Swatchdog MCP Server

Give your AI agents a visual system guide so they write brand-consistent styles instead of guessing color values, radii, and layouts.

See Swatchdog in Action

Tested live: claude.ai connector run →

Full test transcript, all green: part-b-transcript.txt

https://github.com/user-attachments/assets/6ae8b242-d6c7-4a29-8892-27b067835308

Swatchdog

Swatchdog is a Model Context Protocol (MCP) server that runs on-demand CSS design-token drift checks for AI coding assistants (Claude Code, Claude Desktop, Cursor, Google Antigravity, and other MCP-aware clients — plus claude.ai via a connector).

When your agent asks, Swatchdog checks the generated CSS against a chosen design pack — or your own tokens — and returns the off-token color, radius, spacing, and type values with the closest matching token to use instead. The check is on-demand and the loop is yours: Swatchdog reports drift; it does not intercept writes or change your code.

Why Swatchdog?

AI coding agents are fast, but blind to your design system — they invent off-brand spacing, off-palette colors, and ignore your scales.

Swatchdog adds an on-demand drift check to the agentic loop. When connected:

1. The agent detects your design tokens (or you pick a pack).
2. The agent calls Swatchdog to check the CSS it generated.
3. Swatchdog returns precise suggestions (e.g., "Use radius.sm (5px) instead of 7px").
4. The agent applies the fixes on a new Git branch for your review.

Core Features

1. Pack Mode (Preset Checks)

Check your styles against a curated Swatchdog design family — Workbench, Showcase, Terminal, plus the free Studio sandbox. Fast, zero-configuration checks.

2. BYO Mode (Bring Your Own Tokens)

Check styles against your project's own custom design system.

• The agent extracts tokens locally from files like tailwind.config.js or your CSS variables and passes them as parameters.
• Content-only and stateless — your source code and custom tokens are never stored.
• Free to try on the connector endpoint (no key — shared, rate-capped lane), or uncapped with any paid key, including the standalone $12 drift-check license.

3. Connector Endpoint (keyless) — new

A dedicated, trimmed BYO-only surface built for chat clients:

• One read-only tool — check_design_drift(reference_tokens, code) — returning a compact, machine-first report sized for an agent's fix-and-recheck loop.
• No key required: keyless calls ride a shared free lane (best-effort, rate-capped with a clean 429 + Retry-After at capacity). A paid key in the Authorization header gets its own uncapped lane.
• In claude.ai: add it as a custom connector (Settings → Connectors → Add custom connector, URL below, no credentials).

4. Intelligent Drift Suggestions

Swatchdog doesn't just flag drift; it maps each off-token value to the closest valid token:

• Color: off-palette hex/rgb → the nearest theme token.
• Radius: off-scale border-radii → the nearest radius token.
• Spacing & Typography: off-scale padding/margin, font-size, and font-family.

Note: checks currently cover hex and standard color formats. HSL-channel representation and complex multi-file token resolution are planned for Phase 2.

Supported tools

check_drift — main endpoint. Checks a CSS string against a pack or a custom token set.

• content (string, required) — the CSS/markup to check.
• paletteId (string, optional) — a pack id (e.g. studio-blue-hour). Pack mode.
• tokens (object, optional) — your own token set. BYO mode (any paid key).
• source (string, optional) — telemetry tag (pack, css, tailwind).

check_design_drift — connector endpoint. BYO-only, keyless-friendly.

• reference_tokens (object, required) — your design tokens (e.g. {"color":{"primary":"#b06ed0"},"radius":{"md":"6px"}}).
• code (string, required) — the CSS/markup to check.
• Returns per-violation: axis · found value · expected token + value · location.

Access & pricing (one-time, no subscriptions)

If a free caller attempts a premium or BYO check on the main endpoint, the tool returns a structured JSON upgrade payload with where to get a key — at swatchdog.dev.

Privacy & telemetry

All checks are on-demand and transient.

No source code, files, or custom tokens are ever stored on our servers. We log minimal usage metadata — the source tag, which pack, and the finding outcome, tied to the account the key belongs to — never your license key, your source code, or your tokens.

Created and maintained by swatchdog.dev · privacy policy: swatchdog.dev/privacy.html · support: hey@swatchdog.dev

Client Integration

Main endpoint (packs + BYO, bearer key — the free sandbox key shown):

{
  "mcpServers": {
    "swatchdog-sandbox": {
      "type": "http",
      "url": "https://swatchdog-mcp-970396648818.us-central1.run.app/mcp",
      "headers": { "Authorization": "Bearer swt_sandbox_studio" }
    }
  }
}

Connector endpoint (In process: Pending Connector Submission Approval BYO-only, no key needed — add a key to remove the rate cap):

{
  "mcpServers": {
    "swatchdog-check": {
      "type": "http",
      "url": "https://swatchdog-connector-970396648818.us-central1.run.app/mcp"
    }
  }
}

[In process: Pending connector submission approval] In claude.ai: Settings → Connectors → Add custom connector → paste the connector URL above — no credentials needed.