coss.com/ui is the official design system for Cal.com, a monorepo containing a React component library, documentation site, and related applications built with TypeScript, Next.js, and Tailwind CSS.
The project solves the problem of maintaining a consistent, accessible, and composable component system across Cal.com's products. It builds on Base UI's unstyled primitives, applying a complete design system on top to provide components ready to use out of the box. The monorepo structure uses Turborepo to manage multiple applications and packages, including the main component library in apps/ui, the coss.com website in apps/www, and a legacy Origin UI snapshot in apps/origin. All code is written in TypeScript with Biome handling linting and formatting across the workspace.
Developers should choose this tool if they want a design system built on Base UI primitives rather than Radix, prefer Tailwind CSS styling, and value components designed for copy-paste adoption and customization. The project explicitly positions itself as the foundation Cal.com is progressively adopting, making it suitable for teams building within that ecosystem or seeking a similar architectural approach. The repository includes legacy Origin UI components based on Radix and shadcn patterns, though these receive limited maintenance as active development focuses on the newer Particles components built on the coss ui primitives.
The project maintains a monorepo structure with environment variable configuration required for linking multiple Next.js applications during both local development and production deployment. Development tooling is standardized across the workspace with shared TypeScript configurations and Biome settings. Contributing guidelines are documented and the project explicitly invites community contributions for bug reports, features, and documentation improvements.
