The old model for selling data was a subscription. You paid once a month and queried as much as you wanted. That assumes a person is deciding what to buy. Agents change the shape of the transaction, because they pay for each question on their own, at a price agreed for that call.
That only works if the payment rail is built for software.
x402
x402 takes an old idea and makes it usable. The HTTP status "402 Payment Required" becomes a real flow. A request arrives, the server responds with a price, the client pays, and the request completes. There are no accounts to set up and no invoices to reconcile. The payment happens inside the same request and response the client was already making.
For an agent, this fits well. It can learn the price, decide, and pay in one exchange.
MPP
MPP covers the wider path for machine-to-machine payment. It defines how a client settles an agreed amount for a specific piece of work. Combined with an up-front quote, it turns a data request into a clean transaction with an agreed price and a settled payment.
Why per-request pricing fits agents
A subscription cannot say that one specific answer is worth one specific amount. Per-request pricing can. The agent asks, receives a quote, and pays for exactly what it uses. Testril supports x402 and MPP so that exchange works without extra steps, which is why we price every request before it runs.