Pay for a metered API call using the HTTP 402 / x402 payment pattern

Steps

1. Make an initial HTTP request to the resource endpoint; the server responds with 402 Payment Required and a payment descriptor in the response body or a WWW-Authenticate / X-Payment header describing what is owed: amount, currency, payment destination, and a nonce or invoice reference.
2. Parse the payment descriptor; validate that the requested amount is within your agent's authorized spend policy before proceeding — treat a 402 from an unknown endpoint as untrusted until validated.
3. Construct a payment using a method the server accepts (commonly a blockchain micropayment, a signed payment channel update, or a pre-authorized PSP token — the descriptor specifies the mechanism); sign or authorize the payment proof.
4. Retry the original request with the payment proof attached (typically in an X-Payment or Authorization header, or as a query parameter per the server's specification); include the original nonce to bind the payment to this specific request.
5. If the server returns 200 with the resource, record the payment proof and response in your audit log; if it returns 402 again (e.g., proof rejected or nonce expired), do not retry more than once without human review.
6. Track cumulative spend per endpoint across agent sessions to enforce budget caps; a 402-enabled API can drain a budget if the agent calls it in a tight loop.