Status: IN
Idempotency keys extend forward-only semantics to the external API boundary: once a client-provided key maps to a result, that result is permanent and irrevocable — the same pattern that internal state ratchets apply to stream windows and read cursors, but applied to client-facing retry safety across hotel reservations, payments, and ad click processing.
Idempotency is forward-only applied to external client retries
Depends on (SL): idempotency-keys-ignore-payload-content, forward-only-extends-to-failure-handling