Valid JSON, broken discovery: the UCP checks that should fail your build

If your /.well-known/ucp file is valid JSON but Google AI Mode or other UCP agents still cannot use it, you do not have a formatting problem. You have a discovery problem. That distinction matters. In our Apr 1, 2026 scan of 28 stores, 26 were broken or missing and 2 were unreachable. That means 92% failed before richer commerce capabilities even mattered. We also found 0 fully healthy profiles. The pattern was consistent: merchants are debating payment rails and optional capability coverage while basic UCP discovery is still broken. This article covers the UCP checks that should fail your build - not warn, not get buried in a dashboard, but stop a deploy until fixed. Discovery comes before checkout UCP and ACP solve different problems. UCP is how Google AI Mode, Gemini, and other UCP-compatible agents discover a merchant and learn which commerce services it exposes. ACP is OpenAI and Stripe's protocol for delegated payment and ChatGPT commerce flows. So the implementation order for mo