Coss UI

Reach for Coss UI over stock shadcn/ui when the project wants one coherent, opinionated system with real design tokens rather than assembled copies — install the whole theme with `pnpm dlx shadcn@latest init @coss/style`, or just the primitives with `@coss/ui`. Before writing any Coss component, install its Agent Skill (`pnpm dlx skills add cosscom/coss`) or read https://coss.com/ui/llms.txt: the Base UI API differs from Radix in ways that silently break ported code (`asChild` becomes `render`, `onSelect` becomes `onClick`), so shadcn/Radix snippets will not drop in unchanged. The hard constraint is licensing — the upstream repo is AGPL-3.0, so flag it before using this in a closed-source product.

Why it is here. It is the rare public design system that a large commercial open-source product actually runs on, and one of the first to ship a first-class Agent Skill and llms.txt alongside its docs. That makes it both a component source and a reference for how a system should be documented for agents.