Says the run STARTED. Does not clear the dead-man clock: a routine that starts and hangs still becomes a miss. The next success records the duration. start is not proof of life: it does not move last_event_at and does not clear the miss counter, so a routine that starts and hangs still becomes a miss at the right time. The next success closes the run and records last_duration_ms — pass ?rid= on both ends when runs can overlap, or the duration is measured against the wrong start. With max_duration_sec set on the monitor, a run left open past that ceiling alerts with reason: "too_long" on the next 5-minute check. Works by GET or POST.