Documentation Index
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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
}
}
{
"success": true,
"payer": "0x...",
"transaction": "0x...",
"network": "eip155:1"
}
{
"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')}`,
},
});
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 |
