Uswds MCP Server

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)

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

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.