x402 client integration

The hosted Flare path was exercised with @x402/fetch and @x402/evm version 2.19.0.

Install

npm install @x402/fetch@2.19.0 @x402/evm@2.19.0 viem

Expected challenge

Fetch once without a signer and decode PAYMENT-REQUIRED. Require the exact tuple you intend to pay:

const endpoint = 'https://app.proofrails.com/v1/x402/premium/fx-lookup';
const first = await fetch(endpoint, {
  method: 'POST',
  headers: { 'content-type': 'application/json' },
  body: JSON.stringify({ base_ccy: 'USD', quote_ccy: 'FLR' }),
});

if (first.status !== 402) throw new Error(`Expected 402, got ${first.status}`);

const encoded = first.headers.get('PAYMENT-REQUIRED');
if (!encoded) throw new Error('Missing PAYMENT-REQUIRED');

const challenge = JSON.parse(Buffer.from(encoded, 'base64').toString('utf8'));
const option = challenge.accepts.find((item) =>
  item.network === 'eip155:14' &&
  item.asset.toLowerCase() === '0xe7cd86e13ac4309349f30b3435a9d337750fc82d'.toLowerCase()
);

if (!option) throw new Error('Expected Flare USD₮0 option is missing');
if (option.amount !== '1000') throw new Error('Unexpected amount');
if (option.payTo.toLowerCase() !== '0x0A617D605a1010a74B8dA756E48D75bCef110ef4'.toLowerCase()) {
  throw new Error('Unexpected recipient');
}
if (option.extra?.facilitator?.toLowerCase() !== '0xa78F15ee5a1Ff1D89F6AD782a5f9b81f7C2aA4aE'.toLowerCase()) {
  throw new Error('Unexpected facilitator');
}

Paid request

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

const account = privateKeyToAccount(process.env.PAYER_PRIVATE_KEY);
const paidFetch = wrapFetchWithPaymentFromConfig(fetch, {
  schemes: [
    { network: 'eip155:14', client: new ExactEvmScheme(account) },
  ],
});

const response = await paidFetch(endpoint, {
  method: 'POST',
  headers: { 'content-type': 'application/json' },
  body: JSON.stringify({ base_ccy: 'USD', quote_ccy: 'FLR' }),
});

if (!response.ok) throw new Error(`${response.status}: ${await response.text()}`);

const paymentHeader = response.headers.get('PAYMENT-RESPONSE');
if (!paymentHeader) throw new Error('Missing PAYMENT-RESPONSE');

const payment = decodePaymentResponseHeader(paymentHeader);
const result = await response.json();

console.log({ result, transaction: payment.transaction, receiptId: payment.receiptId });

Complete the evidence workflow

Store transaction, receiptId, receiptUrl and verifierUrl with the agent task. Poll receiptUrl until status becomes anchored, then download bundle_url and verify it.

A duplicate authorization nonce is rejected with HTTP 409 and nonce_already_used. Do not create a second payment merely because receipt processing or anchoring is still pending.