@x402/core Custom Server

Demonstrates how to implement x402 payment handling manually without using pre-built middleware packages like @x402/express or @x402/hono.

import { x402ResourceServer, HTTPFacilitatorClient } from "@x402/core/server";
import { ExactEvmScheme } from "@x402/evm/exact/server";

const resourceServer = new x402ResourceServer(
  new HTTPFacilitatorClient({ url: facilitatorUrl }),
).register("eip155:84532", new ExactEvmScheme());

// In your request handler:
if (!paymentHeader) {
  const paymentRequired = resourceServer.createPaymentRequiredResponse([requirements], resource);
  res.status(402).set("PAYMENT-REQUIRED", encode(paymentRequired)).json({});
  return;
}

const paymentPayload = decode(paymentHeader);
const verifyResult = await resourceServer.verifyPayment(paymentPayload, requirements);
if (!verifyResult.isValid) return res.status(402).json({ error: verifyResult.invalidReason });

// Execute handler, then settle
const settleResult = await resourceServer.settlePayment(paymentPayload, requirements);
res.set("PAYMENT-RESPONSE", encode(settleResult));

Prerequisites

• Node.js v20+ (install via nvm)
• pnpm v10 (install via pnpm.io/installation)
• Valid EVM address for receiving payments
• URL of a facilitator supporting the desired payment network, see facilitator list

Setup

1. Copy .env-local to .env:

cp .env-local .env

and fill required environment variables:

• FACILITATOR_URL - Facilitator endpoint URL
• EVM_ADDRESS - Ethereum address to receive payments

2. Install and build all packages from the typescript examples root:

cd ../../
pnpm install && pnpm build
cd servers/custom

3. Run the server

pnpm dev

Testing the Server

You can test the server using one of the example clients:

Using the Fetch Client

cd ../../clients/fetch
# Ensure .env is setup
pnpm dev

Using the Axios Client

cd ../../clients/axios
# Ensure .env is setup
pnpm dev

These clients will demonstrate how to:

1. Make an initial request to get payment requirements
2. Process the payment requirements
3. Make a second request with the payment token

Example Endpoint

The server includes a single example endpoint at /weather that requires a payment of 0.001 USDC on Base Sepolia to access. The endpoint returns weather data for a given city.

HTTP Headers

Request Headers

When submitting payment, include one of these headers (both are supported for backwards compatibility):

Header	Protocol	Description
PAYMENT-SIGNATURE	v2	Base64-encoded JSON payment payload
X-PAYMENT	v1	Base64-encoded JSON payment payload

Example request with payment:

GET /weather HTTP/1.1
Host: localhost:4021
PAYMENT-SIGNATURE: eyJwYXltZW50IjoiLi4uIn0=

Response Headers

Header	Status	Description
PAYMENT-REQUIRED	402	Base64-encoded JSON with payment requirements
PAYMENT-RESPONSE	200	Base64-encoded JSON with settlement details

Response Format

Payment Required (402)

HTTP/1.1 402 Payment Required
Content-Type: application/json; charset=utf-8
PAYMENT-REQUIRED: <base64-encoded JSON>

{"error":"Payment Required","message":"This endpoint requires payment"}

The PAYMENT-REQUIRED header contains base64-encoded JSON with the payment requirements:

{
  "x402Version": 2,
  "error": "Payment required",
  "resource": {
    "url": "http://localhost:4021/weather",
    "description": "Weather data",
    "mimeType": "application/json"
  },
  "accepts": [
    {
      "scheme": "exact",
      "network": "eip155:84532",
      "amount": "1000",
      "asset": "0x036CbD53842c5426634e7929541eC2318f3dCF7e",
      "payTo": "0x...",
      "maxTimeoutSeconds": 300,
      "extra": {
        "name": "USDC",
        "version": "2",
        "resourceUrl": "http://localhost:4021/weather"
      }
    }
  ]
}

Successful Response (with payment)

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
PAYMENT-RESPONSE: <base64-encoded JSON>

{"city":"San Francisco","weather":"foggy","temperature":60,"timestamp":"2024-01-01T12:00:00.000Z"}

