@fastxyz/x402-facilitator

Payment verification and settlement for the x402 HTTP Payment Protocol.

Supports:

• EVM — EIP-3009 transferWithAuthorization verification and settlement
• Fast — Ed25519 transaction certificate verification

Use Cases

• Run a payment facilitator service
• Verify x402 payment proofs on the network
• Settle EVM EIP-3009 authorizations or Fast transaction certificates
• Mount facilitator API routes in an existing Express app

Out of scope: Paying for 402-protected content → @fastxyz/x402-client · Protecting API routes → @fastxyz/x402-server

Installation

pnpm add @fastxyz/x402-facilitator

Quick Start

Standalone Server

import express from 'express';
import { createFacilitatorServer } from '@fastxyz/x402-facilitator';
import { arbitrumSepolia } from 'viem/chains';

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

app.use(
  createFacilitatorServer({
    evmPrivateKey: '0x...',
    evmChains: {
      'arbitrum-sepolia': {
        chain: arbitrumSepolia,
        rpcUrl: 'https://sepolia-rollup.arbitrum.io/rpc',
        usdcAddress: '0x75faf114eafb1BDbe2F0316DF893fd58CE46AA4d',
      },
    },
    fastNetworks: {
      'fast-testnet': {
        rpcUrl: 'https://api.fast.xyz/proxy-rest',
        committeePublicKeys: ['abc123...', 'def456...'], // Ed25519 public keys used to verify Fast network transactions
      },
    },
  }),
);

app.listen(4402, () => console.log('Facilitator running on :4402'));

Mount in Existing App

import express from 'express';
import { createFacilitatorRoutes } from '@fastxyz/x402-facilitator';

const app = express();
const routes = createFacilitatorRoutes(config);

for (const route of routes) {
  app[route.method](route.path, route.handler);
}
// Exposes: POST /verify, POST /settle, GET /supported

API

createFacilitatorServer(config)

Create Express-compatible middleware that serves the facilitator endpoints.

import express from 'express';
import { createFacilitatorServer } from '@fastxyz/x402-facilitator';

const app = express();
app.use(express.json());
app.use(createFacilitatorServer({ evmPrivateKey: '0x...', evmChains: { ... } }));
app.listen(4402);

createFacilitatorRoutes(config)

Create route descriptors (method, path, handler) that you can attach to an existing app.

import { createFacilitatorRoutes } from '@fastxyz/x402-facilitator';

const routes = createFacilitatorRoutes(config);
for (const route of routes) {
  app[route.method](`/facilitator${route.path}`, route.handler);
}

Routes:

• POST /verify — verify a payment payload against a requirement
• POST /settle — settle a verified payment on the network
• GET /supported — list supported payment kinds

verify(payload, requirement, config)

Verify a payment payload against a payment requirement. Checks signature validity on the network (EIP-3009 for EVM, Ed25519 for Fast). Returns a VerifyResponse indicating whether the payment proof is valid and covers the required amount.

import { verify } from '@fastxyz/x402-facilitator';

const result = await verify(payload, requirement, config);
if (result.isValid) {
  console.log('Payment verified for payer:', result.payer);
} else {
  console.log('Verification failed:', result.invalidReason);
}

settle(payload, requirement, config)

Settle a previously verified payment on the network. For EVM payments, calls transferWithAuthorization to actually move the USDC. For Fast payments, records the settlement on the network. Returns a SettleResponse.

import { settle } from '@fastxyz/x402-facilitator';

const settlement = await settle(payload, requirement, config);
if (settlement.success) {
  console.log('Payment settled:', settlement.txHash);
}

getNetworkId(network)

Derive the chain ID number (e.g. 421614 for Arbitrum Sepolia) from a network name.

Note: The getNetworkId function returns a chain ID number (e.g., 421614 for Arbitrum Sepolia), not the network ID string. The example below shows how to map a network name to a chain ID using the evmChains config.

// import { arbitrumSepolia } from 'viem/chains'; // Uncomment if not already imported
import { getNetworkId } from '@fastxyz/x402-facilitator';

// The function returns a chain ID number (requires evmChains config to map network names to IDs)
// Without config, unknown networks return 0; with config, returns the chain ID number (from viem's chain definitions)
const id = getNetworkId('arbitrum-sepolia', {
  evmChains: {
    'arbitrum-sepolia': { chain: arbitrumSepolia, rpcUrl: '...', usdcAddress: '0x...' },
  },
});
// → 421614 (Arbitrum Sepolia chain ID)

FacilitatorConfig

interface FacilitatorConfig {
  evmPrivateKey?: `0x${string}`;
  evmChains?: Record<string, FacilitatorEvmChainConfig>;
  fastNetworks?: Record<string, FacilitatorFastNetworkConfig>;
  debug?: boolean;
}

interface FacilitatorEvmChainConfig {
  chain: Chain; // viem Chain object
  rpcUrl?: string;
  usdcAddress: `0x${string}`;
  usdcName?: string;
  usdcVersion?: string;
}

interface FacilitatorFastNetworkConfig {
  rpcUrl: string;
  committeePublicKeys: string[]; // Ed25519 public keys used to verify Fast network transactions
}

Common Pitfalls

1. DO NOT omit evmPrivateKey when settling EVM payments — the facilitator needs a key to call transferWithAuthorization.
2. DO NOT omit committeePublicKeys for Fast networks — required for Ed25519 signature verification. These are the Ed25519 public keys used to verify Fast network transactions.
3. DO NOT pass chain IDs as strings — evmChains values require a viem Chain object (import from viem/chains).
4. DO NOT assume built-in networks — all networks must be explicitly configured in FacilitatorConfig.

Design

• No hardcoded network configs — all chain info provided via FacilitatorConfig
• Dual-network support — EVM (EIP-3009) and Fast (Ed25519) in a single service
• Flexible deployment — standalone server or mounted routes