Exactly-once side effects

Tue, 08 Sep 2026 00:00:00 +0000

Part of my AI Security Engineering series, a local email-to-calendar agent I build, then attack, then harden. If you have not set up the lab yet, start with Set up your lab. The lessons are on GitHub, and the agent itself lives at t0ul/emaildrop.

When a retry is dangerous

The calendar chapter met this once already: re-ingest an email and the same event lands twice. There the cost was a duplicate on a screen. This post is the version where the cost is real, because the agents that matter do not only write calendars, they take actions, send a message, issue a refund, move money, and they retry those actions all the time. A request times out and the caller tries again. A process crashes mid-task and resumes. A client never saw the response and resends. Retrying is correct behavior and usually free, right up until the thing being retried has a side effect, at which point once-per-attempt is a bug with a dollar figure.

Idempotency on t0ul

Exactly-once side effects

When a retry is dangerous