The PAYMENT-RESPONSE header contains base64-encoded JSON with the settlement details:

{
  "success": true,
  "transaction": "0x...",
  "network": "eip155:84532",
  "payer": "0x...",
  "requirements": {
    "scheme": "exact",
    "network": "eip155:84532",
    "amount": "1000",
    "asset": "0x036CbD53842c5426634e7929541eC2318f3dCF7e",
    "payTo": "0x...",
    "maxTimeoutSeconds": 300,
    "extra": {
      "name": "USDC",
      "version": "2",
      "resourceUrl": "http://localhost:4021/weather"
    }
  }
}

Payment Flow

The custom implementation demonstrates each step of the x402 payment flow:

1. Request Arrives — Middleware intercepts all requests
2. Route Check — Determine if route requires payment
3. Payment Check — Look for PAYMENT-SIGNATURE or X-PAYMENT header
4. Decision Point:
   • No Payment: Return 402 with requirements in PAYMENT-REQUIRED header
   • Payment Provided: Verify with facilitator
5. Verification — Check payment signature and validity
6. Handler Execution — Run protected endpoint handler
7. Settlement — Settle payment on-chain (for 2xx responses)
8. Response — Add settlement details in PAYMENT-RESPONSE header

Key Implementation Details

Defining Payment Requirements

const routeConfigs: Record<string, RoutePaymentConfig> = {
  "GET /weather": {
    scheme: "exact",
    price: "$0.001",
    network: "eip155:84532",
    payTo: evmAddress,
    description: "Weather data",
    mimeType: "application/json",
  },
};

Checking for Payment

const paymentHeader = (req.headers["payment-signature"] || req.headers["x-payment"]) as
  | string
  | undefined;

if (!paymentHeader) {
  const paymentRequired = resourceServer.createPaymentRequiredResponse([requirements], {
    url: `${req.protocol}://${req.get("host")}${req.originalUrl}`,
    description: routeConfig.description,
    mimeType: routeConfig.mimeType,
  });
  const requirementsHeader = Buffer.from(JSON.stringify(paymentRequired)).toString("base64");

  res.status(402);
  res.set("PAYMENT-REQUIRED", requirementsHeader);
  res.json({
    error: "Payment Required",
    message: "This endpoint requires payment",
  });
  return;
}

Verifying Payment

const paymentPayload = JSON.parse(Buffer.from(paymentHeader, "base64").toString("utf-8"));
const verifyResult = await resourceServer.verifyPayment(paymentPayload, requirements);

if (!verifyResult.isValid) {
  res.status(402).json({
    error: "Invalid Payment",
    reason: verifyResult.invalidReason,
  });
  return;
}

Settling Payment

const settleResult = await resourceServer.settlePayment(paymentPayload, requirements);
const settlementHeader = Buffer.from(JSON.stringify(settleResult)).toString("base64");
res.set("PAYMENT-RESPONSE", settlementHeader);

Middleware vs Custom Comparison

Aspect	With Middleware (@x402/express)	Custom Implementation
Code Complexity	~10 lines	~150 lines
Automatic Verification	✅ Yes	❌ Manual
Automatic Settlement	✅ Yes	❌ Manual
Header Management	✅ Automatic	❌ Manual
Flexibility	Limited	✅ Complete control
Error Handling	✅ Built-in	❌ You implement
Maintenance	x402 team	You maintain

When to Use Each Approach

Use Middleware (@x402/express, @x402/hono) when:

• Building standard applications
• Want quick integration
• Prefer automatic payment handling
• Using supported frameworks (Express, Hono)

Use Custom Implementation when:

• Using unsupported frameworks (Koa, Fastify, etc.)
• Need complete control over flow
• Require custom error handling
• Want to understand internals
• Building custom abstractions

Adapting to Other Frameworks

To use this pattern with other frameworks:

1. Create middleware function for your framework
2. Check for payment requirements per route
3. Use x402ResourceServer to verify/settle payments
4. Intercept responses to add settlement headers

The pattern in index.ts can be adapted to any Node.js web framework.