zkAPI brings privacy-focused API billing to Ethereum mainnet

The Ethereum Foundation and Open Anonymity Project introduced zkAPI on October 1, making an experimental system for privately funding metered API use available on Ethereum mainnet. Instead of tying every request to a conventional account and payment method, the software lets a user deposit funds into a vault and prove that a private balance can cover subsequent usage.

The release matters beyond artificial-intelligence services. Its architecture attempts to make Ethereum a settlement layer for repeated digital-service purchases without placing each request onchain or exposing which deposit funded a particular session. The developers identify model inference as the first use case, while describing potential applications for blockchain data queries, media generation, bandwidth and machine-to-machine services.

Payment and requests take separate paths

Under the current workflow, a user funds a private note and generates a zero-knowledge proof locally when requesting service. The billing server verifies that proof, checks that sufficient value is available and issues a short-lived, spending-capped credential. In the direct-key mode, prompts travel from the user’s device to the inference provider rather than through the billing server.

After the credential expires, the provider records actual usage in a signed receipt. The billing system applies that charge to the private balance and returns a newly signed state. The public protocol documentation says no funds move onchain for each request; the vault is used for funding and eventual settlement. Unused value can be withdrawn through a normal close or an escape process if the operator does not cooperate.

The cryptographic design uses commitments to represent deposits, proofs to demonstrate authorization and nullifiers to stop the same private state from being spent twice. The active repository describes Groth16 proofs, note-bound commitments and a Merkle-tree ledger. Those mechanisms are intended to prove solvency without showing the billing server which public deposit belongs to the requester.

Privacy stops short of the request itself

The system separates payment identity from usage, but it does not encrypt information from the company performing the requested computation. An AI provider still receives prompts and produces responses. It can also observe network information such as an internet address and may correlate sessions through timing, writing style, repeated context or other details contained in the requests.

The distinction makes “privacy-focused billing” more accurate than anonymous or confidential inference. The payment server is designed not to see prompts in direct-key mode, while the inference provider is designed not to learn the funding identity. Proxy mode is easier to operate but gives the relay visibility into traffic, adding another privacy tradeoff.

The implementation remains experimental

The active documentation describes the current browser system as settling in native ETH. The Ethereum Foundation’s launch post also discusses ETH, USDC and other assets as possible credits and links to a USDC vault, so broader asset support should not be assumed to apply uniformly across the current version and every interface.

The repository explicitly labels the protocol experimental. Its documentation says the checked-in proving setup was produced by a single party, without a multiparty ceremony, and that the elliptic-curve construction is not post-quantum. Matching circuit files, proving keys, verifier contracts and client configuration are therefore material security dependencies.

Repository acceptance testing covers deposits, authorization, settlement and withdrawals, but its documented end-to-end process uses a mocked provider and price oracle. That testing supports the existence of the implementation; it does not establish production capacity, adoption or independent security assurance.

None of the reviewed launch materials supplied a transaction count, funded-value total, paying-user count, API-volume measure or external audit result for the mainnet release. The defensible conclusion on October 2 is consequently narrow: zkAPI is live as an experimental Ethereum-based billing system with meaningful payment-link privacy, not a demonstrated replacement for conventional API accounts or a complete shield for request contents.