App-server: expose atomic idle-only turn admission

What variant of Codex are you using?

CLI / app-server v2 on current main (e766f7598993ce37cf61b9c26c80cc2ba3a4f2d7).

What feature would you like to see?

An experimental app-server request that atomically starts a turn only when a specific thread is idle.

Today, turn/start is intentionally start-or-steer: if a turn becomes active before Core admits the request, the input can be steered into that turn. A client can read an idle status first, but thread/read → turn/start is not atomic. That makes turn/start unsafe for background or event-driven clients that must never alter an active human-owned turn.

A narrow v2 shape would be:

turn/startIfIdle {
  threadId,
  expectedCwd,
  clientUserMessageId?,
  input
}

The admission should happen inside the same ordered Core turn-input path as ordinary input and either:

• start one turn and return the existing TurnStartResponse with its actual turn ID; or
• reject without steering, interrupting, queuing, applying turn settings, recording input, or making a provider request.

The useful fail-closed reasons are:

• thread is busy;
• pending trigger work exists;
• Plan mode is active for this client admission;
• the effective turn cwd differs from expectedCwd;
• the thread is not persistence-enabled.

The cwd check must be part of the same reserved admission operation; checking it in app-server before submission would leave another race.

Core now has most of the required primitive: PR 38275 added ordered CodexThread::start_turn_if_idle / TurnInputMode::StartIfIdle. The remaining work is a small precondition extension plus the experimental app-server protocol/processor surface, schemas, docs, and focused fake-provider regressions.

This does not require a new delivery queue, retry loop, fallback to turn/start, approval response, or replacement client.

Additional information

We ran into this while building a small resident app-server integration at @clevvi: inbound work should wake an idle Codex thread, but must leave an active TUI turn completely alone.

Related, but not duplicates:

• #20312 asks for a broader native event-driven wake facility.
• #36866 documents the existing start-or-steer behavior and turn-ID consequences of turn/start.
• #34767 shows why multi-client admission needs a single authoritative turn boundary.

I have a focused current-main patch and deterministic no-real-model tests in progress. Happy to open the PR if this API shape is welcome.