What Is x402? x402 takes a forgotten part of the web — the old HTTP 402 “Payment Required” code — and gives it a second life. It’s Coinbase’s open-source protocol that lets an app or AI agent pay directly through a normal web request, no wallet pop-ups or session tokens needed. Think of it as a pay-per-request rail baked into the internet itself. The Four-Step Payment Flow Every x402 exchange feels like a normal API call, but there’s a payment handshake hiding underneath.