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idempotency-enforcer

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Designs client-supplied idempotency keys, deduplication storage, and replay semantics so at-least-once delivery and client retries return the first result instead of double-charging or double-shipping. Use when someone asks "how do I stop duplicate charges on retry", "add an Idempotency-Key header to this endpoint", "the queue redelivered a job and we shipped twice", or before exposing any unsafe POST behind a retrying client or queue. Do NOT use for inbound third-party webhook handlers whose dedup is keyed on a provider event ID - use webhook-receiver-hardener instead and reference this skill only for the storage pattern; do NOT use for designing the client-side retry/backoff policy itself - use rate-limit-handler instead.

skills/idempotency-enforcer/SKILL.md

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Oct 13, 2026
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# Idempotency Enforcer

Make the second arrival of any non-reversible write a no-op that replays the first result. Without this, every retrying client, queue redelivery, and impatient double-click is a potential duplicate charge, duplicate shipment, or duplicate email - and the failure only shows up in production, under load, as a customer complaint.

## Operating procedure

Steps 1-3 are design decisions; 4-7 are the runtime protocol. Do them in order - the storage decision (step 3) depends on the scope decision (step 2), and the replay rules (step 6) are meaningless until intent is persisted atomically (step 5).

### Step 1: Gather inputs

Collect before designing. Label guesses as guesses.

- Which endpoints have non-reversible side effects (payments, orders, emails, inventory)? Only those get the machinery.
- Is the side effect in the same database as the API (one transaction possible) or an external call (a charge, a third-party API)?
- What is the client's maximum retry window - how long after the first attempt can a retry legitimately arrive? Default assumption: 24 hours.
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