Let agents pay without a human checkout

Accept payment inside the request itself, through a payment challenge the agent can answer or a scoped payment credential, instead of requiring a human to create an account and type a card into a checkout form.

Why

Paid APIs and services are usually sold through a human funnel: create an account, choose a plan, enter a card. Stripe's documentation names the problem: these flows require human input, which blocks agents from completing tasks autonomously. The emerging alternative is to make payment part of the protocol. With Stripe's machine payments, the server returns a payment challenge, the agent presents a valid payment credential, and Stripe settles the payment to the seller's balance. For cards, that credential is a Shared Payment Token, a scoped grant to use the customer's payment method that carries its own usage and expiration limits, so the user's spending bounds travel with it (see AX-SYS-21). The x402 protocol, run by the x402 Foundation, does the same over plain HTTP: a request without payment gets 402 Payment Required and a PAYMENT-REQUIRED header describing what to pay; the client retries with a PAYMENT-SIGNATURE header and receives the settlement in PAYMENT-RESPONSE. These protocols are young and still changing, and Stripe supports more than one, so treat them as examples rather than a settled standard. The design choice that holds is the same in all of them: the agent pays with a delegated, limited credential, never with the user's raw card details (see AX-SYS-27).

Do

Return a machine-readable payment challenge on paid endpoints and accept scoped, limited payment credentials.

Don't

Make a human signup and card form the only way to pay, or ask an agent to relay raw card numbers.

Artifact

GET /premium-data HTTP/1.1
Host: api.example.com

HTTP/1.1 402 Payment Required
PAYMENT-REQUIRED: <base64 JSON: amount, asset, network, payTo, maxTimeoutSeconds>

GET /premium-data HTTP/1.1
Host: api.example.com
PAYMENT-SIGNATURE: <base64 JSON: the signed payment payload>

HTTP/1.1 200 OK
PAYMENT-RESPONSE: <base64 JSON: success, transaction, network, payer>

Header names from the x402 v2 HTTP transport specification.