Reasoned resolution; no implementation was inspected and no tests were executed. For effects contained in one durable database transaction, uniqueness must be claimed before applying the mutation, and the completed receipt must commit with the effect. This gives two recovery states: neither persists after rollback, or both persist after commit. A concurrent duplicate can only wait or retry arbitration, then compare the request binding and replay the committed outcome. It must never infer permission to execute from an absent stale read. No separately committed pending record is needed in this model. Retention bounds the guarantee unless expired operations are rejected. For effects in another system, a transactional outbox alone does not close the acknowledgement-loss gap; the receiving effect needs atomic deduplication or equivalent cooperation.

Reasoned schema-rollout extension, without executed tests: keep operation-key uniqueness independent of contract and fingerprint versions. An immutable receipt binds the declared request contract, canonicalization and hash version, digest, and versioned saved outcome. Interpret each retry using its historical contract and defaults. Cross-version replay requires a checked semantic conversion into the receipt's representation before hashing; it cannot silently drop a new field or use current defaults for an old client. This check must work in both directions: an old implicit default conflicts with a newer explicit nondefault, regardless of which request wins concurrent key arbitration. Unsupported readers must route or reject without mutation, so compatible readers precede new writers. Replay adapters transform only the saved outcome. Retain compatibility logic with receipts. If a historical fingerprint erased effect-relevant distinctions, version labels cannot restore proof of equivalence; retained trustworthy request evidence or rejection is necessary.

Independent reasoning review found no concrete error in the versioned replay argument. Two implementation obligations merit explicit wording: an unversioned legacy request must resolve to its historical contract even after deployment defaults change, and a cross-version adapter is a partial conversion that rejects unrepresentable semantics before removing fields. A newer nondefault delivery choice must therefore be rejected before projection into an older representation with only an implicit default. Fingerprints must bind all operation-relevant inputs with a collision-resistant digest. These are deductive checks and clarifications, not executed-test evidence.