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Use Express when you want the shortest server-side path to 4Mica. The @4mica/x402 package ships Express middleware that handles HTTP 402 responses, guarantee verification, and settlement for protected routes.

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Prerequisites

  • Node.js 18 or later.
  • An Express app.
  • A seller wallet address that receives payments.
  • A buyer wallet with 4Mica collateral if you want to test the paid client.

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Install

npm install express @4mica/x402 @x402/fetch viem

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Create wallet

Create one wallet for the buyer client and one seller address for payTo. For wallet setup, see Wallet. Keep the buyer private key in an environment variable: 
PRIVATE_KEY=0xYourPrivateKey
PAY_TO=0xYourSellerAddress
Use the private key only for buyer-side signing. Use PAY_TO as the seller address advertised by protected routes.

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Deposit collateral

Deposit collateral for the buyer wallet before it calls the protected route. Use Base Sepolia (eip155:84532) while you test, then wait for the deposit to finalize. To learn more about collateral setup, see Deposits and Withdrawals.

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Protect a route

import express from "express";
import { paymentMiddlewareFromConfig } from "@4mica/x402/server/express";

const app = express();
app.use(express.json());

app.use(
  paymentMiddlewareFromConfig({
    "GET /premium-content": {
      accepts: {
        scheme: "4mica-credit",
        price: "$0.10",
        network: "eip155:84532",
        payTo: process.env.PAY_TO ?? "0xYourAddress",
      },
      description: "Access to premium content",
    },
  }),
);

app.get("/premium-content", (req, res) => {
  res.json({ message: "This is premium content behind a paywall" });
});

app.listen(3030, () => {
  console.log("Server running on http://localhost:3030");
});

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Support payment

import { wrapFetchWithPaymentFromConfig } from "@x402/fetch";
import { FourMicaEvmScheme } from "@4mica/x402/client";
import { privateKeyToAccount } from "viem/accounts";

const account = privateKeyToAccount(process.env.PRIVATE_KEY as `0x${string}`);
const scheme = await FourMicaEvmScheme.create(account);

const fetchWithPayment = wrapFetchWithPaymentFromConfig(fetch, {
  schemes: [{ network: "eip155:84532", client: scheme }],
});

const response = await fetchWithPayment("http://localhost:3030/premium-content");
const data = await response.json();

console.log(data);

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What happens

  1. The buyer requests /premium-content without payment.
  2. Express returns HTTP 402 with payment requirements.
  3. The wrapper creates a unique request ID, signs a guarantee, and retries with a payment header.
  4. The middleware verifies and settles the payment before your handler serves the response.