Unofficial MCP Server for USWDS

uswds-mcp

Make AI-generated government UI actually USWDS-compliant.

An MCP server that gives Claude, Cursor, Copilot, Windsurf and any other MCP client the official U.S. Web Design System markup, a validator that knows every real USWDS class, and a page composer that outputs accessible, validated pages.

Unofficial and independent. Not affiliated with, endorsed by, or maintained by GSA, TTS, or the official USWDS team. See NOTICE.md.

The problem

LLMs write USWDS from memory. They invent usa-btn, forget the usa-overlay that makes the mobile menu work, put usa-card markup without its container, and wire up accordions to nothing. The result looks plausible and is subtly broken.

uswds-mcp closes that loop:

recommend_uswds_structure  →  get_component_markup  →  compose_uswds_page  →  validate_uswds_markup
        (what to use)          (official HTML)         (accessible page)       (fix until 0 errors)

Without	With uswds-mcp
Class names recalled from memory	Official HTML rendered from the @uswds/uswds templates (178 snippets, ~70 components and page templates, every variant)
"Looks right"	Validator rejects any usa-* class that is not in the USWDS stylesheet, with a did-you-mean
Missing structure goes unnoticed	BEM, required children, ARIA wiring, form control structure, header overlay, scripts, heading order, duplicate ids, link/image/button names
Stock template output	compose_uswds_page builds pages from your real sections; zero axe-core violations in CI

What a validator finding looks like

{
  "severity": "error",
  "rule": "unknown-class",
  "message": "\"usa-alrt\" is not a class defined by USWDS.",
  "selector": "div.usa-alrt",
  "snippet": "<div class=\"usa-alrt usa-alert--eror\">",
  "suggestion": "Did you mean \"usa-alert\"?"
}

Findings carry the offending element, a fix suggestion, the component to look up, and a docs link, so the model can repair its own output.

Quick start

npx -y uswds-mcp

Add it to your MCP client:

{
  "mcpServers": {
    "uswds": { "command": "npx", "args": ["-y", "uswds-mcp"] }
  }
}

Claude Code: claude mcp add uswds -- npx -y uswds-mcp

Setup for Claude Desktop, Cursor, VS Code, Windsurf and others: docs/CLIENTS.md. Ready-made configs are in examples/.

No network, API key, or ingest step needed: the data ships in the package.

Tools

Tool	Use
get_component_markup	Official HTML for a component/page template and its variants (accordion, footer + slim, sign-in, ...). Understands everyday names (dropdown → select).
compose_uswds_page	Build a full page from structured sections: hero, content, alert, summary box, cards, process list, step indicator, accordion, table, form, contact.
validate_uswds_markup	Validate fragments or full pages: real class names, component structure, forms, ARIA, accessibility basics.
find_uswds_classes	Look up real classes, including utilities and responsive variants (margin top 2, tablet grid col 6).
generate_uswds_page	Quick start from free-text requirements; returns an editable section spec, bracketed placeholders, and its own validation.
recommend_uswds_structure	USWDS-first structure for a service or page.
search_uswds	Search docs, accessibility and usage guidance (with synonym expansion).
get_component / get_pattern / get_template	Structured guidance, plus the canonical markup for components.
get_uswds_integration_recipe	Framework setup for Next.js, Vite/React, static HTML, Rails, Drupal.
validate_uswds_project_setup	Catch wrong CSS import paths, missing scripts, CDN use, copied dist, global CSS risk.

All tools are read-only. See docs/TOOLS.md for arguments and recommended sequences.

Resources: uswds://component/{slug}, uswds://pattern/{slug}, uswds://template/{slug}, uswds://token/{category}, uswds://package/{name} Prompts: build_agency_website, build_service_page, audit_uswds_page, convert_page_to_uswds, integrate_uswds_in_project

Example

"Build a page where residents renew a fishing permit: eligibility, fees table, steps, FAQ, contact."

The model calls compose_uswds_page and gets back a complete page with banner, skipnav, header (with usa-overlay), breadcrumbs, summary box, striped table with caption and scoped headers, process list, accordion, contact details, slim footer and identifier, plus a list of the placeholders that still need real content, and a validation result.

Prefer to see it first? Run the scorecard:

npm run eval

scenario                        unknown classes  errors  warnings  axe violations
'Apply for housing assistance'  0                0       0         0
'Renew a fishing permit'        0                0       0         0
...

Does it help?

In a small test (5 tasks, same model, with and without the server) pages built with uswds-mcp had 0 validator errors on 5 of 5 pages, versus 1 of 5 without it. The model did not invent class names in either case; what it missed without the tools was structure, such as the header overlay that makes the mobile menu work, banner internals, and accordion button types. It is a small, single-run sample, so see the caveats and method in docs/COMPARISON.md.

How it works

1. Ingest (npm run ingest): renders every official @uswds/uswds twig template with its JSON fixtures into canonical HTML, extracts all class names from the official stylesheet, and indexes the official docs (uswds-site).
2. Validate against ground truth: the validator is tested against every official snippet (they must pass) and against a corpus of 30 typical LLM mistakes (they must be caught).
3. Stay current: docs and markup come from the same USWDS version (enforced by a test), and a monthly workflow re-ingests and opens a PR.

Current data: USWDS 3.14.0.

Limits

• Static analysis. It cannot judge color contrast, reading order on rendered pages, or real screen-reader behavior. USWDS components do not by themselves make a site Section 508 compliant; keep testing with axe, keyboard and screen readers.
• Custom (non-usa-) classes are allowed and not validated.
• Upstream fixture text in angle brackets (e.g. <Project title>) is placeholder content.

Develop

npm install
npm test         # unit, self-consistency over official markup, MCP end-to-end, axe-core
npm run eval
npm run ingest   # refresh from upstream

See CONTRIBUTING.md and the CHANGELOG.

License

MIT. See NOTICE.md for USWDS attribution and the licensing notes for indexed USWDS material.