pay-per-call API

MPP gives the shapes names, which is more useful than it sounds. It defines a charge as a one-time payment; a session as metered pay-as-you-go, for usage that is frequent or unpredictable; and a subscription as a recurring fixed payment, for access that renews on a schedule. Pay-per-call is the charge shape, and naming the three is what lets an operator see they are choosing between them.

What makes it work over HTTP is that the price is part of the ordinary exchange. The endpoint answers an unpaid request with a payment challenge naming what it costs; the client pays and retries; the resource is served. No account exists before the call, and the alternative — account creation, subscription selection and payment entry — is the sequence Stripe's machine-payments documentation names as what blocks agents from completing tasks autonomously.

The shape is being applied outside APIs as well. Cloudflare's pay per crawl prices access to content per fetch, with the price in the response to an unpaid crawler — the same exchange, with a page where the API call would be.