The Ethereum Foundation on Thursday launched zkAPI, a system that lets users pay for AI models and other metered APIs without revealing who they are, according to a blog post published Thursday.
The Foundation built zkAPI with the Open Anonymity (OA) Project, an open-source AI privacy effort, and it is live on Ethereum mainnet, the Foundation said in a blog post.
“Every AI API call today carries an identity,” the post said. “Your API key points to an account, the account to a payment method, and every prompt you send joins the record attached to both. The provider can connect years of your usage into a single profile.”
“It’s like a generalization of OA unlinkable inference that also abstracts payments away,” Ken Liu, a Stanford computer science PhD candidate who works on the Open Anonymity Project, said in an X post on Thursday afternoon. He added that “more infrastructure should exist where privacy and sovereignty is foundational and people are in control.”
How zkAPI works
To use zkAPI, users must first deposit tokens such as ETH (ETH) or USDC into a vault contract on Ethereum, which records the user’s balance as a private note, according to the post. To spend the tokens, software on the user’s device generates a zero-knowledge proof which shows that its request is backed by a funded note, though it does not reveal which note or deposit funds it.
A zkAPI server checks the zero-knowledge proof and issues a temporary API key with a spending cap. The user then sends their prompts directly to the AI model provider along with that key, and when it expires, the server deducts the usage from the user’s private balance.
A serial number called a “nullifier” is also published each time a user pays for AI usage. “A user who tries to spend the same balance twice produces a duplicate nullifier, which exposes the attempt and nothing else,” the post states.
The client works with existing AI tools, per the post. “The client exposes the standard OpenAI and Ollama APIs on your own machine,” the post states. “Existing apps, editors, and chat clients work by pointing them at localhost.”
Use cases and limitations
The post lists AI chat and agents as the first use case. Other listed applications include blockchain RPC queries, image and video generation, VPN bandwidth and machine-to-machine payments between AI agents.
The zkAPI system does not provide network anonymity, however, according to the post. The gateway can potentially correlate requests sent from a stable IP address, and users seeking stronger privacy may route traffic through Tor, an anonymity network that relays traffic through several servers and is commonly used by so-called “dark web” sites, per the post.
Sessions can also be re-linked through the content of prompts, such as personal details, writing style or conversation history. The zkAPI project’s GitHub repository describes the protocol as experimental.
Design co-authored by Vitalik Buterin
zkAPI implements “ZK API Usage Credits,” a design Davide Crapis and Ethereum co-founder Vitalik Buterin published on the Ethereum Research forum on Feb. 11.
Crapis leads the Ethereum Foundation’s dAI team, which the Foundation formed in September 2025 to improve Ethereum’s capabilities as a settlement and coordination layer for AI. The team also developed ERC-8004, a standard for AI agent identity that launched on mainnet in January.
The blog post announcing zkAPI was written by Vittorio Rivabella of the dAI team.
Buterin pointed to cryptographic payment mechanisms for AI services as part of his near-term vision for Ethereum’s role in AI in a February X post.