on-chain vs off-chain

A machine payment over x402 keeps the whole exchange with the endpoint off-chain. The client reads a payment challenge, signs an EIP-3009 authorization for the terms it names, and sends the signature; the endpoint and its facilitator check that message without asking any chain anything. The submission at the end is the single step that touches the network.

The reason for the split is cost and delay. An on-chain write costs gas and waits to be confirmed, and a payment worth a fraction of a cent cannot carry either for every field the two sides have to agree on. EIP-2612's own motivation says the same thing from the token side: without a signed message, using a token inside a contract takes two transactions and the user has to hold the chain's native token to pay for them.

The two halves also differ in what they leave behind. An off-chain signature exists between the parties that exchanged it, so a third party has no way to know one was made. An on-chain settlement is a public record anybody can query afterwards, naming amounts and addresses without naming what was bought. Neither half by itself says what an endpoint was charging at a given moment.

That is the gap an outside reading fills. A payment challenge is stated off-chain and nothing retains it by default, so what an endpoint quoted on a given day exists afterwards if somebody asked and kept the answer.