Submitting a job and having it build and run. Measured every ten minutes by a tiny REAL job through the same pipeline as a customer's - validation, build, registry push, fairness queue, worker pull, run - and hourly by a job whose build runs a quarter of an hour, which is how a build that outlives its own push credentials is caught here instead of in your job. Red by default on unknown failure modes; a sold-out fleet stays green, since a full queue is healthy rather than broken.