The original 1991 HTTP spec reserved status code 402 for digital payments. Credit cards need minimum transaction floors. SWIFT needs human sign-off at every hop. Neither works when a machine needs to pay $0.002 for an API call and retry in milliseconds.
x402 builds on that status code. It pairs the 402 response with gasless stablecoin transfers, letting AI agents pay for resources programmatically. Three signing schemes cover the major chains and assets: EIP-3009 for USDC on Base and Arbitrum, Permit2 for USDC and USDT on all EVM chains, and native fee payer abstraction for Solana.
EIP-3009 (TransferWithAuthorization): A standard built into USDC that lets a wallet holder sign a transfer off-chain. The signed authorization can be submitted by anyone. The signer never pays gas. Supported on Base and Arbitrum only.
Permit2: Uniswap's universal approval contract. Works with any ERC-20 token, including USDC, USDT, and tokens without built-in gasless transfer support. The token holder approves the Permit2 contract once on-chain, then signs individual transfer permits off-chain for each payment. CoinCircuit submits the permit and covers gas. Supported on Base, Arbitrum, and BSC.
Solana fee payer abstraction: Solana transactions natively support separate fee payers. The agent builds a transfer, designates CoinCircuit's wallet as the fee payer, and partially signs. CoinCircuit adds its fee payer signature and submits. Works with SOL, USDC, and USDT on Solana.
Gasless payment: The agent signs an authorization to move tokens from their wallet to a deposit address. The resource server (your backend) submits this signed payload to CoinCircuit, which settles it on-chain and covers gas. The agent only needs a wallet and a signing key.
Resource server: Your API backend. You create the payment session, return the 402 response to the agent, receive the signed payload back, and call CoinCircuit to settle it.
Your server creates a payment session. The agent signs a payment authorization using the appropriate scheme for their chain and asset. Your server submits the signed payload to CoinCircuit for on-chain settlement.
For a step-by-step guide on building signed payloads for each scheme, see How to Sign a Gasless Stablecoin Payment.
1. Agent requests your resource
The agent sends a request to your API:
2. Your server creates a payment session
Your backend calls CoinCircuit to create a checkout session:
The response includes a deposit address, the required USDC amount, and a session reference.
3. Your server responds with 402
Return the payment details to the agent:
Your server keeps the session reference internally for settlement.
4. Agent signs the payment
The agent signs a gasless payment authorization using the scheme that matches its chain and asset. Every scheme produces a flat payload with from, to, value, and scheme-specific fields.
The agent sends the signed payload back to your server:
5. Your server settles via CoinCircuit
Your backend submits the signed payload to CoinCircuit for on-chain settlement:
CoinCircuit validates the session requirements, submits the transaction on-chain, and covers gas. The response includes the transaction hash and block confirmation.
6. Your server releases the resource
Once you receive a successful settlement response, deliver the resource to the agent.
Before settling, you can dry-run the payment to catch problems early. Call the verify endpoint with the same payload:
This runs scheme-specific checks without touching the blockchain. For EIP-3009: authorization timestamps, nonce replay, and sender balance. For Permit2: deadline and sender balance. Returns 200 if the payment will succeed, or 400 with per-check pass/fail results.
All EVM networks offer sub-second block times and gas costs under a cent. Solana settles in under a second with fees below $0.001.
The agent signs the authorization, but your server submits it. Two reasons:
You control the session lifecycle. By submitting through your backend, you tie the settlement to your checkout session, validate the amount, and confirm payment before releasing the resource.
The agent stays simple. The agent only needs a wallet and signing capability. No CoinCircuit API key, no gas tokens, no blockchain node. Sign and hand off.
Developer friendly API. Instant settlements. No hidden fees.
How to Sign a Gasless Stablecoin Payment Get Started NowScheme
Asset
Chains
eip3009
USDC
Base, Arbitrum
permit2
USDC, USDT
Base, Arbitrum, BSC
solana
SOL, USDC, USDT
Solana
GET /api/premium-dataPOST https://api.coincircuit.io/api/v1/payments
x-api-key: sk_live_your_key
{
"amount": "1.00",
"currency": "USD",
"asset": "USDC",
"chain": "base",
"customer": {
"email": "agent@example.com"
}
}HTTP/1.1 402 Payment Required
{
"paymentRequired": {
"chain": "base",
"asset": "USDC",
"amount": "1000000",
"depositAddress": "0xabc...def"
}
}POST /api/premium-data/pay
{
"scheme": "eip3009",
"payload": {
"from": "0xAgentWallet...",
"to": "0xabc...def",
"value": "1000000",
"validAfter": "0",
"validBefore": "1774055094",
"nonce": "0xrandom32bytes...",
"signature": "0xabcd...1234"
}
}POST https://api.coincircuit.io/api/v1/payments/agent/settle
x-api-key: sk_live_your_key
{
"sessionReference": "cs_ref_abc123",
"scheme": "eip3009",
"chain": "base",
"asset": "USDC",
"payload": {
"from": "0xAgentWallet...",
"to": "0xabc...def",
"value": "1000000",
"validAfter": "0",
"validBefore": "1774055094",
"nonce": "0xrandom32bytes...",
"signature": "0xabcd...1234"
}
}Agent -> GET /api/premium-data
Server -> CoinCircuit: POST /payments (creates session)
Server -> Agent: 402 (deposit address, amount)
Agent -> Signs payment (eip3009 / permit2 / solana)
Agent -> Server: POST /pay (signed payload)
Server -> CoinCircuit: POST /payments/agent/settle
Server -> Agent: 200 (resource delivered)POST https://api.coincircuit.io/api/v1/payments/agent/verify
x-api-key: sk_live_your_key