Nerio is an open-source React design system that installs as editable source code, not a locked UI package. It ships 46 accessible components (WCAG 2.2 AA), 950 semantic tokens with themes, light/dark modes and density, and a CLI that adds, diffs and safely updates components without overwriting your edits. A read-only MCP server and llms.txt let AI agents discover and assemble your UI. Built on React, Tailwind CSS v4 and Base UI. MIT licensed.