Idempotent means doing something twice has the same effect as doing it once. This is essential for APIs, agents, and any system where retries can happen. Here’s why, and how to get it right.
Retrying a read is free. Retrying a write can charge a card twice, send two emails, or book two rooms — and the model has no idea it happened. Retry safety is a property you build into the tool, not a flag you set on the loop. Here’s why at-least-once delivery is the default you’re actually running, how to derive a stable idempotency key from an agent’s intent, and a dedup wrapper that makes any write safe to retry.