UCP Cart, Catalog, and Identity Linking: What to Validate (and What Can Wait)

Google shipped the first major UCP update on March 19, 2026 - three new capabilities: Cart , Catalog , and Identity Linking . If you're a merchant or platform developer implementing UCP, you now have more to validate. But here's the thing most guides won't tell you: not all three capabilities have the same stability level. Cart and Catalog are draft specs. Identity Linking is stable. Treating them the same way in your validation pipeline is a mistake. This post breaks down what changed, what to validate strictly, what to treat as warnings, and provides a minimal checklist for keeping your /.well-known/ucp profile working. What Changed on March 19 Capability Namespace Status What It Does Cart dev.ucp.shopping.cart Draft Multi-item basket management before checkout Catalog dev.ucp.shopping.catalog Draft Real-time product search, variants, pricing, stock Identity Linking Part of core spec Stable OAuth 2.0 account linking for loyalty/member pricing Cart and Catalog are published under ucp.