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2414a6f98b
### Summary
Fix session_state_changed: idle handler to check session.cancelled,
ensuring the prompt response returns stopReason:
"cancelled" instead of "end_turn" when the session was interrupted.
### Problem
When a client sends session/cancel, the cancel() method sets
session.cancelled = true and calls query.interrupt(). The
SDK then transitions to idle state and emits a session_state_changed
event with state: "idle". However, the handler
for this event returns the prompt response immediately without checking
session.cancelled:
```
case "session_state_changed": {
if (message.state === "idle") {
return { stopReason, usage: sessionUsage(session) }; // stopReason defaults to "end_turn"
}
break;
}
```
Since stopReason is initialized as "end_turn" and may not have been
updated to "cancelled" by a preceding result
message (the result handler at line 601 does check session.cancelled),
the prompt response can incorrectly report
stopReason: "end_turn" when the turn was actually cancelled.
This causes clients (e.g. JetBrains AI Assistant) to treat the turn as
completed normally rather than cancelled,
leaving the frontend UI stuck in a "running" state.
### Fix
Check session.cancelled in the session_state_changed: idle handler,
mirroring the checks already present in the done
handler (line 464) and the result handler (line 601):
```
case "session_state_changed": {
if (message.state === "idle") {
if (session.cancelled) {
stopReason = "cancelled";
}
return { stopReason, usage: sessionUsage(session) };
}
break;
}
```
Per the ACP spec, upon receiving session/cancel the agent must respond
to the original session/prompt request with
StopReason::Cancelled. This fix ensures that obligation is met
regardless of which message type triggers the prompt
response.
### Test plan
- Start a session, send a prompt, then immediately cancel — verify the
prompt response returns stopReason: "cancelled"
and the client UI transitions out of "running" state
- Start a session, send a prompt, let it complete normally — verify
stopReason remains "end_turn" (no false
cancellation)
- Existing test suite passes (npm run test:run)
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