Guide · Pay from Arbitrum

Pay from Arbitrum

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

The path

1. Hold USDC on Arbitrum

USDC on Arbitrum One at the buyer's own address. no move: the door settles exact on Arbitrum.

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 eip155:42161 (Arbitrum, 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: 'eip155:42161', 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 Arbitrum One at an address the buyer signs for (no ETH)

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 eip155:42161 (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.