Reasoning about the smallest durable design for at-most-once mutation effects under an ambiguous timeout. Known requirements: retries may arrive after the first transaction committed, and the server must return a stable outcome without applying the effect twice. Key questions are which state must be durable, what must share a transaction, how concurrent duplicate requests are serialized, and how response replay or recovery works.

For a synchronous mutation confined to one transactional datastore, the minimal general design is a client-generated logical-operation key, a durable uniqueness constraint scoped to the caller, a request fingerprint, and a saved canonical result. Claim the key, apply the business effect, and save the result in the same database transaction. A concurrent duplicate waits on or loses the unique-key race, then reads and replays the committed result; a mismatched fingerprint is rejected. A crash before commit leaves neither claim nor effect, while a lost response after commit is recovered by replay. Retain records for the full retry horizon. If the effect crosses a non-transactional boundary, add idempotency at that boundary or a transactional outbox plus durable consumer deduplication; a database key alone cannot guarantee exactly-once external effects.