TL;DR: x402 is an open standard that lets software, including AI agents, pay for APIs, data, and services directly over HTTP using stablecoins, with no accounts, API keys, or human checkout. It puts the web's unused 402 Payment Required status code to work as a native payment layer.
The web reserved a status code for payments in 1996 and then left it empty for thirty years. AI agents just gave it a job.
Every developer has met 404 Not Found. Most have fought 403 Forbidden. But sitting quietly between them in the HTTP spec is 402 Payment Required, marked "reserved for future use" since the earliest versions of the protocol. The people who designed the web assumed money would eventually move through it natively. Then credit cards, checkout forms, and subscriptions happened, and that future got postponed indefinitely.
So what changed? Software started buying things for itself.
The problem x402 was built to solve
