codex prompt --session delays first text event by ~190s on a prompt where codex exec completes in ~208s

[monkeygold]

Summary

I am seeing a large session-specific delay before the first text event when running the same Codex prompt via ACPX session transport versus exec.

The issue reproduces both:

• inside a local wrapper (Idea OS) with an external 180s timeout
• outside the wrapper with direct ACPX benchmark commands and a 240s timeout

The dominant extra latency is before the first streamed event, not in session close.

Environment

• acpx: 0.3.1
• node: v22.22.1
• OS: Linux ubuntu-8gb-nbg1-1 6.8.0-106-generic x86_64

Prompt characteristics

This is a real production-style prompt used as a text-generation-only engineer task:

• system_prompt_len=3096
• user_message_len=6712
• text_only_prompt_len=10094
• source run id: 2026-03-21-web-agentic-009

Reproduction

Exec path

npx --yes acpx@latest \
  --approve-reads \
  --format quiet \
  --timeout 240 \
  codex exec "<same 10094-char prompt>"

Session path

npx --yes acpx@latest \
  --format json \
  --json-strict \
  --cwd /opt/idea-os \
  codex sessions ensure --name bench-65c98fac8228

npx --yes acpx@latest \
  --approve-reads \
  --format json \
  --json-strict \
  --cwd /opt/idea-os \
  codex prompt --session bench-65c98fac8228 --file -

npx --yes acpx@latest \
  --format json \
  --json-strict \
  --cwd /opt/idea-os \
  codex sessions close bench-65c98fac8228

Observed results

Direct ACPX benchmark outside wrapper, timeout 240s

• exec: completed in 208239ms
• session: completed in 216148ms
• session.ensure_duration_ms=8096
• session.first_event_ms=190740
• session.final_event_ms=216148
• session.close_duration_ms=1222
• session.poll_count=3321
• session.first_event_type=text
• session.final_event_type=done

JSON excerpt:

{
  "metadata": {
    "source_run_id": "2026-03-21-web-agentic-009",
    "measure_timeout_seconds": 240,
    "system_prompt_len": 3096,
    "user_message_len": 6712,
    "text_only_prompt_len": 10094
  },
  "exec": {
    "transport": "exec",
    "status": "completed",
    "duration_ms": 208239
  },
  "session": {
    "transport": "session",
    "status": "completed",
    "ensure_duration_ms": 8096,
    "first_event_type": "text",
    "final_event_type": "done",
    "first_event_ms": 190740,
    "final_event_ms": 216148,
    "close_duration_ms": 1222,
    "poll_count": 3321,
    "timed_out": false
  }
}

Same prompt inside wrapper with external timeout 180s

• exec: external timeout at about 180.158s
• session: external timeout at about 180.857s
• despite the outer timeout, the session runtime snapshot still shows:
  • start_session=10132ms
  • send_input=21ms
  • first_event=143870ms
  • final_event=169349ms
  • close_session=1049ms
  • poll_count=1692
  • first_event_type=text
  • final_event_type=done

Relevant snapshot excerpt:

{
  "status": "failed",
  "agent_runtime_summary": {
    "latest": {
      "provider": "acpx",
      "transport": "session",
      "final_status": "completed",
      "first_event_type": "text",
      "final_event_type": "done",
      "poll_count": 1692,
      "timed_out": false,
      "session_phase_timings_ms": {
        "start_session": 10132,
        "send_input": 21,
        "first_event": 143870,
        "final_event": 169349,
        "close_session": 1049
      }
    }
  }
}

Interpretation

• the dominant session overhead is before the first event
• close_session is small and does not explain the gap
• after the first text event, the session run finishes in about 25.4s
• the same pattern reproduces outside my wrapper, so this does not look like a local instrumentation bug

Expected behavior

For the same prompt, codex prompt --session should not spend ~190s before the first text event while codex exec completes in ~208s overall.

Question

Is there a known bottleneck in the ACPX Codex session path before first streamed output, or anything obvious I should inspect around session dispatch / load / stream initialization?