Pay an x402 API from Node.js

This is the complete client. It calls a paid endpoint, gets the 402 Payment Required challenge, signs a gasless USDC authorization from your wallet, retries, and returns the result — one round-trip, no API keys, no account signup. This exact pattern runs in production; you can test it against a live $0.001 endpoint below.

Install

npm install @x402/fetch @x402/evm viem

The client

// pay.mjs — node pay.mjs
import { createWalletClient, http } from "viem";
import { privateKeyToAccount } from "viem/accounts";
import { base } from "viem/chains";
import { wrapFetchWithPaymentFromConfig } from "@x402/fetch";
import { ExactEvmScheme } from "@x402/evm";

// Any Base wallet holding a little USDC. Never hardcode the key.
const account = privateKeyToAccount(process.env.PRIVATE_KEY);
const walletClient = createWalletClient({ account, chain: base, transport: http() });
const signer = {
  address: account.address,
  signTypedData: (msg) => walletClient.signTypedData({ account, ...msg }),
};

// fetch, but it pays 402s automatically.
const paidFetch = wrapFetchWithPaymentFromConfig(fetch, {
  schemes: [{ network: "eip155:8453", client: new ExactEvmScheme(signer) }],
});

// A live $0.001 endpoint: OFAC sanctions screen for any wallet.
const res = await paidFetch(
  "https://api.anchor-x402.com/v1/screen?address=0xd8dA6BF26964aF9D7eEd9e03E53415D37aA96045",
);
console.log(res.status, await res.json());

That’s the whole integration. paidFetch behaves exactly like fetch for free endpoints and transparently handles the payment dance for paid ones.

What actually happens

First request → the server replies 402 with a JSON challenge listing accepts[]: network (eip155:8453 = Base mainnet), asset (USDC), amount, and payTo.
Sign → the client signs an EIP-3009 transferWithAuthorization — an off-chain signature, so you pay no gas; the facilitator settles it on-chain.
Retry → the same request goes out again with a PAYMENT-SIGNATURE header. The server verifies, settles, and returns the result plus a payment-response header containing the settlement details.

Funding the wallet

The wallet only needs USDC on Base (no ETH — settlement is gasless for the buyer). For agents, Coinbase’s Agentic Wallet (npx awal@latest) is the fastest path; any EOA whose key you hold works the same.

Troubleshooting

402 again after paying, body error: "payment_invalid" — the authorization was rejected: insufficient USDC balance, expired, or already used. The body includes a retry_hint; sign a fresh authorization and retry (don’t resubmit the old one).
Client throws before ever paying — check the challenge’s accepts[].network. Current @x402/evm expects CAIP-2 (eip155:8453). Some older servers still emit the legacy label "base"; those need a v1 client or a server-side fix.
Want to inspect a challenge without paying? curl -i the endpoint — the full challenge JSON is in the body and the payment-required header (base64).

Try it against 16 live endpoints

Everything in the anchor-x402 catalog works with the client above — sanctions screening, wallet intel, tx decoding, token prices, on-chain anchoring, from $0.001/call. Machine-readable index: llms.txt · OpenAPI · selling instead? Accept x402 payments in Python →