x402 lets a server respond to a request with payment terms (an HTTP 402); the client signs a payment locally (commonly USDC 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.
The v2 spec, launched in early 2026, moved payment data into HTTP headers (PAYMENT-REQUIRED, PAYMENT-SIGNATURE, PAYMENT-RESPONSE), identifies networks with facilitator-pluggable CAIP-2 IDs (e.g. eip155:8453 for Base, plus Solana), and added discovery and wallet-based identity. Governance moved to the Linux Foundation's x402 Foundation (operational July 2026), whose 40 members include Coinbase, Cloudflare, Google, Stripe, Visa, Mastercard and AWS. Coinbase reported 100M+ transactions through the protocol's first six months.
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.
Payment data moved into HTTP headers, networks are identified by CAIP-2 IDs (EVM and non-EVM), facilitators and payment schemes became pluggable, and the protocol added API discovery and wallet-based identity.