Guide · Pay from Arc

Pay from Arc

A buyer on Arc pays for inference with the USDC it already holds on Arc. Each call is paid with x402, with no key, no gas and no account with us.

The path

1. Hold USDC on Arc

USDC on Arc at the buyer's own address. no move: the door settles exact on Arc (a buyer on Base may also cross with our CCTP router, never needed to pay).

2. Call, paid with x402

Without a key the call answers 402. The PAYMENT-REQUIRED header holds the terms: one exact entry per network the door settles, the amount, and the address that is paid. Pick the entry for eip155:5042 (Arc, USDC). You never type the payee: it is read from the challenge.

curl -i -X POST https://coord.taifoon.dev/v1/chat/completions \
  -H "content-type: application/json" \
  -d '{"model":"llama-3.3-70b","messages":[{"role":"user","content":"hello"}],"max_tokens":64}'

Any x402 v2 client pays it. In TypeScript, with the key of the address that holds the USDC:

import { wrapFetchWithPaymentFromConfig } from '@x402/fetch';
import { ExactEvmScheme } from '@x402/evm';
import { privateKeyToAccount } from 'viem/accounts';

const account = privateKeyToAccount(process.env.EVM_PRIVATE_KEY as `0x${string}`);
const pay = wrapFetchWithPaymentFromConfig(fetch, {
  schemes: [{ network: 'eip155:5042', client: new ExactEvmScheme(account) }],
});

const r = await pay('https://coord.taifoon.dev/v1/chat/completions', {
  method: 'POST',
  headers: { 'content-type': 'application/json' },
  body: JSON.stringify({"model":"llama-3.3-70b","messages":[{"role":"user","content":"hello"}],"max_tokens":64}),
});
console.log(await r.json()); // an OpenAI chat completion

The client signs one transfer authorization (EIP-3009). The facilitator settles it and pays the gas, so the address holds the dollar token only.

A key is free and not needed to pay with x402. A call sent with a key is paid from that key's balance; a call sent without one is paid with x402. Each model also has its own path, /v1/chat/completions/<model>. Prices lists every priced wall.

What it takes

• USDC on Arc at an address the buyer signs for

Nothing from Taifoon: no key, no registration, no account and no spec before the first paid call. The money lands per call in vault:taifoon:kms.

How this was checked

the door's 402 challenge offers exact on eip155:5042 (read 2026-10-22T18:04:25Z). Proven by a quote or a challenge: yes. Quotes and challenges only: nothing was deposited, signed or paid to check this page.