x402 Definition.

An open payment protocol launched by Coinbase in May 2025 that revives HTTP status code 402 Payment Required to build payments directly into web requests. It enables per-request micropayments settled in stablecoins (USDC), with no account and no redirect, which makes it a rail of choice for AI agents and pay-per-use APIs (including pay-per-crawl). It is one of the building blocks of a web where machines pay machines.

See also

→ Stablecoin→ Pay-per-crawl→ AI payment agent

Where this term appears

GuideAgentic payments around the worldGuideAgentic payments: the fundamentalsGuideStablecoins and machine-to-machine settlementGuideAgentic commerce: scenarios for 2030CoursePreparing for agentic paymentsCourseAgentic payments: the fundamentalsCourseAgentic payments: integration and readinessCourseStablecoins for payments

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