Coordinator tasks bypass _run_attempt, so their _claim placeholder in _running was never cleared: each completed coordinator permanently reduced the worker process effective capacity until it could claim nothing, while heartbeat stayed fresh and the capacity gate returned silently. This caused the 2026-10-28 40-person stall (autoscale-1 had one leaked slot, then claimed coordinator A and filled up) and the old 18-person C-6 never-claim (autoscale-1 leaked both A and B). Fix: wrap the run_one coordinator branch in try/finally _running_clear, so normal and exception paths both release. Also log capacity-gate refusals (worker_claim_refused_capacity) throttled to once per 30s; throttling applies to the log only, refusal always returns False. Tests: coordinator completion releases slot, next claim succeeds, capacity-log throttle; both leak tests fail on pre-fix code.
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