Glossary
x402
An open protocol revived by Coinbase in 2025 that uses the long-reserved HTTP 402 "Payment Required" status to let agents and APIs settle micropayments, including in stablecoins.
A revival of HTTP’s reserved 402 status code as a real-world payment flow. When an API returns 402 Payment Required, an accompanying header describes the price and accepted payment rails (USDC on Base or Solana, Lightning, etc.). A client agent reads the header, signs and sends the payment, retries the request with proof, and gets the resource.
The protocol matters for agenticagentsAn informal descriptor for AI systems that pursue multi-step goals via tool use, planning, and self-correction, rather than single-turn question-answering. Open full entry commerce: agents can pay for API calls, content, or compute without prearranged accounts. Coinbase’s 2025 launch focused on stablecoin settlement; later work added support for traditional rails and for Lightning-style off-chain channels.
Related: L402 (the Lightning-native predecessor from Lightning Labs)
and Google’s AP2 (agenticagentsAn informal descriptor for AI systems that pursue multi-step goals via tool use, planning, and self-correction, rather than single-turn question-answering.
Open full entry payment protocol). All three address the
same problem from different starting points; whether they converge or
fragment is still open.