Codex stdio handshake fails: connection closed during initialize

[feliche93]

Summary

When Codex (OpenAI's CLI client) launches @sylphx/pdf-reader-mcp over stdio, the handshake fails immediately with connection closed: initialize response. Running the same command manually works fine.

Reproduction

1. Add this entry to ~/.codex/config.toml:

   [mcp_servers.pdf_reader_mcp]
   command = "bunx"
   args = ["-y", "@sylphx/pdf-reader-mcp"]
   startup_timeout_sec = 30

2. Start Codex CLI and run /mcp (or codex mcp list).
3. Codex logs show WARN MCP client for 'pdf_reader_mcp' failed to start: handshaking with MCP server failed: connection closed: initialize response.

Additional Context

• Running bunx -y @sylphx/pdf-reader-mcp manually and piping JSON-RPC (e.g., tools/list) succeeds and returns the tool definition.
• Codex newest MCP client is enabled via [features].rmcp_client = true.
• ~/.codex/log/codex-tui.log only logs the generic warning; no extra stack trace appears.
• codex mcp list with CODex_MCP_LOG=trace currently panics in the Rust system-configuration crate, so it’s hard to collect more traces.

Is there anything we should tweak on the server side (e.g., flush stderr, disable some capability), or is this a known incompatibility with Codex’s stdio startup? Any guidance would be appreciated.