The Ethereum Foundation's Oct. 1 announcement that zkAPI is running on mainnet gives the private AI-payment design coauthored by Davide Crapis and Vitalik Buterin a concrete financial-control test: how does a user get unspent money back when the billing server stops cooperating?

zkAPI, a billing system for metered APIs, documents an onchain withdrawal route that does not require the server's clearance. But a user's ability to recover a balance depends on which spending state they hold and whether they can start an exit in time. The implementation's pause powers and expiring notes put boundaries around that control.

Open Anonymity built the implementation with the Ethereum Foundation . Crapis and Buterin published the underlying design on Feb. 11 ; the foundation's October announcement credits the team that turned it into software and contracts. Etherscan records the announcement-linked vault's creation on Sept. 30 , a day before the announcement.

The announcement describes its linked vault as holding USDC credits. The current mainnet manifest identifies that same vault as native ETH, with balances accounted for in whole gwei. Its explorer activity also shows ETH-valued deposits and close payouts.

CryptoSlate's February coverage examined the proposal. The mainnet implementation now gives withdrawal rights, deadlines and settlement dependencies practical significance.

Under the current protocol , a deposit funds a note. The wallet keeps the private spending state locally and uses proofs to authorize metered service. Individual API requests do not each move money onchain. As usage is settled, the server signs a successor state representing the remaining balance. The practical dividing line is between an unused spending state and a predecessor that has already authorized a request: the latter can be challenged if used to seek an escape payout.

That arrangement makes the spending state central to recovery. The vault can check a withdrawal proof against its rules, while the wallet must still possess the information needed to prove the balance it wants to withdraw. Preserving the note and its recovery records is essential to proving a withdrawal balance after an interruption.

The cooperative route, called mutual close, begins with server clearance. A separate server signing key authorizes the withdrawal, and the wallet includes that signature inside its proof. The vault checks the proof and pays the remaining balance to a destination bound into it. That destination can differ from the address that originally funded the note.