UCP v2026-04-08 Spec Update

On April 9th, the UCP Technical Council shipped v2026-04-08 — the first spec bump since January's 2026-01-23 release. It's the largest single release in the protocol's history: 26 new features, 6 breaking changes, 19 documentation updates, and contributions from 15 first-time contributors. This isn't a patch. It's the release where UCP stops being a checkout-and-order protocol and starts becoming a full commerce platform. Here's what changed, what it means, and what you should do about it. The headline features Carts are now a first-class capability The most consequential addition is formal cart support ( #73 ). The dev.ucp.shopping.cart capability gives agents the ability to create, read, update, and manage persistent shopping carts — the workflow that dominates human e-commerce but has been entirely absent from agent commerce until now. We wrote in March that only 2 out of 2,832 verified stores declared cart capabilities. That number was low because the spec itself hadn't formalized