Assay
Assay is a paid-probe rating service for x402 services, published by Nominal Labs. Its own site describes it as spending real USDC buying from machine-payable services, checking what comes back, and publishing the scores with an on-chain receipt for each rating.
Assay pays the services it rates: its site states that it spends USDC buying from machine-payable services and publishes a score for each. Every rating is described as carrying an on-chain receipt, so the payment behind a score can be found on the chain it settled on.
Its site names the wallet it pays from, which is what lets probe402 attribute its payments without inference. A payment made to test whether a route answers is a payment for which any route would have done, and that is the sense in which probe402 lists it as a grader rather than as a buyer.
→ probe402 names the graders whose payments do not count toward its own demand reading on the method page, and this is one of them.
Sources
Related buyer · payment challenge · USDC · settlement