ADR 0166: Parent-judged subagent lifetime

• Status: Accepted for implementation
• Date: 2026-09-06
• Deciders: PI-Desktop core
• Related: D328, ADR 0089, ADR 0119, ADR 0129, issue discussion on GitHub #44 follow-on (parent closing kills unfinished delegates)

Context

ADR 0089 made Task non-blocking and told the parent to TaskWait or TaskStop before ending a turn. The safety net was to abort leftover delegates on parent agent_end. ADR 0119 / ADR 0129 then armed a 300-second idle watchdog and a 6-hour duration cap.

That combination failed in practice: the parent cannot see the delegate's live work, TaskWait returns after at most 900 seconds, the model treats that as permission to finish, and agent_end kills the still-running delegate. Long compiles, tests, and audits died because the parent closed, not because the work was done.

Prompting the model to wait again does not fix this. Waiting is an event loop; models are not reliable event loops.

Decision

1. Do not time-kill a delegate. Idle and duration watchdogs are not armed. idle-timeout / max-duration still parse so existing documents load, but they have no effect. Explicit maxTurns remains a definition backstop. Concurrency stays capped at 10.
2. Do not abort on parent idle. agent_end / turn_end while delegates are running are swallowed. The durable turn stays open. User Stop, TaskStop, runtime dispose, and a parent fatal error (D352 / ADR 0189) abort a delegate.
3. Deliver reports when they finish. After the parent loop idles with running delegates, the runtime waits for them and prompts the parent with the joined reports (not shown as a user bubble). The parent then judges: integrate, start more work, or TaskStop.
4. Give the parent a heartbeat, not the transcript. TaskList and a timed-out TaskWait include agent, status, elapsed seconds, turns, tool calls, and last tool name. Process rows stay out of the parent context.

Consequences

• A long delegate can outlive the parent's last tool call. The user still sees a live turn (Stop remains available) until the reports are delivered and the parent actually finishes.
• The parent no longer has to poll correctly for the job to survive.
• A looping delegate without maxTurns can run until the user Stops it.
• ADR 0119 / ADR 0129 watchdog policy is withdrawn for killing. Their "silence vs slowness" analysis remains historical.

Alternatives rejected

• Progress file the parent Reads (Claude Code). Puts live transcript into the parent context and requires the model to remember to poll.
• Keep abort-on-end and only raise TaskWait. The model still closes.
• Survive the turn with no auto-resume. The user has to send 继续.