Quickstart for Sellers

This guide walks you through integrating with x402 to enable payments for your API or service. By the end, your API will be able to charge buyers and AI agents for access.

Prerequisites

Before you begin, ensure you have:

• A crypto wallet to receive funds (any EVM-compatible wallet, e.g., CDP Wallet)
• A Coinbase Developer Platform (CDP) account and API Keys
• Node.js and npm installed
• An existing API or server

We have pre-configured examples available in our repo, including examples for Express, Next.js, and Hono. We also have an advanced example that shows how to use the x402 SDKs to build a more complex payment flow.

1. Install Dependencies

Express

Install the x402 Express middleware package.

npm install x402-express
npm install @coinbase/x402 // for the mainnet facilitator

2. Add Payment Middleware

Integrate the payment middleware into your application. You will need to provide:

• The Facilitator URL or facilitator object. For testing, use https://x402.org/facilitator which works on Base Sepolia. For mainnet, you can use the facilitator from @coinbase/x402.
• The routes you want to protect.
• Your receiving wallet address.

Express

Testnet

Full example in the repo here.

import express from "express";
import { paymentMiddleware, Network } from "x402-express";

const app = express();

app.use(paymentMiddleware(
"0xYourAddress", // your receiving wallet address 
{  // Route configurations for protected endpoints
    "GET /weather": {
        // USDC amount in dollars
        price: "$0.001",
        network: "base-sepolia",
    },
    },
{
    url: "https://x402.org/facilitator", // Facilitator URL for Base Sepolia testnet. 
}
));

// Implement your route
app.get("/weather", (req, res) => {
res.send({
    report: {
    weather: "sunny",
    temperature: 70,
    },
});
});

app.listen(4021, () => {
console.log(`Server listening at http://localhost:4021`);
});

This is the interface for the payment middleware config:

interface PaymentMiddlewareConfig {
description?: string;               // Description of the payment
mimeType?: string;                  // MIME type of the resource
maxTimeoutSeconds?: number;         // Maximum time for payment (default: 60)
outputSchema?: Record; // JSON schema for the response
customPaywallHtml?: string;         // Custom HTML for the paywall
resource?: string;                  // Resource URL (defaults to request URL)
}

When a request is made to this route without payment, your server will respond with the HTTP 402 Payment Required code and payment instructions.

3. Test Your Integration

To verify:

1. Make a request to your endpoint (e.g., curl http://localhost:3000/your-endpoint).
2. The server responds with a 402 Payment Required, including payment instructions in the body.
3. Complete the payment using a compatible client, wallet, or automated agent. This typically involves signing a payment payload, which is handled by the client SDK detailed in the Quickstart for Buyers.
4. Retry the request, this time including the X-PAYMENT header containing the cryptographic proof of payment (payment payload).
5. The server verifies the payment via the facilitator and, if valid, returns your actual API response (e.g., { "data": "Your paid API response." }).

4. Error Handling

• If you get an error stating Cannot find module 'x402-hono/express' or its corresponding type declarations., add the tsconfig.json from the Hono example to your project.
• npm install the dependencies in each example

Next Steps

• Looking for something more advanced? Check out the Advanced Example
• Get started as a buyer

For questions or support, join our Discord.

Summary

This quickstart covered:

• Installing the x402 SDK and relevant middleware
• Adding payment middleware to your API and configuring it
• Testing your integration

Your API is now ready to accept crypto payments through x402.