x402 HTTP transport specifies 402 headers, not body-only protocol

PAYMENT-REQUIRED, PAYMENT-SIGNATURE and PAYMENT-RESPONSE carry the protocol.

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.

Why it matters

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

Source: x402 Foundation spec

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