Ethereum zkAPI is an experimental system designed to separate metered API billing from a user’s billing identity. A zero-knowledge proof can show that a permitted spend is covered by a private balance; it is not itself an Ethereum payment, a transfer of funds, or a general-purpose authorization to use an API. In zkAPI, proof verification, the API service’s permission to issue a lease, usage settlement, and on-chain vault transactions are distinct steps.
What Ethereum zkAPI is designed to do
zkAPI is a protocol for paying for metered API use while limiting the link between a deposit and the API usage it funds. Its design represents a funded balance as a private note. Client software proves that a valid note can cover an allowed spend, and a server can verify that proof without learning which deposit funded it.
That is a narrower claim than saying API use is anonymous. The system aims to separate the billing link; it does not hide every part of a request, make the network anonymous, or prevent an inference provider from seeing the content it processes.
Ethereum.org describes a zero-knowledge proof as a way to prove a statement is valid without revealing the statement itself. In zkAPI, the relevant statement concerns membership of a private note and whether it has enough value for a proposed spend. A nullifier helps prevent that note from being spent more than once. The proof lets a verifier check protocol conditions; it does not move ETH or compel a provider to accept a request.
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How a zkAPI request works
The documented direct flow separates the user’s client, the authorization service, the API provider and the Ethereum vault. The client keeps the note secrets and creates the proof locally; the service verifies it and grants a bounded lease; usage is then recorded and settled according to the system’s rules.
- Fund the vault. A user deposits funds through an on-chain vault operation. The resulting balance is represented as a private note. The asset description depends on the version and source: the Ethereum Foundation’s October 1, 2026 launch post describes ETH, USDC and other credits, while the current v2 protocol documentation describes native-ETH funding.
- Create a proof on the client. The client uses the private note data to produce evidence that the note is valid and can cover the requested spend. Current documentation says the browser SDK runs its prover in WebAssembly (WASM), keeping the note secrets on the client in the described flow.
- Obtain a bounded API lease. The authorization server verifies the proof, checks active protocol state and, if the checks pass, issues a short-lived API key with a spending cap. In the current v2 documentation, leases are for OpenRouter and use native-ETH billing quotes.
- Send requests and record usage. With a direct runtime-key approach, the client sends prompts to the provider rather than through the zkAPI relay. The provider records metered use in a signed receipt, and the server settles measured usage against the private balance. The Foundation also describes a proxy mode: in that mode, zkAPI relays the request and can see its traffic.
- Withdraw or use recovery paths. The project documentation describes on-chain vault withdrawals and escape or challenge mechanisms. Those are vault and protocol operations, not proof creation or API-key issuance.
The lease is the service’s permission for a particular API use under its rules. The proof supports the authorization check; the vault separately handles funding, settlement state and withdrawal. Treating those as one “payment authorization” obscures what each component does.
Who can see what
Privacy depends on which party is being discussed and which operating mode is used. The intended separation in the direct flow does not mean that every party is blind to every part of a transaction.
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| Party | What it can learn | What the design aims to conceal or does not hide |
|---|---|---|
| zkAPI authorization server | Whether a proof is valid, plus session spend and settlement information needed to issue and manage a lease. | In the described direct design, it does not receive prompts and should not learn which note or deposit authorized a spend. |
| Inference provider | The prompts, responses and ordinary request context needed to serve the API request. | The billing identity behind the lease is intended to be hidden. Prompt content, repeated context and writing patterns may still make sessions correlatable. |
| Ethereum public chain | Public vault activity such as deposits, closes and withdrawals. | The launch description says the chain does not show what a balance paid for. Funding and exit transactions themselves are not hidden. |
| Network observers and services | IP address and timing metadata may remain observable. | zkAPI is not network anonymity. The Foundation suggests Tor with a fresh circuit as an additional layer for users who need it. |
The privacy boundary also changes in proxy mode: because the relay handles the request, it can see traffic that the direct runtime-key path sends from the client to the provider. Neither mode conceals prompts from the provider that performs inference.
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A proof answers a verification question: does the claimant satisfy the stated conditions without exposing the private witness? In zkAPI, verification supports the claim that a spend is valid under the protocol. It is not an Ethereum transaction, does not itself debit a vault, and does not independently grant access to an API.
Several separate decisions and actions follow. The server must accept the proof and issue a lease; the provider must accept requests under that lease; measured usage must be recorded and settled; and any funding or withdrawal on Ethereum requires the relevant vault operation. A proof is therefore evidence used within an authorization flow, not a stand-alone payment instruction.
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A useful, limited analogy is a verifiable ticket that shows a bounded charge can be covered. The lease is the service’s permission to use an API, and the vault is where funds and withdrawals are managed. The analogy does not describe every protocol detail, but it keeps the proof, service permission and funds movement distinct.
Asset and provider claims depend on the version
Descriptions of what zkAPI supports are not fully interchangeable across the launch announcement and the current protocol documentation. The Ethereum Foundation’s October 1, 2026 post describes ETH, USDC and other credits, and says the vault holds USDC. The Open Anonymity Project’s current v2 documentation, accessed October 2, 2026, instead describes native ETH funding and OpenRouter leases.
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| Source and date | Funding or asset description | Provider or scope described |
|---|---|---|
| Ethereum Foundation launch post, October 1, 2026 | ETH, USDC and other credits; the post says the vault holds USDC. | Introduces the system as private usage credits for APIs and describes both direct and proxy approaches. |
| Open Anonymity Project protocol documentation, accessed October 2, 2026 | Current v2 documentation specifies native ETH. | Documents v2 leases for OpenRouter against native-ETH billing quotes. |
Do not combine these descriptions into a claim that every asset or provider is available in every version. The version, asset and provider supported by a particular deployment must be checked against its own documentation.
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Cryptography, setup assumptions and project status
The current documentation identifies protocol v2 circuit revision zkapi-v2-note-bound-v1. It specifies Groth16 over BN254, Poseidon hashes, Baby-JubJub commitments, Schnorr signatures and a 32-level Merkle tree. The API reference gives an encoded proof size of 256 bytes; the reference does not establish that this figure predicts total request overhead, latency or operating cost.
The protocol depends on compatible circuit, proving and verifying artifacts, vault deployment and server keys. A deployment using mismatched artifacts or keys will not implement the documented protocol consistently. The project says its checked-in proving keys use a single-party setup without a multiparty ceremony, and that the construction is not post-quantum. These are material trust and cryptographic limitations.
The repository labels zkAPI experimental. The reviewed project materials do not establish an independent security audit, and they provide no independent performance or security benchmark. A listed withdrawal or escape mechanism should not be mistaken for evidence that the system has been audited or that every recovery scenario has been independently validated.
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Potential uses and what is not established
The Foundation lists AI chat and agents, blockchain RPC, image and video generation, VPNs and bandwidth, and machine-to-machine services as possible metered applications. These are suggested use cases, not evidence of widespread third-party adoption. The reviewed sources establish no user count, adoption ranking, cost savings, throughput result or independent performance comparison.
For a reader evaluating a real deployment, the practical questions are whether the desired provider and asset are supported by that version, whether the direct or proxy mode fits the privacy requirement, and whether the operator’s setup and recovery assumptions are acceptable. The proof’s validity alone does not answer those operational questions.
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