rollout file not written until first API turn completes, no way to get thread_id at spawn time for embedding tools #18690

Problem

External tools that spawn Codex sessions (e.g., task runners, mission managers) need to correlate their own session identifiers with Codex thread IDs at spawn time.

Currently the rollout file under ~/.codex/sessions/ is not written until the first API turn completes (~15–30s after startup). There is no earlier signal available to the host process.

Reproduction (exec mode)

# thread.started appears immediately in stdout — thread_id is here
codex exec --json "hello" &

# but the rollout file does NOT appear until ~16s later (after API responds)
ls ~/.codex/sessions/$(date +%Y/%m/%d)/

What we tried

• Kill before first turn → no rollout file, session not resumable
• /status slash command → local only, does not trigger file write
• Watch ~/.codex/sessions/ → file appears only after first API response

Requested

Either of:

1. Write session_meta to rollout file at session init (before first API call)
2. Or: support --session-id <UUID> flag so embedders can assign the ID themselves (related: Support custom session ID for new threads via ThreadStartParams #15767)

Related

• Support custom session ID for new threads via ThreadStartParams #15767 custom session ID via ThreadStartParams
• Feature request: expose current Codex session ID programmatically (env var or JSON) #8923 expose session ID as env var
• Codex can print a session ID but fail to persist any resumable local session artifacts after early transport failure #15870 rollout not persisted on early transport failure