fix(background-agent): decrement spawn budget on task completion, cancellation, error, and interrupt#2725
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code-yeongyu merged 2 commits intocode-yeongyu:devfrom Mar 25, 2026
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…cellation, error, and interrupt rootDescendantCounts was incremented on every spawn but never decremented when tasks reached terminal states (completed, cancelled, error, interrupt, stale-pruned). This made maxDescendants=50 a session-lifetime quota instead of its intended semantics as a concurrent-active agent cap. Fix: add unregisterRootDescendant() in five terminal-state handlers: - tryCompleteTask(): task completes successfully - cancelTask(): running task cancelled (wasRunning guard prevents double-decrement for pending tasks already handled by rollbackPreStartDescendantReservation) - session.error handler: task errors - promptAsync catch (startTask): task interrupted on launch - promptAsync catch (resume): task interrupted on resume - onTaskPruned callback: stale task pruned (wasPending guard) Fixes: code-yeongyu#2700
…nt on task completion Tests prove rootDescendantCounts is never decremented on task completion, cancellation, or error — making maxDescendants a lifetime quota instead of a concurrent-active cap. All 4 tests fail (RED phase) before the fix. Refs: code-yeongyu#2700
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Confidence score: 5/5
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Auto-approved: Fixes a significant resource leak in the subagent spawn budget. Includes defensive guards against double-decrementing for pending tasks and adds comprehensive regression tests.
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@code-yeongyu please merge/add this PR |
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Summary
Fixes #2700 — subagents stop spawning after 50 total per session.
Root Cause
rootDescendantCountsinBackgroundManagerincrements on every spawn but only decrements on pre-start rollback or root session deletion. Task completion, cancellation, error, and interrupt never decrement the counter. This makesbackground_task.maxDescendants(default: 50) a session-lifetime quota instead of its intended semantics as a concurrent-active agent cap.After 50 total spawns across a session (completed + cancelled + errored), no new subagents can be created — even if all 50 have finished.
Fix
Add
this.unregisterRootDescendant(task.rootSessionID)in five terminal-state handlers:tryCompleteTask()— task completes successfullycancelTask()— running task cancelled (withwasRunningguard: pending tasks already decrement viarollbackPreStartDescendantReservationto prevent double-decrement)session.errorhandler — task errorspromptAsynccatch instartTask— task interrupted on launchpromptAsynccatch inresume— task interrupted on resumeonTaskPrunedcallback — stale task pruned (withwasPendingguard)Tests
Four regression tests added to
manager.test.ts:Verification
Before fix: spawn 51st agent after 1st completes → blocked with maxDescendants=50 error
After fix: spawn 51st agent after 1st completes → succeeds
Full test suite: 121 pass, 3 fail (3 pre-existing failures in unrelated
session.error > retry pathtests, present before this change)Summary by cubic
Fixes spawn budget leaks in
BackgroundManagerby decrementing root descendant counts when tasks end. Restoresbackground_task.maxDescendantsto act as a concurrent cap so new subagents can spawn after others finish.Written for commit 031503b. Summary will update on new commits.