app-server fs/watch misses append updates to active rollout JSONL on macOS

What issue are you seeing?

On macOS, app-server fs/watch does not emit fs/changed while an active Codex rollout JSONL is being appended to.

The rollout file size and modification time continue changing as reasoning, tool-call, and output items are written, but the app-server connection receives no corresponding fs/changed notification. A notification may appear around a user-message or turn boundary, but incremental turn items do not reliably trigger one.

This prevents clients that use the file notification as a signal to call thread/read from updating the active turn in real time.

What steps can reproduce the bug?

1. Start codex app-server over stdio and send initialize.
2. Start or resume a thread and locate its active rollout JSONL under the Codex sessions directory.
3. Subscribe to the exact absolute file path:

{ "method": "fs/watch", "id": 44, "params": {
  "watchId": "0195ec6b-1d6f-7c2e-8c7a-56f2c4a8b9d1",
  "path": "/absolute/path/to/rollout.jsonl"
} }

4. Continue the turn so Codex appends reasoning, tool-call, command-output, and assistant-output records to the same file.
5. Observe that the file size and mtime change, but no fs/changed notification is emitted for those appends.

Control checks from the same machine:

• The same app-server watcher emits fs/changed for a newly created file in a watched temporary directory, so the subscription and connection are working.
• fswatch using its default macOS backend also misses the active rollout appends.
• fswatch -m kqueue_monitor on the same rollout file reports every append immediately.

The FSEvents/kqueue comparison is diagnostic evidence, not a confirmed root cause.

What is the expected behavior?

After the watcher debounce interval, each modification or batch of modifications to the watched file should emit:

{ "method": "fs/changed", "params": {
  "watchId": "0195ec6b-1d6f-7c2e-8c7a-56f2c4a8b9d1",
  "changedPaths": ["/absolute/path/to/rollout.jsonl"]
} }

This should work for append-only writes while the writer keeps the file open, not only for create, replace, rename, or turn-boundary activity.

Additional information

• Codex CLI: 0.148.0 (current latest release at time of report)
• Platform: Darwin 25.5.0 arm64 arm
• macOS: 26.5.2 (25F84)
• Architecture: Apple Silicon
• Installation: Homebrew
• codex doctor --json: app-server check is OK; the unrelated overall failure is only because the noninteractive diagnostic shell has TERM=dumb.
• The app-server documentation says watching a file emits fs/changed for that path. The observed behavior differs specifically for sustained append activity on the active rollout file.

Start at the app-server fs/watch implementation and reproduce the append-only rollout JSONL case on macOS, comparing its watcher behavior with fswatch -m kqueue_monitor. Trace how file modifications become fs/changed notifications and add or update coverage for sustained appends. Done means watched files emit the expected path notification after the debounce interval while they remain open.