MCP: daemon handler crashes on initialize requests (HTTP 500, "Received type number (0)")

[calionauta]

Summary

The daemon's MCP handler crashes when handling initialize requests in executor v1.5.40 (linux-arm64), so MCP requests cannot be handled via either documented transport. The internal daemon transport (e.g. POST /executions, GET /tools) works as expected; the issue appears to be isolated to MCP session handling.

Reproduction

# 1) stdio bridge
printf '%s\n' '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"probe","version":"1.0"}}}' \
  | executor mcp --scope /some/scope --elicitation-mode model
# -> Executor MCP bridge daemon transport error: Streamable HTTP error: Error POSTing to endpoint:

# 2) direct HTTP (with the daemon bearer token)
curl -X POST http://127.0.0.1:4789/mcp \
  -H 'Content-Type: application/json' \
  -H 'Authorization: Bearer <daemon token>' \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"probe","version":"1.0"}}}'
# -> HTTP 500 {"jsonrpc":"2.0","error":{"code":-32603,"message":"Internal server error"},"id":null}

Daemon error log

[executor] MCP session mode {"clientCapabilities":null,"elicitationSupport":{"form":false,"url":false},"elicitationMode":"model","resumeEnabled":true}
[mcp] handleRequest error: Error: The first argument must be of type string, Buffer, ArrayBuffer, Array, or Array-like Object. Received type number (0)
    at nNe (/$bunfs/root/main.js:89835:51338)
    at ~effect/Utils/internal (/$bunfs/root/main.js:8:1707)

Observations

• Crash is independent of payload: reproduced with empty params, with/without capabilities/clientInfo, and with protocolVersion 2024-10-07, 2024-11-05, 2025-06-18.
• Auth is not the issue: without the bearer token the endpoint returns 401; with it the request reaches the handler and crashes (500).
• Reproduced on both the default port (4788) and a custom --port; the /mcp route exists in both cases.
• executor daemon status always reports "Daemon not running at http://localhost:4788" even when a daemon is running on that port (default-port check may be a separate issue).

Environment

• executor 1.5.40 (linux-arm64), daemon run via executor daemon run --foreground --port 4789