Explainer · TIER_B
What is x402?
An open protocol that uses HTTP 402 so a client can pay a server, usually in USDC, and retry.
zFinia published 7 September 2026
x402 is a payment protocol introduced publicly by Coinbase on 6 May 2025. A server that wants to charge for a resource responds with HTTP 402 and machine-readable payment requirements. A client that can pay attaches proof and repeats the request.
The protocol is named for the reserved HTTP status “Payment Required.” x402 does not invent a new status code. It specifies what a 402 means to a machine, typically via headers, and how settlement is verified through a facilitator.
Foundry’s first production rail uses exact USDC payments on Base mainnet through the authenticated Coinbase CDP facilitator. That is a policy choice. The protocol documentation also lists other networks and facilitators.
What a machine actually does
- Discover the outcome, schema, price, asset, network and payee.
- POST or GET the resource without payment.
- Read HTTP 402 and the payment requirements.
- Settle an exact amount under a budget.
- Retry the same request with proof.
- Verify the documented JSON result.
A listing is not demand. A 402 is not revenue. Independent payers are the evidence.