Glossary
What is x402?
x402 is an HTTP-native payment protocol, introduced by Coinbase in 2025, that revives the reserved HTTP status code 402 Payment Required. When an agent requests a paid resource, the server answers 402 with machine-readable payment instructions; the agent attaches signed payment proof (typically stablecoins) and retries — no account, card or human checkout involved.
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The web has carried a placeholder for machine payments since the 1990s: HTTP 402, reserved but never standardised. x402 fills it in. The flow is a plain HTTP round trip — request, 402 with price and payment details, signed retry, content — which makes it natural for agents paying per API call, per document or per inference, at prices too small for card rails.
Governance has moved toward neutrality: an x402 Foundation was launched with Cloudflare, and the standard sits under Linux Foundation governance with backing from payment and cloud majors (as of August 2026). Volume has been dominated by machine-to-machine micropayments on chains such as Base and Solana. x402 also plugs into AP2 as its crypto payment extension.
For B2B software evaluation — AgentDoor's territory — x402 matters as infrastructure: it is how a buyer's agent could pay per-query for a paid data room, a benchmark report or a vendor's metered API during evaluation, without procurement raising a purchase order for every 10-yen request.
Frequently asked questions
- Does x402 require cryptocurrency?
- In practice today, yes — settlement runs on stablecoins over public chains, which is what makes instant, account-less machine payments feasible. The protocol itself is a payment negotiation layer over HTTP, and its instructions describe where and how to pay.
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