Who pays the gas.

A buyer on this rail holds USDG and nothing else. No ETH, no faucet, no top-up in a second currency before the first payment can happen — which matters, because an agent that has to acquire gas before it can transact is an agent that cannot start.

That works because settlement is relayed. The buyer signs; somebody else submits.

The mechanism

1. The buyer signs, offlinean EIP-3009 TransferWithAuthorization: from, to, value, a time window, and a random nonce. Signing costs nothing and touches no chain.
2. The seller hands it to the facilitatorwhich verifies the signature and the payer’s balance against the chain, then submits transferWithAuthorization from Aeron’s relayer account.
3. The relayer pays the gasin ETH, from its own balance. It is the transaction’s sender, so it is the account the network charges.
4. The USDG moves payer → payToinside that same transaction. The relayer is never a party to the transfer and never holds the money — it carries the transaction, not the value.

One transaction, two roles: the payer authorises, the relayer pays. This is not a paymaster or an account-abstraction bundler; it is a property of the token standard, which is why it works with an ordinary EOA and no smart-contract wallet.

What is covered

There is no approval step to sponsor: EIP-3009 carries its own authorization, so a buyer never sends an approve transaction and never needs gas for one.

What it costs Aeron

About 102,500 gas per settlement, paid in ETH by the relayer. Measured on chain rather than estimated: the first settlement through this facilitator burned 102,543 gas at 0.046232 gwei, which is 0.0000047 ETH. The relayer address and its live balance are public:

curl -s https://x402.aeron.sh/status | jq '{relayer, relayerEth, settlement}'

{
  "relayer": "0x2b94e4188da990EcF84d13A5AbE69BC9523015A7",
  "relayerEth": "0.100773054387092",
  "settlement": "enabled"
}

Divide one by the other and you have the capacity number: at that gas price, one ETH covers roughly 211,000 settlements. relayerEth is therefore the number that turns the rail off. When it reaches zero, /settle fails and every buyer sees a payment failure — nothing is lost and nothing is charged, but nothing works either. If your product depends on this facilitator, alert on it the way you would alert on a certificate expiry.

Gas price moves, so read any capacity figure as an estimate at the price of the day, and read the balance yourself.

Access policy

The reason is plain enough to state: /settle has no authentication today, so the relayer’s gas is spendable by anyone who can sign a valid transfer — including someone paying themselves in a loop, which costs them nothing and costs Aeron a transaction every time. An open window is the fastest way to find out what people build on this. Credits are what let it keep running once they have.

Until then the allowance is shared, and so is the cost of wasting it. Deliberate abuse closes the window early for everybody, and Aeron will close it early if that is what abuse makes necessary.

If you are integrating now

• Build against it. Nothing about the request or response shape changes when credits arrive; only where the gas is drawn from.
• Handle a settle failure properly — charge nothing, deliver nothing, surface errorReason. That is the same code path that will handle an empty credit balance in October.
• Watch this page, or watch /status. Credits will be visible in both before they are enforced.

If you would rather not depend on it

Run your own facilitator with your own relayer and your own gas policy. It is a single Cloudflare Worker, MIT licensed, and it settles the same payments to the same token: aeronlabs/aeron-facilitator. Buyers do not have to change anything — the wallet talks to whichever facilitator the seller names.

Keep the relayer’s balance small either way. It is a hot key that signs without a prompt, and the only thing standing between it and a bad day is how little is in it.