Ethereum Foundation's zkAPI Live on Mainnet, Enables User Balance Recovery

The Ethereum Foundation's zkAPI is now live on mainnet, marking an important milestone for private AI-payment design.

Co-authored by Davide Crapis and Vitalik Buterin, zkAPI provides a billing system for metered APIs, allowing users to send unspent money back to their treasury in case of server cooperation issues.

The implementation includes two routes for withdrawal: mutual close, which requires server clearance, and escape withdrawal, which does not. The user's ability to recover a balance depends on the spending state they hold and whether they can initiate an exit before the deadline.

Each spending state has a nullifier, preventing reuse. To challenge an escape, a challenger supplies an original request proof with the same nullifier as the attempted withdrawal. If successful, the pending payout is cancelled, and the note is restored to the active set.