## Summary

- Advertise and implement standard, unstable `systemPrompt` support on
ACP v1 and v2 session lifecycle requests. Support text-only **append**
mode; reject override, custom modes, and non-text content before
lifecycle side effects.
- Preserve AIR's existing instruction baseline. Append standard client
text after AIR `customInstructions` when present, otherwise after
configured Codex developer instructions. Leave Codex base instructions
and workspace context intact. Requests without standard prompts retain
legacy AIR behavior.
- Carry known instruction state through new/resume/load/fork and through
provider restarts/app-server recovery. Omitted/null prompts preserve
known instructions while the adapter session remains open; clients
should resend after closing/restarting the adapter.
- For changed prompts on an idle, persisted thread, detach/reload under
the same ID. Verify an actual unload instead of acknowledging an
override Codex silently ignored. Check persisted history (a planned
rollout path alone is not proof), block racing inputs/config changes,
and restore delivery after rejected/failed changes.
- Do not interrupt foreground work or background terminals, force
compaction, restart the global app-server, rewrite history, or repurpose
collaboration-mode instructions. Standard subagent support remains
separate; the AIR subagent contract is unchanged.

## Timing and limits

**New sessions apply the prompt on their first turn. Changed prompts on
existing sessions and forks become model-visible at the next
compaction.** Reload updates configuration but retained history still
contains the previous developer message until context is rebuilt. This
is the deferral allowed by the ACP system-prompt specification, similar
to the Claude adapter's documented behavior.

Changed prompts on busy sessions, sessions with background terminals, or
live sessions without a saved rollout are explicitly rejected with
`-32602`; finish the work and retry, or create a new session for
immediate instructions. A runtime that keeps the existing thread loaded
also produces an error rather than false success. Busy-session changes
are not automatically queued in this package.

## Validation

- `npm run typecheck` — passed, including examples.
- `npm test` — **1,859 passed**, 36 skipped; includes 38 system-prompt
wire tests across v1/v2, concurrency guards, runtime refusal,
restoration after failure, empty-content clearing, and provider-restart
preservation.
- `npm run build` — passed.
- `npm run test:integration` — **7 passed** against real Codex
app-server with a local mock model. New v1/v2 tests inspect actual
developer messages for new, changed resume, fork, full-history replay,
and compaction, preserving AIR and configured baselines and keeping
rejected unused sessions usable.
- `npm run codex-test` live-model legacy smoke — passed.
- Additional live-model ACP smoke: standard system prompt specified
`system-prompt-live-ok`, user prompt was `Greet me.`, and the model
returned exactly the system-prompt marker with `end_turn`.
- `git diff --check` — passed.
- Verified all 33 AIR scenario snapshot changes add only the
`systemPrompt` capability. The pinned compatibility baselines are
unchanged; their allowed feature-delta rule accounts for the new
advertisement.

The full live-model E2E suite was not run. Specification checked at
`agent-client-protocol@4034a8cbc99bc48fd4579f70a3407b07907e0eb6`;
backend behavior verified on Codex 0.160.1.

## Follow-up observed during testing

The existing `/compact` command can answer on compaction-item completion
shortly before Codex releases the underlying turn. The new integration
fixture waits for the separate runtime turn completion before issuing
the next probe. Changing that command's completion semantics is outside
this package.