run_all previously sized its pool from the ready snapshot at round start (min(capacity, ready_count)). If a round started with only 1 ready task (e.g. a just-claimed PM), only 1 thread ran; that thread entered a marker wait (up to 3600s) after its model turn, and children created/retried mid-round had no free thread to claim them. run_loop is blocked joining that round, so no new round starts; if the other worker had already scaled down, the queued child waited out the full wait timeout unclaimed. Fix: always start capacity threads. Idle threads exit immediately when no task is ready and nothing is active, and poll every 0.2s while something is active, so mid-round arrivals are claimed. Test: run_all with capacity=2, 1 initial task whose mocked run_one creates a child then polls for it; without the fix the PM poll times out in 5s (1 result), with the fix both tasks dispatch in the same round (0.2s).
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