x402: How AI Agents Pay for API Calls with Crypto Micropayments

X402 is an HTTP payment protocol that lets AI agents pay for individual API calls without ever creating an account, entering a credit card, or requesting an API key. When an agent requests a paid endpoint, the server returns 402 Payment Required with a JSON envelope describing the facilitator, token, amount, and network. The agent signs a stablecoin payment with its wallet, retries the same request with an X-PAYMENT header, and gets the response in a single HTTP round trip.