Build an x402 Payment Server

Create an Express server that charges USDC for API access using the x402 payment protocol. Callers pay per-request in USDC on Base — no accounts, API keys, or subscriptions needed.

How It Works

x402 is an HTTP-native payment protocol. When a client hits a protected endpoint without paying, the server returns HTTP 402 with payment requirements. The client signs a USDC payment and retries with a payment header. The facilitator verifies and settles the payment, and the server returns the response.

Confirm wallet is initialized and authed

npx awal@latest status

If the wallet is not authenticated, refer to the authenticate-wallet skill.

Step 1: Get the Payment Address

Run this to get the wallet address that will receive payments:

npx awal@latest address

Use this address as the payTo value.

Step 2: Set Up the Project

mkdir x402-server && cd x402-server
npm init -y
npm install express x402-express

Create index.js:

const express = require("express");
const { paymentMiddleware } = require("x402-express");

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

const PAY_TO = "<address from step 1>";

// x402 payment middleware — protects routes below
const payment = paymentMiddleware(PAY_TO, {
  "GET /api/example": {
    price: "$0.01",
    network: "base",
    config: {
      description: "Description of what this endpoint returns",
    },
  },
});

// Protected endpoint
app.get("/api/example", payment, (req, res) => {
  res.json({ data: "This costs $0.01 per request" });
});

app.listen(3000, () => console.log("Server running on port 3000"));

Step 3: Run It

node index.js

Test with curl — you should get a 402 response with payment requirements:

curl -i http://localhost:3000/api/example

API Reference

paymentMiddleware(payTo, routes, facilitator?)

Creates Express middleware that enforces x402 payments.

Route Config

Each key in the routes object is "METHOD /path". The value is either a price string or a config object:

// Simple — just a price
{ "GET /api/data": "$0.05" }

// Full config
{
  "POST /api/query": {
    price: "$0.25",
    network: "base",
    config: {
      description: "Human-readable description of the endpoint",
      inputSchema: {
        bodyType: "json",
        bodyFields: {
          query: { type: "string", description: "The query to run" },
        },
      },
      outputSchema: {
        type: "object",
        properties: {
          result: { type: "string" },
        },
      },
    },
  },
}

Route Config Fields

Supported Networks

Patterns

Multiple endpoints with different prices

const payment = paymentMiddleware(PAY_TO, {
  "GET /api/cheap": { price: "$0.001", network: "base" },
  "GET /api/expensive": { price: "$1.00", network: "base" },
  "POST /api/query": { price: "$0.25", network: "base" },
});

app.get("/api/cheap", payment, (req, res) => { /* ... */ });
app.get("/api/expensive", payment, (req, res) => { /* ... */ });
app.post("/api/query", payment, (req, res) => { /* ... */ });

Wildcard routes

const payment = paymentMiddleware(PAY_TO, {
  "GET /api/*": { price: "$0.05", network: "base" },
});

app.use(payment);
app.get("/api/users", (req, res) => { /* ... */ });
app.get("/api/posts", (req, res) => { /* ... */ });

Health check (no payment)

Register free endpoints before the payment middleware:

app.get("/health", (req, res) => res.json({ status: "ok" }));

// Payment middleware only applies to routes registered after it
app.get("/api/data", payment, (req, res) => { /* ... */ });

POST with body schema

const payment = paymentMiddleware(PAY_TO, {
  "POST /api/analyze": {
    price: "$0.10",
    network: "base",
    config: {
      description: "Analyze text sentiment",
      inputSchema: {
        bodyType: "json",
        bodyFields: {
          text: { type: "string", description: "Text to analyze" },
        },
      },
      outputSchema: {
        type: "object",
        properties: {
          sentiment: { type: "string" },
          score: { type: "number" },
        },
      },
    },
  },
});

app.post("/api/analyze", payment, (req, res) => {
  const { text } = req.body;
  // ... your logic
  res.json({ sentiment: "positive", score: 0.95 });
});

Using the CDP facilitator (authenticated)

For production use with the Coinbase facilitator (supports mainnet):

npm install @coinbase/x402

const { facilitator } = require("@coinbase/x402");

const payment = paymentMiddleware(PAY_TO, routes, facilitator);

This requires CDP_API_KEY_ID and CDP_API_KEY_SECRET environment variables. Get these from https://portal.cdp.coinbase.com.

Testing with the pay-for-service Skill

Once the server is running, use the pay-for-service skill to test payments:

# Check the endpoint's payment requirements
npx awal@latest x402 details http://localhost:3000/api/example

# Make a paid request
npx awal@latest x402 pay http://localhost:3000/api/example

Pricing Guidelines

Checklist

• Get wallet address with npx awal@latest address
• Install express and x402-express
• Define routes with prices and descriptions
• Register payment middleware before protected routes
• Keep health/status endpoints before payment middleware
• Test with curl (should get 402) and npx awal@latest x402 pay (should get 200)
• Announce your service so other agents can find and use it