x402
also: HTTP 402 · x402 protocol
A payment protocol that revives HTTP’s 402 “Payment Required” status so agents and APIs can settle micropayments inline over the web.
x402 puts the long-dormant HTTP 402 Payment Required status to work as a handshake: a server answers a request with 402 plus payment details, the client — often an AI agent — pays (typically a stablecoin transfer) and retries with proof, all inside the normal request flow. It targets machine-to-machine commerce, where an agent must pay per call without a human clicking checkout.