Catalog Endpoints Do Not Establish a Checkout Contract for AI Agents

A product catalog can show an agent what items exist but grants no ability to purchase them, and this boundary is often overlooked in integrations. A QA run of one gateway found five store actions, covering product search and operations on existing orders, with no order-creation or checkout action discovered and no payment handlers advertised. The author argues this supports only a narrow conclusion about discovered capabilities, not whether purchases are possible or safe to add. Real checkout requires much more than product lookup, including cart construction, price freshness, payment authorization, confirmation, and idempotency. The author recommends that discovery separate read and mutation capabilities, enforce these boundaries server-side, and test both advertised and absent actions.