x402 for Sellers

Charge for your API in USDC, per request, with no accounts and no signup. A facilitator running at facilitator.fretchen.eu verifies each payment and settles it on-chain, so your server never has to touch a blockchain directly. I run it, but as the whole protocol is open and permissionless you can use it too.

• 0.01 USDC flat per settlement — no percentage, no minimum, no monthly fee.
• Optimism and Base, mainnet and testnet. Other chains on request.
• Open source and self-hostable — no lock-in. If this facilitator goes away, the code and your integration both still work.

What you'll need

• A server you can run code on, and an endpoint worth charging for.
• An EVM wallet — Optimism and Base are Ethereum layer-2 networks, so any Ethereum wallet works. This address receives the payments.
• A little ETH on that network, for one transaction: the approval below. Nothing after that.
• About 1 USDC of allowance for fees — USDC is a dollar stablecoin, pegged 1:1 to the US dollar. That covers roughly 100 settlements.
• To rehearse on testnet: testnet ETH and a buyer holding testnet USDC — see below.

The buyer pays no gas — they sign, they do not transact. The facilitator pays the gas to settle. You pay gas exactly once, for the approval below.

Quick start

Three things happen on your server: you quote a price, you verify, and you settle. The resource is delivered in between — after the payment is known good, before it is taken.

1. Return a 402 with your price

When a request arrives without payment, answer 402 and say what you want. Set scheme to exact — x402's word for how the money moves; this one settles one authorized amount, once, per request. Set network to eip155:10 (Optimism, in the standard eip155:<chainId> form), payTo to your wallet, and amount in USDC units — 6 decimals, so 100000 is $0.10.

{
  "x402Version": 2,
  "accepts": [{
    "scheme": "exact",
    "network": "eip155:10",
    "amount": "70000",
    "asset": "0x0b2C639c533813f4Aa9D7837CAf62653d097Ff85",
    "payTo": "0xYourSellerAddress",
    "maxTimeoutSeconds": 60,
    "extra": { "name": "USD Coin", "version": "2" }
  }],
  "facilitatorUrl": "https://facilitator.fretchen.eu"
}

2. Approve the facilitator for the fee

The fee is collected after settlement with ERC-20 transferFrom, so the facilitator needs an allowance. Keep it small — about 1 USDC, roughly 100 settlements. The spender is a hot settlement wallet, and a large standing allowance is a standing risk. Every /verify response tells you how many settlements you have left, so you can top up before it runs out.

3. Verify, deliver, settle

paymentPayload is the decoded PAYMENT-SIGNATURE header from the buyer; paymentRequirements is the same object you put in accepts[0] above.

// 1. Verify — before you spend anything on the resource
const verifyRes = await fetch("https://facilitator.fretchen.eu/verify", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ paymentPayload, paymentRequirements })
});

// /verify answers 200 even when the payment is bad. Check isValid, not the status code.
const { isValid, invalidReason } = await verifyRes.json();
if (!isValid) return new Response(invalidReason, { status: 402 });

// 2. Deliver the resource
const result = await generateYourResource(request);

// 3. Settle — the money moves here
const settleRes = await fetch("https://facilitator.fretchen.eu/settle", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ paymentPayload, paymentRequirements })
});
const { success, transaction } = await settleRes.json();

return new Response(JSON.stringify(result), { status: 200 });

That is the whole integration.

Try it on testnet first

Rehearse the full flow on OP Sepolia before any real money is involved. The facilitator treats testnet exactly like mainnet — including the fee and the approval — so a testnet run exercises the same code path you will ship. Pick the testnet in the approval widget above.

You will need testnet ETH to send the approval — get some from the Superchain faucet. Your test buyer needs testnet USDC to pay you with — Circle's faucet covers both OP Sepolia and Base Sepolia.

Three values change:

{
  "network": "eip155:11155420",
  "asset": "0x5fd84259d66Cd46123540766Be93DFE6D43130D7",
  "extra": { "name": "USDC", "version": "2" }
}

On testnet the USDC contract's EIP-712 domain name is USDC, not USD Coin. The signature is bound to that name, so if you change only the network and the asset, every payment fails verification — and it fails after your server has already done the expensive work. Both testnets use USDC; both mainnets use USD Coin.

The exact scheme documented here works on all four networks. Batch-settlement, if you go beyond exact later, is not deployed on OP Sepolia — use Base Sepolia to rehearse that one.

Fee model

0.01 USDC per settlement, flat, taken after the payment succeeds — no percentage, no minimum. On a $0.07 request that is about 14%; on a $1 request, 1%. Card processors start around $0.30 per transaction, so they can't price a seven-cent request at all — a flat fee can.

The amount and the facilitator's address are advertised in /supported under facilitatorFees, so a client can read them rather than trust this page.

API reference

Three endpoints at facilitator.fretchen.eu. Both POST endpoints take the same body: paymentPayload and paymentRequirements. The only scheme supported is exact, with USDC, via EIP-3009 transferWithAuthorization: the buyer signs an authorization for one specific amount, recipient and expiry, and pays no gas — the facilitator submits the transaction. Nothing here ever holds your buyer's funds or gets blanket access to them.

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POST /verify

Checks the signature, the balance, the recipient and the expiry — off-chain, so it costs nothing. Call it before you deliver. Note that a rejected payment still comes back as HTTP 200 — branch on isValid, not the status code.

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POST /settle

Submits the payment on-chain via EIP-3009 transferWithAuthorization. Call it after the resource is delivered. The on-chain hash comes back as transaction.

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GET /supported

Networks, schemes and fees the facilitator currently accepts.

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Supported networks

For the other side of the same exchange — what your buyers sign and how they call you — see x402 for buyers.