> ## Documentation Index
> Fetch the complete documentation index at: https://docs.primev.xyz/llms.txt
> Use this file to discover all available pages before exploring further.

# Fastx402

> x402 payment facilitator on Ethereum mainnet powered by FAST RPC preconfirmations

Fastx402 is an [x402](https://www.x402.org/) 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](https://github.com/primev/mainnet-x402-facilitator) |

***

## 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](/v1.2.x/get-started/fastrpc) 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](https://eips.ethereum.org/EIPS/eip-3009) `transferWithAuthorization` function for gasless, signature-based payments. The flow:

1. An agent encounters a `402 Payment Required` response from a resource server
2. The agent signs an EIP-712 `TransferWithAuthorization` message authorizing a USDC transfer
3. The resource server forwards the signed payment to Fastx402's `/settle` endpoint
4. Fastx402 verifies the signature, checks balances and nonces, then submits the transaction via FAST RPC
5. FAST RPC preconfirms the transaction in \~100-200ms
6. 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:**

```json theme={null}
{
  "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):**

```json theme={null}
{
  "success": true,
  "payer": "0x...",
  "transaction": "0x...",
  "network": "eip155:1"
}
```

**Response (failure):**

```json theme={null}
{
  "success": false,
  "error": "insufficient_funds",
  "payer": "0x..."
}
```

### POST /verify

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

```json theme={null}
{
  "isValid": true,
  "payer": "0x..."
}
```

### GET /supported

Discover facilitator capabilities.

```json theme={null}
{
  "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:

```typescript theme={null}
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](https://eips.ethereum.org/EIPS/eip-8004):

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