zkAPI keeps prompts awayy from payment servers, but AI providers can read submitted content and may connect sessions through repeated details.
The Ethereum Foundation and the Open Anonymity Project have developed zkAPI, a zero-knowledge protocol that lets users pay for AI services and other paid APIs without linking their identity to their requests.
Live on Ethereum mainnet, the system separates payment records from the prompts and queries sent to service providers, although it does not hide all network activity or the content users submit.
How zkAPI Separates Payments From API Requests
The Open Anonymity Project announced its collaboration with the Ethereum Foundation in a September 25, 2026, post introducing its public AI chat services, OA-chat. The foundation also promoted zkAPI on October 2, describing it as “a means for private AI.”
The protocol was originally proposed by Ethereum researcher Davide Crapis and co-founder Vitalik Buterin, while the Open Anonymity team built the client, server, smart contracts, and browser integration.
Users first deposit ETH into the ZkAPIVault contract on Ethereum mainnet. The deposit is then turned into a commitment via the use of a Merkle tree, which is a cryptographic data structure, while the spending balance is kept as a private note on the user’s device.
At the start of a session, the browser generates a zero-knowledge proof that proves the user has sufficient funds to pay and has not yet spent the money. Payment verification is done without knowing the specific deposit being used for the payment and the user’s identity.
Once the verification is complete, an API key with a spending cap is issued, with the prompt going straight from the browser to the AI provider. When the API key expires, you get a receipt of usage. The system then deducts that amount from the private balance, with a one-way serial number called a nullifier preventing the same funds from being spent twice.
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The protocol uses Groth16 proofs, BN254 cryptography, and Poseidon hashing to make the payment system both private and verifiable, and users can withdraw their remaining funds directly through the vault contract, even if the zkAPI servers stop operating.
What zkAPI Does Not Hide
The protocol hides the payment link, not the content. A provider still sees prompts and network details such as an IP address, and can try to match sessions by timing. The developers suggest Tor with a fresh circuit per session, since reused conversation history, writing style or personal details can let a provider relink individual sessions.
This project sits close to Buterin’s recent interests. Recall that late last month he shared a longer-term Ethereum roadmap that placed greater emphasis on cryptographic proofs and verification. zkAPI applies that general approach to everyday payments, allowing a service to verify spending without receiving a conventional account identity.
