zkAPI launches private usage credits for metered APIs

The Ethereum Foundation and the Open Anonymity project announced zkAPI, a payment system that lets developers pay for metered APIs — starting with AI inference — without revealing who is paying. It runs on Ethereum Mainnet today.

Users deposit credits (ETH, USDC, etc.) into an onchain vault once; the balance then exists as a private note in a 32-level Merkle tree. Software on the user's device produces a zero-knowledge proof authorizing bounded spend, so the API provider sees requests and the payment layer sees the spend, but neither can link them. Spends use Groth16 on BN254 with Poseidon hashing, nullifiers prevent double-spending, and an onchain escape hatch lets funds be withdrawn even if the servers disappear.

• A client exposes standard OpenAI and Ollama APIs locally, so existing apps work unchanged; a simpler proxy mode is also offered
• Applicable to Blockchain RPC, image/video generation, VPNs, and machine-to-machine payments
• Limitations: no network anonymity (Tor recommended) and prompts can still fingerprint a user

zkAPI implements the "ZK API usage credits" design by Davide Crapis and Vitalik Buterin published on Ethereum Research.