How does a 402 payment work?

The x402 HTTP transport spec states that servers signal payment with HTTP 402 and a PAYMENT-REQUIRED header, clients send PAYMENT-SIGNATURE, and servers return PAYMENT-RESPONSE. Response bodies are an implementation concern; protocol information lives in headers.

• Call the resource with a documented JSON body.
• Receive HTTP 402 if unpaid.
• Pay the exact advertised amount on the advertised rail.
• Retry the same request with proof.
• Verify the documented result schema.

Buyer-conformance bugs often come from stuffing metadata into the wrong layer. Header-valid 402 beats keyword-stuffed JSON.