x402 Cross-Chain Facilitator

x402-compliant facilitator for cross-chain payments with Relay Network

The facilitator verifies payment signatures (EIP-3009 for USDC, Permit2 for all other ERC-20s), executes token transfers, and handles cross-chain bridging via Relay Network. Merchants receive USDC on Base regardless of what token/chain the customer pays with.

Overview

The facilitator is the backend engine that powers x402 cross-chain payments:

1. Signature Verification - Verifies EIP-3009 (USDC) and Permit2 SignatureTransfer (all other ERC-20s) signatures
2. Token Collection - Executes Permit2.permitTransferFrom() or transferWithAuthorization() to collect tokens
3. Swap & Bridge - Uses Relay Network to swap any token → USDC and bridge to Base
4. Settlement - Delivers USDC to merchant on Base

Key Features

• x402 Protocol Compliant - Works with standard x402 clients (x402-axios, x402-fetch)
• Gasless for Customers - Customers sign permits, facilitator pays gas (ERC-20 tokens)
• Any Token Support - Accept ETH, WETH, USDC, DAI, or any ERC-20 token (via Permit2)
• Any Chain Support - 10+ chains including Ethereum, Arbitrum, Base, Polygon, BNB Chain
• Instant Settlement - 2-3 second bridging via Relay liquidity pools
• Native Token Support - Accept native ETH/BNB/MATIC (customer sends tx)

Supported Payment Methods

| Payment Type | Gasless? | How It Works | |--------------|----------|--------------| | USDC | ✅ Yes | EIP-3009 TransferWithAuthorization | | WETH, DAI, any ERC-20 | ✅ Yes | Permit2 SignatureTransfer (one-time approve required) | | Native ETH/BNB | ❌ No | Customer sends tx to Relay directly |

Deployment

Option 1: Docker (Recommended)

docker build -t x402-facilitator .

docker run -d \
  -p 3001:3001 \
  -e BASE_RPC_URL=https://mainnet.base.org \
  -e SETTLER_PRIVATE_KEY=0xYourPrivateKey \
  --name x402-facilitator \
  x402-facilitator

Option 2: Node.js

pnpm install
pnpm build
pnpm start

Configuration

Environment Variables

# Required
SETTLER_PRIVATE_KEY=0xYourPrivateKey   # Wallet to pay gas for settlements
BASE_RPC_URL=https://mainnet.base.org  # Base RPC for settlement

# Optional
PORT=3001

Configuration Options

| Variable | Description | Required | |----------|-------------|----------| | SETTLER_PRIVATE_KEY | Private key for facilitator wallet (pays gas) | ✅ Yes | | BASE_RPC_URL | Base network RPC endpoint | ✅ Yes | | PORT | HTTP server port (default: 3001) | ❌ No |

Note: The facilitator wallet needs ETH on each supported chain to pay gas for settlements.

API Endpoints

POST /verify

Verify a payment signature (Permit2 for non-USDC ERC-20, EIP-3009 for USDC, or native ETH via Relay status).

Request (Permit2 — WETH/DAI/any ERC-20):

{
  "paymentPayload": {
    "x402Version": 1,
    "scheme": "exact",
    "network": "arbitrum",
    "payload": {
      "permit2": {
        "owner": "0xCustomerAddress",
        "spender": "0xFacilitatorWalletAddress",
        "token": "0x82aF49447D8a07e3bd95BD0d56f35241523fBab1",
        "amount": "500000000000000000",
        "nonce": "123456789",
        "deadline": "1735200000"
      },
      "signature": "0x..."
    }
  },
  "paymentRequirements": {
    "scheme": "exact",
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "maxAmountRequired": "1000000",
    "network": "base",
    "payTo": "0xMerchantAddress"
  }
}

Request (EIP-3009 — USDC):

{
  "paymentPayload": {
    "x402Version": 1,
    "scheme": "exact",
    "network": "base",
    "payload": {
      "authorization": {
        "from": "0xCustomerAddress",
        "to": "0xFacilitatorWalletAddress",
        "value": "1000000",
        "validAfter": "0",
        "validBefore": "1735200000",
        "nonce": "0x..."
      },
      "signature": "0x..."
    }
  },
  "paymentRequirements": {
    "scheme": "exact",
    "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913",
    "maxAmountRequired": "1000000",
    "network": "base",
    "payTo": "0xMerchantAddress"
  }
}

Response:

{
  "isValid": true,
  "payer": "0xCustomerAddress"
}

POST /settle

Execute the payment: collect tokens, swap/bridge via Relay, deliver to merchant.

