Fastx402

Fastx402 is an x402 payment facilitator that enables AI agents and applications to pay for resources on Ethereum mainnet using USDC. It achieves sub-second settlement through FAST RPC preconfirmations, solving the slow finality problem that has left agents unable to access Ethereum’s stablecoin liquidity for real-time payments.

Property	Value
Endpoint	`https://facilitator.primev.xyz`
Network	Ethereum Mainnet (Chain ID: 1)
Asset	USDC (`0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48`)
Settlement	FAST RPC (~100-200ms preconfirmation)
Fees	0%
Source	GitHub

Why Fastx402

Traditional Ethereum payments require 12+ seconds for finality, making them unsuitable for real-time API access. Existing x402 facilitators only work on Base and Solana. Fastx402 changes this:

Sub-second settlement — ~1.2s end-to-end via FAST RPC preconfirmations (vs 12+ seconds)
Zero gas for agents — Agents only need USDC, no ETH required. Gas is fully sponsored via the relay wallet
Mainnet liquidity — Access Ethereum’s largest stablecoin issuance and DeFi ecosystem
x402 compatible — Drop-in replacement for any x402 facilitator
Gasless signatures — Uses EIP-3009 transferWithAuthorization so agents never submit transactions

How It Works

Fastx402 uses USDC’s native EIP-3009 transferWithAuthorization function for gasless, signature-based payments. The flow:

An agent encounters a 402 Payment Required response from a resource server
The agent signs an EIP-712 TransferWithAuthorization message authorizing a USDC transfer
The resource server forwards the signed payment to Fastx402’s /settle endpoint
Fastx402 verifies the signature, checks balances and nonces, then submits the transaction via FAST RPC
FAST RPC preconfirms the transaction in ~100-200ms
The resource server receives confirmation and grants access

Agent                     Resource Server              Fastx402                  FAST RPC / L1
 │                              │                          │                          │
 │  GET /resource               │                          │                          │
 │ ──────────────────────────>  │                          │                          │
 │                              │                          │                          │
 │  <── 402 Payment Required    │                          │                          │
 │      (paymentRequirements)   │                          │                          │
 │                              │                          │                          │
 │  (sign EIP-3009 authorization locally)                  │                          │
 │                              │                          │                          │
 │  GET /resource + signature   │                          │                          │
 │ ──────────────────────────>  │                          │                          │
 │                              │  POST /settle            │                          │
 │                              │ ──────────────────────>  │                          │
 │                              │                          │  transferWithAuthorization│
 │                              │                          │ ──────────────────────>   │
 │                              │                          │  <── preconfirmed         │
 │                              │  <── { success: true }   │                          │
 │                              │                          │                          │
 │  <── 200 OK + resource       │                          │                          │

API Reference

POST /settle

Verify a payment signature and settle it on-chain via FAST RPC. Request body:

{
  "paymentPayload": {
    "x402Version": 2,
    "scheme": "exact",
    "network": "eip155:1",
    "payload": {
      "signature": "0x...",
      "authorization": {
        "from": "0x...",
        "to": "0x...",
        "value": "1000000",
        "validAfter": "0",
        "validBefore": "1718000000",
        "nonce": "0x..."
      }
    }
  },
  "paymentRequirements": {
    "scheme": "exact",
    "network": "eip155:1",
    "amount": "1000000",
    "asset": "0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48",
    "payTo": "0x...",
    "maxTimeoutSeconds": 60
  }
}

Response (success):

{
  "success": true,
  "payer": "0x...",
  "transaction": "0x...",
  "network": "eip155:1"
}

Response (failure):

{
  "success": false,
  "error": "insufficient_funds",
  "payer": "0x..."
}

POST /verify

Validate a payment signature without settling (dry run). Same request body as /settle.

{
  "isValid": true,
  "payer": "0x..."
}

GET /supported

Discover facilitator capabilities.

{
  "kinds": [
    {
      "x402Version": 2,
      "scheme": "exact",
      "network": "eip155:1"
    }
  ],
  "extensions": ["bazaar"],
  "signers": {
    "eip155:*": ["0x488d87a9A88a6A878B3E7cf0bEece8984af9518D"]
  }
}

GET /health

Health check endpoint.

EIP-3009 Signing

Agents sign a TransferWithAuthorization typed data message using USDC’s EIP-712 domain:

const signature = await walletClient.signTypedData({
  domain: {
    name: 'USD Coin',
    version: '2',
    chainId: 1,
    verifyingContract: '0xA0b86991c6218b36c1d19D4a2e9Eb0cE3606eB48',
  },
  types: {
    TransferWithAuthorization: [
      { name: 'from', type: 'address' },
      { name: 'to', type: 'address' },
      { name: 'value', type: 'uint256' },
      { name: 'validAfter', type: 'uint256' },
      { name: 'validBefore', type: 'uint256' },
      { name: 'nonce', type: 'bytes32' },
    ],
  },
  primaryType: 'TransferWithAuthorization',
  message: {
    from: agentAddress,
    to: merchantAddress,
    value: BigInt(1_000_000), // 1 USDC (6 decimals)
    validAfter: 0n,
    validBefore: BigInt(Math.floor(Date.now() / 1000) + 900),
    nonce: `0x${randomBytes(32).toString('hex')}`,
  },
});

The relay wallet (0x488d87a9A88a6A878B3E7cf0bEece8984af9518D) submits the transferWithAuthorization call on-chain. The agent never needs ETH — only USDC.

Error Codes

Code	Description
`unsupported_scheme`	Scheme is not “exact”
`unsupported_network`	Network is not “eip155:1”
`unsupported_asset`	Asset is not USDC
`invalid_from_address`	Invalid sender address
`recipient_mismatch`	`authorization.to` does not match `paymentRequirements.payTo`
`insufficient_payment`	Authorization value < required amount
`authorization_expired`	Current time + 60s >= `validBefore`
`authorization_not_yet_valid`	Current time < `validAfter`
`invalid_signature`	EIP-712 signature verification failed
`insufficient_funds`	Payer USDC balance < authorization value
`nonce_already_used`	Nonce already consumed (replay protection)

ERC-8004 Agent Identity

Fastx402 is registered as an on-chain agent via ERC-8004:

Property	Value
Agent ID	23175
Identity Registry	`0x8004A169FB4a3325136EB29fA0ceB6D2e539a432`
Reputation Registry	`0x8004BAa17C55a88189AE136b182e5fdA19dE9b63`
Metadata URI	`/agent.json`

Get Started

Concepts

Developers

Why Fastx402

How It Works

API Reference

POST /settle

POST /verify

GET /supported

GET /health

EIP-3009 Signing

Error Codes

ERC-8004 Agent Identity

​Why Fastx402

​How It Works

​API Reference

​POST /settle

​POST /verify

​GET /supported

​GET /health

​EIP-3009 Signing

​Error Codes

​ERC-8004 Agent Identity

Why Fastx402

How It Works

API Reference

POST /settle

POST /verify

GET /supported

GET /health

EIP-3009 Signing

Error Codes

ERC-8004 Agent Identity