viewpo-mcp

MCP server that gives AI coding assistants eyes into responsive viewport rendering via the Viewpo macOS app.

Capture multi-viewport screenshots, extract DOM layout trees, and compare responsive behaviour across breakpoints — all from your AI assistant.

What it does

AI coding assistants (Claude Code, Cursor, Windsurf, etc.) are blind when working on frontend UI. They write CSS and HTML but can't see the visual result. This creates a slow feedback loop:

AI writes code → you check → you describe what's wrong → AI fixes → repeat

viewpo-mcp closes that loop. The AI can now:

1. Screenshot a URL at multiple viewport widths simultaneously
2. Extract the DOM layout tree with bounding rects and computed styles
3. Compare layouts at two different viewport widths to find responsive issues

Requirements

• Viewpo macOS app with MCP Bridge enabled (Settings → MCP Bridge → Start)
• Node.js 20+

Setup

1. Install

npm install -g viewpo-mcp

Or run directly with npx (no install needed):

npx viewpo-mcp

2. Enable the MCP Bridge in Viewpo

1. Open Viewpo on your Mac
2. Go to Settings → MCP Bridge
3. Click Start to enable the bridge server
4. Copy the auth token (click the copy button next to the token)

3. Configure your AI assistant

Claude Code

Add to your MCP config (~/.claude/mcp.json or project .mcp.json):

{
  "mcpServers": {
    "viewpo": {
      "command": "npx",
      "args": ["-y", "viewpo-mcp"],
      "env": {
        "VIEWPO_AUTH_TOKEN": "<paste token from step 2>"
      }
    }
  }
}

Cursor

Add to your Cursor MCP settings (.cursor/mcp.json):

{
  "mcpServers": {
    "viewpo": {
      "command": "npx",
      "args": ["-y", "viewpo-mcp"],
      "env": {
        "VIEWPO_AUTH_TOKEN": "<paste token from step 2>"
      }
    }
  }
}

Other MCP-compatible assistants

Any assistant supporting the Model Context Protocol can use viewpo-mcp. Set the command to npx -y viewpo-mcp and provide the VIEWPO_AUTH_TOKEN environment variable.

Tools

viewpo_screenshot

Capture screenshots of a URL at one or more viewport widths. Returns base64 JPEG images.

url:       "https://example.com"           (required)
viewports: [{ width: 375, name: "phone" }, (optional, defaults to 1920px desktop)
            { width: 1920, name: "desktop" }]

Common widths: 375 (phone), 820 (tablet), 1920 (desktop).

viewpo_get_layout_map

Extract the DOM layout tree at a given viewport width. Returns element hierarchy with tags, classes, bounding rects, and computed CSS styles.

url:      "https://example.com"    (required)
viewport: 1920                     (optional, default 1920)
selector: ".main-content"          (optional, scope to subtree)

viewpo_compare_viewports

Compare the layout of a URL at two different viewport widths. Returns elements whose size or CSS styles differ between the two viewports.

url:        "https://example.com"  (required)
viewport_a: 375                    (required)
viewport_b: 1920                   (required)
selector:   ".hero"                (optional, scope to subtree)

Environment variables

| Variable | Required | Default | Description | |----------|----------|---------|-------------| | VIEWPO_AUTH_TOKEN | Yes | — | Bearer token from Viewpo Settings → MCP Bridge | | VIEWPO_PORT | No | 9847 | Port the Viewpo bridge server listens on |

How it works

AI Assistant (Claude Code / Cursor / Windsurf)
    | stdio (MCP protocol)
    v
viewpo-mcp (this package)
    | HTTP localhost:9847
    v
Viewpo macOS app (NWListener)
    | WKWebView rendering
    v
Headless browser pool (off-screen, up to 4 concurrent pages)

The Viewpo app runs a local HTTP server on localhost:9847. This MCP server translates tool calls into HTTP requests to that bridge. The app loads pages in headless WKWebView instances with real CSS viewport simulation — media queries fire at the target width, not the physical screen width.

Key advantage: WKWebView bypasses X-Frame-Options entirely. Any website loads, regardless of security headers that would block iframe-based tools.

Example workflow

You: "The hero section looks broken on mobile"

AI:  1. viewpo_screenshot("https://preview.example.com", [{width: 375}, {width: 1920}])
        → Sees the layout at both sizes
     2. viewpo_compare_viewports("https://preview.example.com", 375, 1920)
        → Gets structured diff: ".hero img" is 1200px wide on mobile (overflowing)
     3. Fixes the CSS
     4. viewpo_screenshot("https://preview.example.com", [{width: 375}])
        → Verifies the fix

Development

git clone https://github.com/littlebearapps/viewpo-mcp.git
cd viewpo-mcp
npm install
npm run build     # compile TypeScript
npm run dev       # watch mode

Licence

MIT - see LICENSE.

Links

• Viewpo app - The macOS/iOS app
• Model Context Protocol - MCP specification
• GitHub - Source code