ADR 0189: Parent fatal error aborts leftover delegates

• Status: Accepted for implementation
• Date: 2026-09-08
• Deciders: PI-Desktop core
• Related: D352, D328, ADR 0166, ADR 0089, E2E-155

Context

D328 / ADR 0166 keep a durable turn open when the parent idles with running delegates, then resume the parent with their reports. That path is correct for a parent that stopped calling tools.

A terminal parent provider error is different. After a 429 budget is exhausted, Electron already finishes the durable turn as error and the renderer shows Continue. Leftover delegates — often on another model that is not rate-limited — kept isRunning true, so Continue was rejected as AGENT_BUSY (session already has an active turn). The main conversation looked finished while the sidecar was still busy.

Decision

1. Parent idle still does not abort delegates. D328 is unchanged for a parent that stops calling tools without a fatal error.
2. A terminal parent error does abort leftover delegates. Exhausted rate limits, other terminal provider/stream errors, overflow-recovery failure, mutation-budget termination, and rejected-prompt failures abort running delegates, skip the resume prompt, and emit agent_end.
3. The session becomes idle for Continue. getStatus().isRunning does not count leftover delegates after turnHadError. The failed TurnOutcomeCard stays visible; a later agent_end must not overwrite it with completed.
4. A later parent prompt does not inherit previous-turn delegates. Each prompt() / executeApprovedPlan() bumps a turn epoch, aborts leftover runs from earlier epochs, and only auto-resumes delegates started in the current turn.

No IPC, storage, or host-protocol change.

Consequences

• Continue after a 429 (or other terminal parent error) is accepted even if a Gemini (or other) subagent was still running.
• Work those leftover delegates had not yet reported is aborted rather than written into a dead parent turn.
• Parent idle with live delegates still shows a live turn until reports are delivered.

Alternatives rejected

• Keep leftover delegates running after parent error. The session stays busy and Continue is AGENT_BUSY.
• Detach leftover delegates from isRunning without aborting. A later Continue turn would race them on the workspace.