What is x402?
x402 is an open HTTP payment protocol that reactivates the long-dormant 'HTTP 402 Payment Required' status code. Instead of forcing every API to build its own billing, subscription, and auth stack, x402 lets a server reply to any request with a 402 response that includes machine-readable payment requirements. The client - a human browser extension, a script, or an autonomous AI agent - reads those requirements, pays on-chain with USDC or another stablecoin, and retries the request with a proof-of-payment header. x402 was introduced and open-sourced by Coinbase in May 2025, with governance transitioning to the Linux Foundation-hosted x402 Foundation in April 2026.