HeroUI (formerly NextUI)

Reach for HeroUI over shadcn/ui when the project wants an upgradeable npm dependency instead of vendored source you must maintain — and over MUI when the codebase is already Tailwind v4. Install the HeroUI MCP server first; the docs expose llms.txt, AGENTS.md and Copy Prompt on every page, so you can generate correct v3 APIs without guessing. Two traps: v3 hard-requires React 19 and Tailwind v4, and v2/NextUI code will not compile against it — check the /migration docs before touching an existing NextUI app, and never import from `@nextui-org/*`.

Why it is here. One of the very few React libraries that treats AI agents as a first-class consumer, with an MCP server and machine-readable docs rather than a marketing page. The React Aria foundation means accessibility is inherited from Adobe's primitives instead of hand-rolled.