ArcFX speaks x402 — the HTTP 402 payment standard Circle
co-founded. A client asks for an invoice, gets 402 Payment Required,
signs a gasless authorization, and asks again. No account, no API key, no card.
Every step of the exchange is shown below exactly as it happens on the wire.
This runs against Arc Testnet with test USDC. Payment uses
EIP-3009transferWithAuthorization,
so the payer signs an authorization instead of sending a transaction — the signature moves the money,
and it costs the signer no gas.
1 · What the server accepts
Discovery
Before paying anything, a client asks what this server takes. This is a live call to
/v1/x402/discovery.
GET /v1/x402/discovery
—
2 · Pay an invoice
Invoice to settle
Any invoice number and recipient. The amount is what the agent will be asked to authorize.
Connect a wallet holding Arc Testnet USDC to sign the authorization.
Get test funds from faucet.circle.com.
Protocol trace
Each step is the real request and response, not a simulation.
1
Request the resource
Client asks for the invoice with no payment attached.
2
Server answers 402 Payment Required
Response carries the payment requirements the client must meet.
3
Agent signs an EIP-3009 authorization
A signature over the transfer — not a transaction. Costs no gas.
4
Retry with PAYMENT-SIGNATURE
Same request, now carrying the base64 payment payload.
5
Server verifies and settles
Signature, amount, expiry, recipient and nonce are all checked on-chain.
← 402 response body
—
signed authorization
—
← final response
—
3 · Do it from code
The same exchange with curl. The first call
returns the requirements; the second carries the signed payload.