This example is for learning purposes only. Do not use it for production.

​
Prerequisites

Before setting up a facilitator, ensure you have:
  • Node.js 18+ installed
  • Sei wallet with some SEI for gas fees
  • Private key for your facilitator wallet (testnet recommended for development)

​
Quick Setup

​
1. Install Dependencies

npm install @sei-js/x402 express dotenv

​
2. Environment Configuration

Create a .env file: 
# Required: Your facilitator private key
PRIVATE_KEY=0xYourPrivateKeyHere

# Optional: Server port (defaults to 3000)
PORT=3002
Security Note: Never commit private keys to version control. Use environment variables or secure key management services in production.

​
3. Basic Facilitator Implementation

Create index.ts with the following structure: 
import { config } from "dotenv";
import express from "express";
import { verify, settle } from "@sei-js/x402/facilitator";
import {
  PaymentRequirementsSchema,
  PaymentRequirements,
  evm,
  PaymentPayload,
  PaymentPayloadSchema,
} from "@sei-js/x402/types";

config();

const PRIVATE_KEY = process.env.PRIVATE_KEY;

if (!PRIVATE_KEY) {
  console.error("Missing required environment variables");
  process.exit(1);
}

const { createConnectedClient, createSigner } = evm;

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

type VerifyRequest = {
  paymentPayload: PaymentPayload;
  paymentRequirements: PaymentRequirements;
};

type SettleRequest = {
  paymentPayload: PaymentPayload;
  paymentRequirements: PaymentRequirements;
};

const client = createConnectedClient("sei-testnet");

// Verification endpoint
app.post("/verify", async (req, res) => {
  try {
    const body: VerifyRequest = req.body;
    const paymentRequirements = PaymentRequirementsSchema.parse(
      body.paymentRequirements
    );
    const paymentPayload = PaymentPayloadSchema.parse(body.paymentPayload);
    const valid = await verify(client, paymentPayload, paymentRequirements);
    res.json(valid);
  } catch {
    res.status(400).json({ error: "Invalid request" });
  }
});

// Settlement endpoint
app.post("/settle", async (req, res) => {
  try {
    const signer = createSigner("sei-testnet", PRIVATE_KEY as `0x${string}`);
    const body: SettleRequest = req.body;
    const paymentRequirements = PaymentRequirementsSchema.parse(
      body.paymentRequirements
    );
    const paymentPayload = PaymentPayloadSchema.parse(body.paymentPayload);
    const response = await settle(signer, paymentPayload, paymentRequirements);
    res.json(response);
  } catch (error) {
    res.status(400).json({ error: `Invalid request: ${error}` });
  }
});

// Supported schemes endpoint
app.get("/supported", (req, res) => {
  res.json({
    kinds: [
      {
        x402Version: 1,
        scheme: "exact",
        network: "sei-testnet",
      },
    ],
  });
});

app.listen(process.env.PORT || 3000, () => {
  console.log(
    `Server listening at http://localhost:${process.env.PORT || 3000}`
  );
});

​
4. Run Your Facilitator

npx tsx index.ts
Your facilitator server should start and display: Server listening at http://localhost:3002

​
API Endpoints

Your facilitator exposes three main endpoints:

​
POST /verify

Verifies x402 payment payloads without executing transactions. Request Body: 
{
  paymentPayload: PaymentPayload; // x402 payment data
  paymentRequirements: PaymentRequirements; // Payment requirements
}
Response: 
{
  "valid": true,
  "reason": "Payment payload is valid"
}

​
POST /settle

Settles verified payments by signing and broadcasting transactions to Sei testnet. Request Body: 
{
  paymentPayload: PaymentPayload; // x402 payment data
  paymentRequirements: PaymentRequirements; // Payment requirements
}
Response: 
{
  "transactionHash": "0x...",
  "status": "success"
}

​
GET /supported

Returns supported payment schemes and networks. Response: 
{
  "kinds": [
    {
      "x402Version": 1,
      "scheme": "exact",
      "network": "sei-testnet"
    }
  ]
}