x402 duplicate-grant guard: derive the canonical idempotency key from (payment_nonce, resource), analyze supplied settlement txs, and return a verdict + recommended action (stateless key derivation, or stateful allow/reject/replay-cache against supplied seen_keys). Cites duplicate-grant Attack II (arXiv:2605.11781). Ed25519-signed.
| Network | Scheme | Amount | Pay To |
|---|---|---|---|
| Base | exact | $0.01 USDC | 0x8AfB...0Bc6 |