Ethereum Foundation ships zkAPI, a zero-knowledge way to pay for AI without an identity attached

The Ethereum Foundation and a group called the Open Anonymity Project said on October 1 that they have launched zkAPI, a protocol that lets someone pay for a usage-metered API — AI inference first — without the provider being able to tie the payments to the user or to each other. The team says it is live on Ethereum mainnet now, according to the Foundation's announcement.

The mechanism, as the Foundation describes it: a user deposits credits (ETH, USDC, and the like) into a vault contract once. That balance then lives as a private "note." To spend, software on the user's device generates a zero-knowledge proof asserting that a funded, unspent note covers the request — without revealing which note or which deposit. A zkAPI server verifies the proof and mints a short-lived, dollar-capped API key held only in device memory; prompts then go straight to the AI provider under that key, and the reserved amount is settled against the note via a signed usage receipt when the key expires.

On the cryptography, the Foundation says deposits are stored as commitments in a Merkle tree 32 levels deep, each spend publishes a one-way "nullifier" to prevent double-spends, proofs use Groth16 on the BN254 curve, and hashing uses Poseidon. The vault verifies proofs at deposit, close, and withdrawal, so — per the announcement — a user can exit onchain even if every zkAPI server disappears.

Key facts

• zkAPI launched October 1, 2026, built by the Open Anonymity Project with the Ethereum Foundation; live on Ethereum mainnet (Ethereum Foundation blog).
• Design is based on "ZK API usage credits," published on Ethereum Research by Davide Crapis and Vitalik Buterin (Ethereum Foundation blog).
• Stack: Groth16/BN254 proofs, Poseidon hashing, 32-level Merkle tree; server verifies proofs off-chain, vault verifies onchain (Ethereum Foundation blog).
• A live "ZkApiVault" on mainnet holds USDC credits; a Sepolia testnet deployment also exists (Ethereum Foundation blog).

The real-world read

This is the builders' own launch post, so treat the "it just works" framing as marketing and read the limitations the Foundation itself discloses. By its own account, zkAPI hides only the payment link. The provider still sees prompt contents and network metadata including your IP, and can try to correlate sessions by timing — the post suggests Tor with a fresh circuit per session as a mitigation. It also concedes that reused writing style, personal details, or conversation history can re-fingerprint "anonymous" sessions, forcing a privacy-versus-usefulness tradeoff. There is also a simpler "proxy mode" where the relay does see traffic; the harder-to-run "runtime-key" mode is what delivers the no-intermediary claim. In short: the private-payment layer is the real advance here; anonymity end-to-end is not something the Foundation is promising.

Opinion, and whose

The underlying design is credited to a proposal from Ethereum Research's Davide Crapis and Ethereum co-founder Vitalik Buterin; the Foundation frames private, identity-free API payments as the needed alternative to trusting a billing middleman. No independent security review or adoption figures were offered in the announcement.

Sources

• Ethereum Foundation blog, "Introducing zkAPI" (2026-10-01) — primary announcement and sole source for the mechanism, cryptographic details, mainnet status, and stated limitations. It is the builders' own launch post and reads promotionally in places; claims of security and privacy are theirs and are not independently verified here.

This is news, not financial or security advice.