## Summary
Expose Codex's existing child-turn interruption through standard ACP
`session/cancel`, without opening other child mutation controls.
- Advertise `capabilities.cancel: {}` only for owned, observable
standard children whose current turn can be identified. Revoke with
`cancel: null` when the work ends or becomes unobservable. Idle or
history-only children get no grant.
- Accept the opaque ACP child ID, never a raw provider-ID bypass.
Interrupt that child's current turn and observed owned descendant work,
not its parent or siblings. A later assignment retains the same child
identity and can receive a fresh grant.
- Withdraw affected permission/elicitation requests locally and reject
accepted late replies. Deduplicate in-flight interrupts and permit retry
after provider failure.
- Use receipt-time completion/observability facts and the latest
provider turn ID even while rendering is backpressured. A successful
interrupt acknowledgement does not fabricate a cancelled outcome; actual
provider completion supplies it.
- Suspend existing grants at parent setup admission. Only the latest
successful, non-cancelled setup response can restore them, after its
actual wire write (including the full JSON-RPC batch). Failed attempts
cannot release current child controls.
Standard v1 opt-in, v2 defaults, and AIR's existing contract are
unchanged. Prompt/configuration/load/close/private-extension mutations
remain forbidden for standard children. Copied history cannot acquire
the original runtime's controls. Dependencies and generated app-server
types are unchanged.
## Review scope
19 files, +602/-20 lines, mostly tests/snapshots. Runtime work is
concentrated in the standard child router, the cancel dispatch, and the
successful setup-response boundary. Eight existing standard-child
snapshots gain the intended cancel grant/revocation; AIR and ordinary
parent-session snapshots are unchanged.
## Validation
- `npm run typecheck`
- `npm test`: **2,095 passed**, 40 skipped; final run did not update
snapshots. Includes **25 new v1/v2 wire regressions** (one v2-only batch
case skipped on v1).
- `npm run build`
- `npm run test:integration`: **17 passed** against real Codex 0.164.0
with a local mock Responses API. New v1/v2 cases receive a current
cancel grant after reattachment, send `session/cancel` to the child,
observe Codex's actual cancelled ending, and continue prompting the
parent independently.
- `npm run bundle:all`: all six platform targets compiled.
- `npm run codex-test -- -p 'Reply with exactly child-cancel-ok. Do not
read files or use tools.' -c <isolated-scratch-directory> -o summary`:
live-model smoke completed with `end_turn`.
- `git diff --check`
Wire coverage includes sibling/ancestor isolation, nested descendant
cascade, real-ending confirmation and later work, request withdrawal and
late replies, in-flight deduplication, raw/foreign/stale IDs, unload
under backpressure, newest provider turn targeting, provider-error
retry, loaded foreign fork history, delayed replay responses, failures
before/after provider setup, and whole-batch freshness.
The full live-model E2E suite was not run. Cross-platform binaries were
compiled, not executed on every target OS.
## Protocol / provider evidence
- ACP specification revision `36b3fcc14d1834d6cc2a5ea3f317a49f198c2f79`:
the [subagent RFD cancellation and ownership
rules](https://github.com/agentclientprotocol/agent-client-protocol/blob/36b3fcc14d1834d6cc2a5ea3f317a49f198c2f79/docs/rfds/subagents.mdx#L859-L930)
define the object-valued cancel capability, child-directed cancellation,
descendant policy, actual completion evidence, and pending requests. The
[replay freshness
boundary](https://github.com/agentclientprotocol/agent-client-protocol/blob/36b3fcc14d1834d6cc2a5ea3f317a49f198c2f79/docs/rfds/subagents.mdx#L763-L805)
forbids historical grants and invalidates failed attempts.
- SDK reference revision `c7e07388feed7a530f8211d0dae4951cb6d26d09`,
separately checked for capability/notification shapes. Installed SDK
**2.0.2** decodes the real v1/v2 test wire. No protocol/schema changes
or new RPC methods are needed.
- Codex's existing `turn/interrupt` method is exercised directly by the
real-app-server child cancellation cases, not inferred from schema
alone.
This follows the completed native-subagent extraction; it does not
reopen or merge the superseded combined #655.
