Ethereum's zkAPI Goes Live, Paying for AI Without an Identity

The Ethereum Foundation and the Open Anonymity Project put a payment system called zkAPI on Ethereum mainnet on 1 October, built so that whoever pays for an AI model never has to say who they are. A user deposits ETH or USDC into a vault contract once, and the balance then sits as a private note. Each request draws against it through a zero-knowledge proof that the money is there, with no account name attached. The server checking that proof never sees the prompt, and the model provider never sees the payment. The code is open source, and Decrypt and KuCoin both report it carries an experimental label with no formal audit listed.

How a deposit turns into a key

Four steps, by the Foundation's own account of them. An application talks to a client running on the user's own device. The client sends the zkAPI server a proof of payment that carries neither the prompt nor an identity. The server verifies that proof and mints a short-lived API key capped in dollars, which lives in device memory and nowhere else. Prompts go from the device straight to the provider under that key, and when it expires the real usage is charged against the private balance through signed receipts.

Those keys work through OpenRouter, which routes requests to several hundred models, and with any application written for the OpenAI request format. A browser-based chatbot called OA Chat runs the whole arrangement with nothing to install.

"zkAPI lets you pay for a metered API without being known. Deposit credits into an Ethereum vault once, then authorize bounded usage with zero-knowledge proofs instead of an identity," the Foundation's post said.

The proofs underneath

The Foundation's post is the only account read here that sets out the machinery. zkAPI uses Groth16 proofs, a compact zero-knowledge proof system, on the BN254 curve, with Poseidon hashing and a Merkle tree 32 levels deep. That tree is what lets a deposit be proved a member of the set without being pointed at. Proofs are checked twice. The zkAPI server verifies one off-chain before it issues a key, and the vault contract verifies one on-chain when money actually moves. Decrypt and KuCoin carry the user-facing mechanics and stop short of the curve.

An older design, a newer build

The build is credited to the Open Anonymity Project working with the Foundation, the post to Vittorio Rivabella of its dAI team, and the design behind both to Davide Crapis, who leads that team, with Vitalik Buterin. On the date of that design, accounts split. Decrypt puts the Crapis and Buterin proposal in February 2026 and the ERC-8004 agent standard in April 2026; KuCoin's flash puts the same two in February and August 2025. They do not reconcile. No year for either is asserted here. Crapis has forecast that machines will generate most Ethereum traffic within three to five years, a line that reaches this desk through the KuCoin flash alone.

What the launch leaves open

Nothing published says what zkAPI costs on top of a provider's own metering. Nor how much sits in the vault. OpenRouter is the only provider named as wired in. The Foundation lists uses well beyond chat: blockchain RPC calls, image and video generation, VPNs, machine-to-machine services. Each of those bills by the request, which is the shape this is built for. Other privacy launches this year have shipped with hard limits attached, Aztec's $2,500 cap per transfer among them, and the plumbing for machine payments is being laid in more than one place: Block wired bitcoin's Lightning network into x402, the AI agent payment standard, in September. An experimental label on a contract that holds deposits is the thing to watch next.