Every API call an agent makes today requires an account, an API key, and a credit card on file. That breaks down when you have thousands of agents making millions of microtransactions per day. x402 uses HTTP's own 402 "Payment Required" status code, reserved and unused since the '90s, to let agents pay per-request with USDC stablecoins.
Stripe building their machine payments preview on top of it means merchants get the familiar PaymentIntents API with sales tax, refunds, and reporting built in.