coss.com/ui is the official design system for Cal.com, a collection of React components built on Base UI primitives and styled with Tailwind CSS.
The project solves the problem of building consistent, accessible user interfaces by providing a curated set of composable components ready to use out of the box. Rather than starting from unstyled primitives, developers get Base UI's powerful foundations paired with an opinionated design system. The approach emphasizes copy-paste ownership, allowing teams to customize components for their own needs rather than being locked into a monolithic library.
Teams should adopt this tool if they are building React applications and want a modern component foundation with accessibility built in. It suits projects that value design consistency and the flexibility to modify components without waiting for upstream releases. The project includes a legacy Origin UI collection based on Radix and shadcn patterns, available for use but with limited ongoing maintenance as development focuses on newer Particles components built on the coss ui primitives.
The repository is organized as a TypeScript monorepo using Turborepo, containing multiple applications and packages including the main component library with documentation, the coss.com website, and legacy Origin UI components. Development tooling includes Biome for linting and formatting, with environment variable configuration required for local development across the linked Next.js applications. The project actively welcomes contributions through documented guidelines and is building components in the open.
