Design System MCP

design-system-mcp

An MCP server that gives coding agents ground truth about your React design system, and a linter they can run on their own UI.

[CI] [npm] [License: MIT]

[An agent writes app/settings/danger-zone.tsx, calls check_ui, gets 8 errors and 3 warnings with fixes such as border-[#ef4444] to border-destructive and variant=danger to variant=destructive, applies them, and check_ui reports no problems.]

The problem

Coding agents write UI from their training data, not from your design system. Ask for a settings card in a shadcn/ui project and you get bg-[#ef4444], p-[13px], a native <button> with hand-rolled classes, variant="danger" on a Button that only knows destructive, <Card.Header> in a system that exports CardHeader, and an icon button nobody can name with a screen reader.

The agent cannot see your Storybook or docs site, and TypeScript only catches part of it after the fact. On the demo draft above, tsc reports 3 of the 11 problems (the invalid variant, the unknown prop and the missing member) and has no opinion on hex colors, off-scale spacing, native elements or accessible names.

design-system-mcp reads your components, tokens and docs, and serves them to the agent over MCP: what exists, which props and variant values are valid, which token to use. It also gives the agent check_ui, a linter it calls on its own output. Every finding has a rule id, a location and a concrete fix, so the agent can correct itself before you review anything.

Quickstart

In a shadcn/ui-style project (components/ui/*.tsx, app/globals.css) no config is needed. Give your agent the tools, check what was extracted, and run the same rules from the terminal:

claude mcp add design-system -- npx -y @dgesteves/design-system-mcpnpx @dgesteves/design-system-mcp inspectnpx @dgesteves/design-system-mcp check "app/**/*.tsx"

Other layouts take a config file. Requires Node.js 22.18 or later.

Setup

The server speaks MCP over stdio. It finds the project from --root, a config file in the working directory, or the workspace roots the client reports.

claude mcp add design-system -- npx -y @dgesteves/design-system-mcp

{  "mcpServers": {    "design-system": {      "command": "npx",      "args": ["-y", "@dgesteves/design-system-mcp"]    }  }}

{  "mcpServers": {    "design-system": {      "command": "npx",      "args": ["-y", "@dgesteves/design-system-mcp", "--root", "${workspaceFolder}"]    }  }}

{  "servers": {    "design-system": {      "type": "stdio",      "command": "npx",      "args": ["-y", "@dgesteves/design-system-mcp", "--root", "${workspaceFolder}"]    }  }}

Any other MCP client: run npx -y @dgesteves/design-system-mcp --root /path/to/app as a stdio server. On native Windows, wrap it as cmd /c npx ....

The server sends usage instructions during the MCP handshake. Clients that ignore them benefit from one line in CLAUDE.md, AGENTS.md or .cursor/rules: "Before writing UI, use the design-system tools. Run check_ui on every file you change and fix all errors."

Tools

All tools are read-only, have zod-validated input schemas with size limits (up to 1,000,000 characters of code for check_ui), and return compact Markdown for the model plus JSON structuredContent (with an output schema) for programs.

Resources: ds://components/{name} (Markdown, with name completion) and ds://tokens (JSON). Prompt: build-with-design-system, which takes a task and walks the agent through search, contract, tokens and check_ui. Claude Code exposes it as /mcp__design-system__build-with-design-system.

What the agent sees, from the demo:

> get_component { "name": "Badge" }
# BadgeA small status label: counts, states ("Active", "Overdue") or categories.
import { Badge } from "@/components/ui/badge"Renders <span> · components/ui/badge.tsx:29 · docs: docs/badge.md
## Props- variant?: "default" | "secondary" | "destructive" | "success" | "outline" = "default"- asChild?: boolean = false — Render the child element with badge styles instead of a `<span>`.- …plus 280 props from React.ComponentProps<"span"> (onClick, id, role, children, aria-*, data-*, …)
## Variantsvariant (default "default")  default      border-transparent bg-primary text-primary-foreground  destructive  border-transparent bg-destructive text-white  success      border-transparent bg-success text-success-foreground  …

> check_ui { "code": "<Button variant=\"primary\" className=\"bg-blue-600 px-[18px]\">Save</Button>" }
snippet.tsx: 2 errors, 1 warning
1:17 error [no-unknown-variant] "primary" is not a valid variant for <Button>.     Allowed: default, destructive, outline, secondary, ghost, link. Did you mean "default"?1:38 error [no-hardcoded-color] `bg-blue-600` is Tailwind's default palette, not a design-system     color. No close token; nearest is muted-foreground (ΔE 0.245). Pick the semantic token that     fits. <Button> already sets bg-* through `variant`; prefer a variant over overriding it.1:50 warning [no-hardcoded-spacing] `px-[18px]` is 18px, which is on the spacing scale: use     `px-4.5`.

