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.

How is swatchdog different from a linter like ESLint?
A linter checks whether your code is valid and well-formatted. swatchdog checks whether your design is on-system. A linter happily passes color: #ff00ff because it's valid CSS — it has no idea magenta isn't in your palette. swatchdog catches exactly that: colors, spacing, radii, and type that have drifted from your reference, even when the code is flawless. Lint catches broken code; swatchdog catches broken design.

Can't I just give my AI agent the design tokens or a design.md?
You can, and it'll still drift. Agents approximate and interpolate even with the rules right in front of them — a tokens file or a design.md is instruction, and having the rules isn't the same as following them. swatchdog verifies conformance in the build loop, where the drift actually happens.

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: [email protected]

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.