Client                    HoudiniSwap API              Facilitator
  |                            |                           |
  |  1. GET /v2/tokens         |                           |
  |--------------------------->|                           |
  |  2. 402 Payment Required   |                           |
  |    (PAYMENT-REQUIRED header)|                          |
  |<---------------------------|                           |
  |                            |                           |
  |  3. Sign USDC authorization|                           |
  |  (EIP-3009, gasless)       |                           |
  |                            |                           |
  |  4. GET /v2/tokens         |                           |
  |    (PAYMENT-SIGNATURE hdr) |                           |
  |--------------------------->|                           |
  |                            |  5. Verify & settle       |
  |                            |-------------------------->|
  |                            |  6. Settlement confirmed  |
  |                            |<--------------------------|
  |  7. 200 OK (data)          |                           |
  |<---------------------------|                           |

npm install @x402/fetch @x402/evm viem

import { x402Client, x402HTTPClient } from "@x402/fetch";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

// Create signer from your wallet's private key
const signer = privateKeyToAccount(process.env.EVM_PRIVATE_KEY as `0x${string}`);

// Create x402 client and register EVM payment scheme
const client = new x402Client();
registerExactEvmScheme(client, { signer });
const httpClient = new x402HTTPClient(client);

const BASE_URL = "https://api-partner.houdiniswap.com";

const x402Fetch = async (url: string, options: RequestInit = {}) => {
  // Initial request
  const res = await fetch(url, options);
  if (res.status !== 402) return res;

  // Parse payment requirements from 402 response
  const paymentRequired = httpClient.getPaymentRequiredResponse(
    (name) => res.headers.get(name),
    undefined,
  );

  // Sign the USDC payment (gasless EIP-3009)
  const payload = await client.createPaymentPayload(paymentRequired);
  const paymentHeaders = httpClient.encodePaymentSignatureHeader(payload);

  // Retry with payment proof
  return fetch(url, {
    ...options,
    headers: { ...options.headers, ...paymentHeaders },
  });
};

// Get tokens — automatically handles 402 → pay → retry
const response = await x402Fetch(`${BASE_URL}/v2/tokens?term=BTC`);
const data = await response.json();
console.log(data.tokens);

const paymentResponse = response.headers.get("payment-response");
if (paymentResponse) {
  const receipt = JSON.parse(Buffer.from(paymentResponse, "base64").toString());
  console.log(receipt);
  // {
  //   "success": true,
  //   "transaction": "0xabc...def",
  //   "network": "eip155:8453",
  //   "payer": "0x..."
  // }
}

import { x402Client, x402HTTPClient } from "@x402/fetch";
import { registerExactEvmScheme } from "@x402/evm/exact/client";
import { privateKeyToAccount } from "viem/accounts";

const signer = privateKeyToAccount(process.env.EVM_PRIVATE_KEY as `0x${string}`);
const client = new x402Client();
registerExactEvmScheme(client, { signer });
const httpClient = new x402HTTPClient(client);

const BASE = "https://api-partner.houdiniswap.com/v2";

const x402Fetch = async (url: string, options: RequestInit = {}) => {
  const res = await fetch(url, options);
  if (res.status !== 402) return res;
  const pr = httpClient.getPaymentRequiredResponse((n) => res.headers.get(n), undefined);
  const payload = await client.createPaymentPayload(pr);
  const headers = httpClient.encodePaymentSignatureHeader(payload);
  return fetch(url, { ...options, headers: { ...options.headers, ...headers } });
};

// Step 1: Look up tokens ($0.0001 each)
const btcRes = await x402Fetch(`${BASE}/tokens?term=BTC&hasCex=true`);
const btcData = await btcRes.json();
const btcToken = btcData.tokens.find((t: any) => t.chain === "bitcoin");

const ethRes = await x402Fetch(`${BASE}/tokens?term=ETH&hasCex=true`);
const ethData = await ethRes.json();
const ethToken = ethData.tokens.find((t: any) => t.chain === "ethereum");

// Step 2: Get a quote ($0.001)
const quoteRes = await x402Fetch(
  `${BASE}/quotes?from=${btcToken.id}&to=${ethToken.id}&amount=0.01`
);
const quoteData = await quoteRes.json();
const quote = quoteData.quotes[0];
console.log(`Quote: ${quote.amountOut} ETH`);

// Step 3: Create exchange ($0.01)
const exchangeRes = await x402Fetch(`${BASE}/exchanges`, {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({
    quoteId: quote.quoteId,
    addressTo: "0xYourEthAddress...",
  }),
});
const exchange = await exchangeRes.json();
console.log(`Order: ${exchange.houdiniId}, deposit to: ${exchange.depositAddress}`);

// Step 4: Poll status ($0.0001 each)
const checkStatus = async () => {
  const res = await x402Fetch(`${BASE}/orders/${exchange.houdiniId}`);
  return res.json();
};

let status = await checkStatus();
const DONE = [4, 5, 6, 7, 8]; // completed, expired, failed, refunded, deleted
while (!DONE.includes(status.status)) {
  await new Promise((r) => setTimeout(r, 30000)); // Wait 30s
  status = await checkStatus();
  console.log(`Status: ${status.status}`);
}

{
  "x402Version": 2,
  "error": "Payment required",
  "resource": {
    "url": "https://api-partner.houdiniswap.com/v2/tokens",
    "description": "HoudiniSwap API - GET /tokens",
    "mimeType": "application/json"
  },
  "accepts": [
    {
      "scheme": "exact",
      "network": "eip155:8453",
      "amount": "100",
      "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
      "payTo": "0x...",
      "maxTimeoutSeconds": 300,
      "extra": {
        "name": "USDC",
        "version": "2"
      }
    }
  ]
}