Rules

Color matches under ΔE 0.02 count as the same color; under 0.1 the fix is offered; beyond that the message names the nearest token but leaves the choice to the agent. Likewise, a spacing or radius step that is off by more than half the value (and more than 4px) is suggested but not auto-fixed. Rules that need tokens are skipped when the design system defines none of that category; a stylesheet that imports tailwindcss brings Tailwind's default spacing unit and radius scale, unless the theme resets that namespace (--spacing-*: initial, --radius-*: initial), in which case only the project's own steps are suggested. Syntax errors are reported as syntax.

Configuration

design-system-mcp.config.json (or .ts, .mjs, .js) in the project root. Every field is optional; the JSON Schema gives editor completion.

{  "$schema": "https://unpkg.com/@dgesteves/design-system-mcp/schema.json",  "components": ["src/components/**/*.tsx"],  "exclude": ["**/*.stories.tsx", "**/*.test.tsx"],  "tokens": ["src/styles/globals.css", { "path": "tokens/*.tokens.json", "prefix": "acme" }],  "docs": ["docs/components/**/*.mdx"],  "importPath": "@acme/ui",  "elements": { "a": "Link" },  "rules": {    "no-hardcoded-spacing": "error",    "no-hardcoded-color": ["error", { "allow": ["#fff"] }],    "icon-button-accessible-name": "off"  }}

Paths and globs are relative to the root and use forward slashes. Windows-style backslashes (components\ui\**\*.tsx, .\tsconfig.app.json) are read as separators, except in a pattern that already uses /, where \ escapes glob syntax (app/\(marketing\)/**). A tsconfig that does not exist is a config error rather than a silent fallback.

Tokens can be W3C DTCG JSON ($type inheritance, aliases, object color and dimension values, $deprecated, modes under $extensions.modes) or CSS custom properties: :root values, .dark / [data-theme] / prefers-color-scheme / @variant dark modes, and Tailwind v4 @theme mappings, with calc() evaluated. Token stylesheets are read as one theme, so .dark can live in its own file; without a :root block the light mode is the base, and a dark mode never is. A DTCG file and the CSS generated from it are merged by custom property.

elements maps a native element to the component that replaces it ({ "a": "Link" }, or a key such as input[type=checkbox] for a non-text input type); a mapped component is treated as a drop-in, so the rename is auto-fixed.

Docs are Markdown or MDX, one file per component, matched by component: frontmatter, the first heading or the file name. Fenced tsx/jsx blocks become examples (title="..." in the fence names them); JSDoc @example tags work too.

CLI flags override the file: --root, --config, --components, --tokens, --docs (repeatable), --no-cache, --no-watch. Run design-system-mcp --help for the rest.

CI

check runs the same rules as check_ui and exits 1 on errors (or on more than --max-warnings warnings):

- run: npx @dgesteves/design-system-mcp check "app/**/*.tsx" "src/**/*.tsx" --format github

--format github prints workflow commands, so findings show up as annotations on the pull request. --format json prints the raw results.

How it works

[Sources (components, tokens, docs, config) flow through the TypeScript checker, a cva parser, token parsers and a docs parser into a cached design-system model with a BM25 index, an OKLCH token index and the lint rules, served by an MCP server over stdio and by the check CLI.]

