You ship a UCP manifest. The validator returns green. The schema parses cleanly. Every required field is present, every URL resolves, every transport responds. You declare the work done and move on. Three weeks later, you find out your store has been quietly failing every agent shopping session. The cart endpoint accepts adds but rejects checkouts. A specific variant ID throws a 400 on update_cart . The agent reaches ready_for_complete and stalls because your payment handler doesn't recognise the token format. None of these issues showed up in static validation. All of them block real users on agent-mediated flows. This post is about how to actually test your UCP implementation — not as a schema document, but as a runtime surface that real frontier agents have to operate against. The short version: schema validation is necessary but not sufficient . The long version is the rest of this post.…