Codex reconnects after SSE idle timeout with local MLX gpt-oss-120b despite Rapid-MLX keepalive comments

When using Codex CLI with a local Rapid-MLX /v1/responses endpoint and the MLX repack model 66ton99/gpt-oss-120b, long-running turns can stall until Codex starts reconnecting and eventually reports:

Reconnecting... 3/5 (18m 52s • esc to interrupt)
  └ Stream disconnected before completion: idle timeout waiting for SSE

Model

• Hugging Face: https://huggingface.co/66ton99/gpt-oss-120b
• Runtime: Rapid-MLX / MLX on Apple Silicon
• API surface: OpenAI-compatible /v1/responses

Observed behavior

• Codex starts a streaming /v1/responses request.
• Rapid-MLX logs SSE keepalive activity while the model is still working.
• Codex still enters reconnect mode after a long idle period.
• Example user-visible error:

Reconnecting... 3/5 (18m 52s • esc to interrupt)
  └ Stream disconnected before completion: idle timeout waiting for SSE

Expected behavior

• Codex should keep the stream alive while the server is still producing SSE keepalive frames, or the server should emit a Responses-compatible heartbeat event that Codex treats as activity.
• Long prefills / long reasoning phases for a 120B local model should not trigger reconnects as long as the server is alive and emitting heartbeat traffic.

Technical notes

• Codex has a stream idle timeout controlled by stream_idle_timeout_ms; default is 300000 ms, i.e. 5 minutes.
• The Codex SSE reader times out when no parsed SSE event arrives before that deadline.
• Rapid-MLX currently emits keepalive as SSE comments, e.g. : keepalive\n\n.
• SSE comments may keep the TCP connection/proxy alive, but Codex may not count them as stream.next() events after EventSource parsing.
• This means Rapid-MLX can be emitting keepalive comments while Codex still considers the Responses stream idle.

Likely root cause

Codex's idle watchdog is based on parsed SSE events, while Rapid-MLX keepalive uses SSE comments. Comments are valid SSE and useful for proxies, but they may be discarded by the parser before Codex's stream.next() activity watchdog sees them.