A Durable Object alarm handler that throws is retried up to six times, then dropped

If the alarm() method throws or rejects, the runtime retries it up to six times with exponential backoff between attempts. After the final failure the alarm is discarded and nothing else happens; there is no dead letter path and no notification. Inside the handler ctx.retryCount reports which attempt is running, which is the hook for giving up gracefully or logging differently on the last try.

Because retries are automatic, an alarm handler performing a non-idempotent action such as charging a card or sending an email must record that it did so in storage and check that record on entry. The retry re-runs the whole handler from the top, so the idempotency check belongs at the very start rather than beside the side effect it guards.