What is x402?

x402 is an open protocol for charging for an HTTP request with a stablecoin payment. The server answers an unpaid request with HTTP 402 and a signed challenge that quotes a price; the caller pays in USDC and retries, and the request succeeds.

x402 turns the long-dormant HTTP 402 “Payment Required” status code into a working payment handshake. It is a wire format, not a SaaS product: a signed challenge that names a price, a payment, and a retry carrying proof of that payment.

Because it is a protocol, any client and any server that speak it interoperate — which is what makes it a natural fit for autonomous agents that need to pay for an API without a human or a stored card.