Request: Same as /verify

Response:

{
  "success": true,
  "transaction": "0xSettlementTxHash",
  "network": "base",
  "payer": "0xCustomerAddress"
}

GET /health

Health check endpoint.

Response:

{
  "status": "ok",
  "type": "x402-facilitator"
}

Payment Flow

ERC-20 Tokens (Gasless for Customer)

1. Customer signs Permit2 SignatureTransfer (or EIP-3009 for USDC)
2. Customer sends X-PAYMENT header with signature
3. Facilitator verifies signature (/verify)
4. Facilitator calls Permit2.permitTransferFrom() to collect tokens (single call)
5. Facilitator calls Relay to swap + bridge → USDC on Base
6. Relay delivers USDC to merchant
7. Facilitator confirms settlement (/settle)

Native ETH/BNB (Customer Pays Gas)

1. Customer sends ETH tx directly to Relay
2. Relay swaps ETH → USDC + bridges to Base
3. Merchant receives USDC
4. Facilitator verifies via Relay status

Supported Chains

| Chain | ID | Native Token | |-------|-----|--------------| | Ethereum | 1 | ETH | | Base | 8453 | ETH | | Arbitrum | 42161 | ETH | | Optimism | 10 | ETH | | Polygon | 137 | MATIC | | BNB Chain | 56 | BNB | | Avalanche | 43114 | AVAX | | zkSync | 324 | ETH | | Linea | 59144 | ETH |

Docker Deployment

Docker Compose

version: '3.8'

services:
  facilitator:
    build: .
    ports:
      - "3001:3001"
    environment:
      - BASE_RPC_URL=https://mainnet.base.org
      - SETTLER_PRIVATE_KEY=${SETTLER_PRIVATE_KEY}
      - PORT=3001
    restart: unless-stopped
    healthcheck:
      test: ["CMD", "curl", "-f", "http://localhost:3001/health"]
      interval: 30s
      timeout: 10s
      retries: 3

Security

Private Key Management

CRITICAL: Never expose your settler private key!

# ✅ Good: Use environment variables
export SETTLER_PRIVATE_KEY=0x...

# ✅ Good: Use Docker secrets
docker run --env-file .env.production x402-facilitator

# ❌ Bad: Hardcode in source
const privateKey = "0x123..." // NEVER DO THIS

Testing

# Start facilitator
pnpm dev

# Test health endpoint
curl http://localhost:3001/health

# Test verify endpoint
curl -X POST http://localhost:3001/verify \
  -H "Content-Type: application/json" \
  -d '{"paymentPayload":{...},"paymentRequirements":{...}}'

Troubleshooting

| Issue | Solution | |-------|----------| | "Permit2 spender mismatch" | Set facilitatorAddress in merchant middleware to match this wallet | | "Signature verification failed" | Check EIP-712 domain parameters match (Permit2 domain is always the Permit2 contract) | | "Insufficient gas" | Fund facilitator wallet with ETH on source chain | | "Relay quote failed" | Verify token/chain is supported by Relay | | "Transfer failed" | Check customer has approved Permit2 contract (token.approve(PERMIT2_ADDRESS, MaxUint256)) |

Changelog

v2.1.0

• Replaced ERC-2612 with Permit2 for all non-USDC ERC-20 tokens (including WETH)
  • Single Permit2.permitTransferFrom() call replaces permit() + transferFrom()
  • Works with any ERC-20 — no need for per-token permit() support
  • Random unordered nonces — no sequential front-running risk
• Added requestId tracking for native ETH payments via Relay status API
• Both /verify and /settle updated for Permit2 flow

v2.0.0 (December 2025)

• Initial release with x402 protocol support
• EIP-3009 for USDC, ERC-2612 for other ERC-20s
• Relay Network cross-chain bridging

License

MIT License

Support

• GitHub Issues: https://github.com/divi2806/x402-cross-bridge-sdk/issues
• Email: [email protected]