume": {
        "method": "mentor_start",
        "via": "mentor_session_token"
      }
    }
  },
  "id": null
}

Detection rule on the client: status == 404 && body.error.code == -32003. Equivalent: body.error.data.reason == "session_expired".

Asks

1. Intercept the 404 response, parse the envelope.
2. On match, run a transparent reinitialize (new mcp-session-id) instead of bubbling the transport error.
3. If a mentor_session_token from the most recent terminal mentor_get_run.result is in scope, follow the resume.via hint and re-issue the current mentor turn via mentor_start with the resume token. The server now keeps that token valid through the entire MCP_SESSION_INACTIVITY_SECS window (+10 min buffer), so it won't 401 on the resume.
4. Surface a single subtle UX cue ("reconnected"); don't tear down the conversation.

Out of scope

• Changing the wire shape (server-side decision).
• Re-architecting the SDK transport layer for general 404 handling — this is a targeted carve-out for the -32003 envelope only.

Links

• Server PR: OutSystems/mcp-server-remote#65