Arc is a library of React components and blocks with motion built in. The core is free and open source (MIT) and installs with the shadcn CLI, so the code lives in your project and you can change anything. Every component has Markdown docs, llms.txt and an MCP server, so Claude Code or Cursor can pick and install the right piece instead of inventing one. Pro adds showpiece motion components and full app screens. New components ship every day.