`Runtime` gets a fourth operation, `canceller()`: a handle that stops the
turn in flight from outside `run`. The ACP backend returns one; claude
returns `None`, because the harness stops a claude turn by signalling the
`claude` process, and that path is unchanged.
Both stops send the agent `session/cancel`:
- `Canceller::cancel()`, when asked from outside. The turn then ends
normally, reported with stop reason `cancelled` whatever reason the agent
gives. opencode 1.15.10, for one, answers a cancelled prompt with
`end_turn` (`acp/agent.ts` `prompt()` always returns `end_turn`).
- The idle watchdog, once no `session/update` has arrived for
`Config::idle_timeout`, the same field claude's watchdog reads. The turn
fails with `AcpError::IdleTimeout`.
An agent that has not answered the prompt 10s after `session/cancel` is
killed (`IdleKilled` / `CancelIgnored`), and the next turn respawns it.
The watchdog is also what ends a turn stuck on a provider HTTP 429.
opencode 1.15.10 retries a retryable provider error with no attempt limit
(`session/retry.ts` `policy`, `session/processor.ts` `Effect.retry`) and
forwards neither `session.status` nor `session.error` over ACP (its
`handleEvent` only handles `permission.asked` and `message.part.*`). So the
ACP client sees nothing at all until the provider recovers. The
`IdleTimeout` message says a silently retried provider error looks like
this.
Refs #4391
bcbeb8ac9c