1. Components. One TypeScript program over the component files, with the project's tsconfig (so path aliases and dependency types resolve). For each exported PascalCase function, forwardRef, memo or class component, and each alias of a library component (const Dialog = DialogPrimitive.Root), the checker gives the props type. Props declared in the project, or by packages such as Radix, are listed with types, required flags, defaults (from destructuring, @default or defaultVariants) and JSDoc. React's DOM attributes are summarised as "…plus 290 props from React.ComponentProps<"button">" but kept in full for linting. When dependency types are missing, extraction falls back to what resolves and marks the props as open, so the linter does not guess.
2. Variants. cva() and tv() calls are read from the AST: values in declaration order, defaults, per-value classes and compound variants. They are linked to a component through VariantProps<typeof x> or a call in its body.
3. Composition. Flat parts (CardHeader next to Card in card.tsx), static members (Card.Header = CardHeader) and Object.assign(Root, { List }) become parent/part relationships. The wrapped native element comes from ComponentProps<"button">, ButtonHTMLAttributes<HTMLButtonElement>, the forwardRef element type, or the rendered JSX (including const Comp = asChild ? Slot : "button").
4. Model. Components, tokens and docs form one JSON model, cached in node_modules/.cache/design-system-mcp and keyed on the sizes and mtimes of the component, token and docs files, the project files the components import and the tsconfig chain, plus the lockfile, the config and the package version. The server answers the MCP handshake immediately and loads in the background; requests wait for the load. File changes trigger a rebuild that reuses the previous TypeScript program, an edited config file is read again, and clients are notified that resources changed.
5. Lint. check_ui parses the snippet on its own (no type-checking), resolves each JSX tag through its imports (named, default, namespace, relative, and barrels such as @/components/ui or ../components/ui) to a design-system component, and runs the rules against the model. Fixes are text edits with offsets, so an agent or a tool can apply them mechanically.

Design decisions

Static analysis, not another model. The agent is already the LLM; what it lacks is ground truth. Everything here is deterministic, takes milliseconds, runs offline, needs no API key, and can be unit-tested rule by rule. The cost: it cannot judge intent, such as whether a Dialog was the right call. That stays with the agent and the reviewer.

The TypeScript checker over react-docgen-typescript. react-docgen-typescript wraps the same API but hides the AST, and cva() parsing, composition and element inference all need it. One program serves all four. The runtime dependency is TypeScript 6, the last release with the JavaScript compiler API; TypeScript 7 (the Go port) does not expose a stable one yet.

Syntactic linting. Agents check fragments they have not saved, often without imports. Type-checking those would need the whole program and would fail on the fragment's missing context. check_ui complements tsc: it catches what types cannot express (tokens, native elements, accessible names) and says what to write instead.

BM25, not embeddings. A design system is tens to a few hundred documents whose vocabulary already lives in names, docs and variant values. Field-weighted BM25 with Porter stemming and a small synonym map ("modal" → Dialog, "delete" → destructive) ranks them well, deterministically, with no model download or API key. It does not understand paraphrases the synonym map does not cover.

OKLCH for nearest colors. Distance in OKLCH (ΔE in OKLab) tracks perceived difference, so the suggested token is the one that looks closest, not the one with the closest hex digits. It also matches how Tailwind v4 and shadcn/ui define colors.

Markdown for the model, JSON for programs. Tool results put compact Markdown in content, which costs fewer tokens than JSON and reads well to a model, and the full JSON in structuredContent for clients and scripts.

Limits

• React only. Fix suggestions are Tailwind classes when the tokens come from a Tailwind theme, otherwise var(--token).
• Linting is per file and syntactic. Class names built at runtime (`bg-${color}-500`) are not checked, and spread props are trusted.
• no-unknown-prop is skipped for components whose props type does not fully resolve (dependencies not installed).
• Composition is inferred from naming and static members; other patterns need explicit exports.
• While the server runs, it rebuilds on changes in the component, token and docs folders, the config file, the tsconfig and the tsconfigs it extends. An edit to another file the components import (a shared lib/types.ts) is picked up on the next start.
• No typography or shadow rules yet, and stdio is the only transport.

Roadmap

• Angular and Web Components extraction: signal and decorator inputs, and the Custom Elements Manifest.
• Storybook import: stories as examples, argTypes as prop docs, CSF as a docs source.
• Figma variables: import variables and modes as tokens, through the REST API or a DTCG export.
• Typography and shadow rules, an ESLint plugin that wraps the same rules, and a Streamable HTTP transport for remote agents.

Development

pnpm installpnpm test          # Vitest: extraction, tokens, every rule, search, CLI, MCP client end to endpnpm build         # tsdown, then schema.jsonpnpm smoke         # spawns dist/cli.js over stdio against the demo and calls every toolpnpm demo:check    # the CLI on examples/shadcn-demo (exits 1: the draft has errors)pnpm assets        # regenerates the images in .github/assets from real check output

Releases use Changesets: add one with pnpm changeset.

License

MIT © Diogo Esteves