Immediately cancelling after prompt is sent hangs agent forever #442
Description
This is in agent-shell, if I:
- Sent prompt
- Immediately cancel with
C-c C-c
Then it hangs forever with the following message:
Session ba806dac-1f1f-46ae-9968-13e7e2bd7160: consuming background task result
According to Claude, this is the root cause:
In `acp-agent.ts`, the `prompt()` method uses a `promptReplayed` flag to distinguish results belonging to the current prompt from results of a previous background task. The flag is set to `true` when the CLI replays the user message (matched by UUID) back through the generator.
When cancel arrives before the user message replay, this is the sequence:
1. `prompt()` enters the `while(true)` loop with `promptReplayed = false`
2. `cancel()` sets `session.cancelled = true` and calls `query.interrupt()`
3. The CLI emits a `"result"` message for the interrupted prompt
4. The `"result"` handler checks `!promptReplayed` first — it's still `false`
5. The result is consumed as a background task result (`break` out of switch, continue loop)
6. The `session.cancelled` check on the next line is never reached
7. `session.query.next()` blocks forever — no more messages will arrive
Relevant code:
case "result": {
// ... usage accounting ...
if (!promptReplayed) {
this.logger.log(`Session ${id}: consuming background task result`);
break; // <-- continues the while loop, skipping the cancelled check below
}
if (session.cancelled) {
return { stopReason: "cancelled" }; // <-- unreachable in this scenario
}
// ...
}Possibly relevant PR:
#423