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Ethereum Foundation puts anonymous pay-per-call AI billing on mainnet with zkAPI
Ethereum Foundation launched zkAPI on mainnet, letting users pay for AI model calls from a private vault without revealing who is paying. The project's own repository still labels the protocol experimental, so early use will come from privacy-minded users and AI agents willing to test it.
The Investor · Invest desk
What happened
- Users deposit ETH or USDC into a vault contract on Ethereum that records each balance as a private note, then prove a request is funded without showing which note pays for it.
- A zkAPI server checks that proof, issues a temporary API key with a spending cap, and deducts the usage from the user's private balance once the key expires.
- The system implements ZK API Usage Credits, a design Vitalik Buterin and Davide Crapis posted to the Ethereum Research forum on February 11, and was built with the Open Anonymity Project.
- The post lists AI chat and agents as the first use case, followed by blockchain RPC queries, image and video generation, VPN bandwidth and machine-to-machine payments between agents.
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Why it matters
- constraint Payment privacy only holds for users who also route traffic through Tor and strip identifying detail from prompts. That setup cost limits the early user base to people who already care about anonymity.
- capability Developers can try it without rewriting their tools, because the local client mimics the standard OpenAI and Ollama APIs and existing apps only need to point at localhost.
- precedent The dAI team, formed in September 2025, has now shipped two mainnet products since January (ERC-8004 and zkAPI), so designs its leads post to the research forum can reach mainnet within months.
In credit terms, the zkAPI server lends. According to the Foundation's post, it authorises spending up to a cap before it settles, and it settles against a balance it has been shown is funded but cannot tie to a payer [4][6]. The payment layer never handles the work itself, because prompts go directly to the model provider [4]. A published serial number stops double-spending. "A user who tries to spend the same balance twice produces a duplicate nullifier, which exposes the attempt and nothing else," said the post, written by Vittorio Rivabella of the Foundation's dAI team [5][17].
The launch materials as reported do not include a fee, the rule linking a key's cap to the note behind it, or how the model provider receiving those prompts gets paid.
"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." [7] zkAPI takes the account and the payment method out of that chain [6]. The prompt stays in it. The post concedes that sessions can still be re-linked through personal details, writing style or conversation history [13]. It also concedes that a gateway can correlate requests from a stable IP address, and it leaves that gap to Tor [12].
I see three ways this develops. In the first, it stays a tool for people who already route through Tor and keep small balances. In the second, a commercial model provider accepts the keys as a billing channel and brings ordinary chat users with it. In the third, the volume comes from software paying software, meaning the agent-to-agent and RPC uses on the post's list [11]. "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 [8].
I'd expect the third path to produce volume first. An RPC query or a payment between two agents carries little of the personal detail, writing style or conversation history that the post says can re-link sessions [13]. That avoids one of the two limits the post names. The other limit is the counter-case: an agent running from a fixed server address is exactly the stable-IP correlation the post describes [12]. If the vault holds little ETH or USDC six months from now [3], none of the three paths has started and the view is wrong.
What to watch
- A commercial AI model provider saying it accepts zkAPI keys, which would test whether ordinary chat users can be reached.
- The GitHub repository dropping its experimental label for the protocol.
- Any change to the protocol that adds network anonymity, so it no longer depends on users routing through Tor.