The Ethereum Foundation launched zkAPI on Ethereum mainnet on October 1, a system built with the Open Anonymity Project that lets users pay for metered APIs with zero-knowledge proofs instead of an account or an API key tied to their identity.
The announcement, posted by Vittorio Rivabella of the Foundation’s dAI Team, describes the flow: a user deposits credits into an on-chain vault once, then authorizes each spend with a proof generated on their own device. The provider sees paid requests, never the billing identity behind them.
The mechanics work like this. Deposits sit as commitments in a 32-level Merkle tree, hashed with Poseidon, so a spend proof can show “this note is one of the valid ones” without naming it. Every spend publishes a nullifier; trying to spend the same balance twice produces a duplicate nullifier, which exposes the double-spend attempt and nothing else. Proofs run on Groth16 over the BN254 curve, and the server verifies spend proofs off-chain.
In runtime-key mode, the server checks a proof and mints a short-lived, dollar-capped API key that lives only in the user’s device memory. When the key expires, the provider records total usage in a signed receipt and the server deducts it from the user’s private note. A simpler proxy mode, where the zkAPI server relays requests to the provider, is easier to run but the relay sees the traffic.
The live ZkApiVault contract holds USDC credits, and a Sepolia testnet deployment is available for testing without real funds. The vault contract verifies proofs at deposit, close and escape, so users can withdraw their credits even if zkAPI’s servers become unavailable or act dishonestly.
zkAPI is the working implementation of “ZK API usage credits”, a design published on Ethereum Research by Davide Crapis and Vitalik Buterin. The Foundation said the same design could later support blockchain RPC queries, image and video generation, VPN bandwidth, and machine-to-machine services.
For developers, a local client exposes standard OpenAI and Ollama APIs on their machine, so existing apps and chat clients work by pointing them at localhost.
The Foundation is upfront about the limits: zkAPI hides the billing identity, not the network path. It does not mask IP addresses, request timing or prompt contents, and providers may still link sessions through reused personal details or writing style. Users who want stronger network privacy can route requests through Tor with a fresh circuit for each session. No launch providers or usage figures have been named yet.
