Idempotency & Duplicate Prevention

When this skill applies

Use this skill when:

• Implementing any PPP endpoint handler that processes payments, cancellations, captures, or refunds
• Ensuring repeated Gateway calls with the same identifiers produce identical results without re-processing
• Building a payment state machine to prevent invalid transitions (e.g., capturing a cancelled payment)
• Handling the Gateway's 7-day retry window for undefined status payments

Do not use this skill for:

• PPP endpoint response shapes and HTTP methods — use payment-provider-protocol
• Async callback URL notification logic — use payment-async-flow
• PCI compliance and Secure Proxy — use payment-pci-security

Decision rules