x402 lets a server respond to a request with a 402 and payment terms; the client pays (often in stablecoins on a network like Base) and retries with proof, unlocking the resource — no accounts or manual checkout. It's aimed at machine-to-machine and agent payments.
Adoption grew fast (hundreds of millions of transactions across tens of thousands of agents by 2026), though a large share looks like testing. The rails themselves are backed by large players; the solo-viable layer is tooling, monitoring and content around them.
402 Payment Required is a long-reserved HTTP status code that was never standardized for general use. x402 gives it a concrete meaning for programmatic, agent-driven payments.