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PageLens

MCP server that gives AI coding agents visual feedback on frontend apps. One command, no config — your AI agent can now see your app, click buttons, read console errors, and diff visual changes.

npx pagelens http://localhost:3000

The Problem

When using AI coding agents (Claude Code, Cursor, Windsurf) for frontend development, the agent is blind. It can edit code but can't see what the app looks like. This creates a painful manual loop:

  1. Agent makes a code change

  2. You look at the browser

  3. You describe the visual issue in words, or screenshot and drag into chat

  4. You copy console errors manually

  5. Repeat dozens of times per session

PageLens eliminates this loop entirely.

Related MCP server: Curupira

How It Works

Your app (localhost:3000)
        |
Headless Chrome (Puppeteer)
        |
PageLens MCP Server
        |
Claude Code / Cursor / Windsurf

PageLens launches a headless browser pointed at your dev server, collects console logs and network errors passively, and exposes tools that any MCP-compatible AI agent can call — screenshots, clicking, typing, DOM inspection, visual diffing — without you doing anything manually.

Why PageLens over Playwright MCP / Browser MCP?

General-purpose browser automation MCPs expose low-level primitives — you get evaluate JavaScript, take screenshot, click element as separate, disconnected actions. PageLens is purpose-built for the AI frontend development loop, which changes the design in ways that matter:

PageLens

General browser MCPs

Setup

npx pagelens <url> — zero config

Requires browser launch management, connection handling

Console/network errors

Collected passively in the background. Agent checks whenever it wants.

Agent must actively poll or set up listeners. Errors between tool calls are lost.

Interaction feedback

Every click, type, navigate automatically returns a screenshot

Agent must remember to take a screenshot after each action

Visual regression

Built-in visual_diff — baseline capture, pixel comparison, diff image, change percentage

Not available. Agent would need to screenshot, store, compare manually.

Visual quality review

visual_audit returns a structured checklist that prompts the agent to critically evaluate layout, contrast, content accuracy

No equivalent. Agents tend to superficially confirm "looks good" without guidance.

Live debugging

toggle_headless to pop open Chrome and watch the agent work in real time

Typically headless-only or requires restart

Multi-route audit

multi_route_screenshot captures multiple pages in one call

Agent must navigate and screenshot each route individually

PageLens doesn't try to be a general browser automation framework. It does one thing — give your AI coding agent eyes on your frontend — and removes every manual step from that loop.

Quick Start

1. Start your dev server

npm run dev
# App running at http://localhost:5173

2. Add PageLens to your MCP config

Note: PageLens isn't on npm yet. See the Development section to install from source, then use the config below with the local path.

Claude Code (.mcp.json in project root):

{
  "mcpServers": {
    "pagelens": {
      "command": "node",
      "args": ["/path/to/PageLens/dist/index.js", "http://localhost:5173"]
    }
  }
}

Cursor (.cursor/mcp.json):

{
  "mcpServers": {
    "pagelens": {
      "command": "node",
      "args": ["/path/to/PageLens/dist/index.js", "http://localhost:5173"]
    }
  }
}

3. Start your agent

That's it. The agent now has access to all PageLens tools. Ask it to "take a screenshot of my app" or "check for console errors" and it just works.

Tools

Observation

Tool

Description

screenshot

Capture the current page as PNG. Optional route to navigate first, fullPage for entire scrollable page.

screenshot_element

Screenshot a specific DOM element by CSS selector.

console_logs

Returns all console output (log, warn, error) since last call. Clears buffer after returning.

network_errors

Returns all failed network requests since last call. Clears buffer after returning.

visual_audit

Screenshot with a guided checklist prompting the agent to critically assess content accuracy, layout, contrast, and polish — not just confirm things render.

Interaction

Tool

Description

click

Click an element by selector. Returns screenshot after click.

type

Type text into an input field. Optional clear to replace existing content. Returns screenshot.

scroll

Scroll the page or a specific element up/down by a given number of pixels. Returns screenshot.

hover

Hover over an element by selector to trigger tooltips, dropdowns, or hover styles. Returns screenshot.

select

Select an option from a <select> dropdown by value. Returns screenshot.

navigate

Go to a URL or path. Returns screenshot of the new page.

set_viewport

Resize to a preset (mobile 375x812, tablet 768x1024, desktop 1280x720) or custom width/height. Returns screenshot.

dom_inspect

Get computed styles, classes, children, and bounding box for an element.

get_page_info

Returns the current URL, page title, viewport size, scroll position, and document dimensions.

Diffing

Tool

Description

visual_diff

Compare current page against a stored baseline. First call captures baseline, subsequent calls return a diff image with changed pixels highlighted and a percentage summary.

multi_route_screenshot

Screenshot multiple routes in one call. Returns labeled images for each route.

Debug

Tool

Description

toggle_headless

Switch between headless and visible browser. When visible, a Chrome window appears so you can watch the agent interact with your app in real time.

CLI Options

pagelens <url> [options]

Options:
  --no-headless        Show the browser window
  --viewport <preset>  Initial viewport: mobile | tablet | desktop (default: desktop)
  -h, --help           Show help

Architecture

src/
├── index.ts              # CLI entry point, arg parsing
├── server.ts             # MCP server, tool registration
├── browser.ts            # Puppeteer lifecycle, passive log/error collection
├── tools/
│   ├── screenshot.ts     # screenshot, screenshot_element, multi_route_screenshot
│   ├── console.ts        # console_logs, network_errors
│   ├── interact.ts       # click, type, navigate, set_viewport
│   ├── inspect.ts        # dom_inspect
│   └── diff.ts           # visual_diff
└── utils/
    └── viewport-presets.ts

Key design decisions:

  • Lazy connection — The MCP server starts immediately. Navigation to your app happens on the first tool call, so PageLens never crashes if your dev server isn't running yet.

  • Passive collection — Console logs and network errors are captured in ring buffers from the moment the browser launches. The agent checks when it wants to, not when events happen.

  • Screenshots after every interactionclick, type, navigate, and set_viewport all return a screenshot so the agent always sees the result of what it did.

  • Baseline storage — Visual diff baselines are stored in memory per route. No filesystem setup needed.

Development

git clone https://github.com/amoghmanral/pagelens.git
cd pagelens
npm install
npm run build

To test locally with Claude Code:

claude mcp add pagelens -- node /path/to/PageLens/dist/index.js http://localhost:3000

Tech Stack

  • TypeScript — Type-safe tool handlers and MCP integration

  • Puppeteer — Headless Chrome automation (bundles Chromium)

  • @modelcontextprotocol/sdk — Official MCP server SDK

  • pixelmatch + pngjs — Pixel-level image comparison for visual diffing

License

MIT

A
license - permissive license
-
quality - not tested
D
maintenance

Maintenance

Maintainers
Response time
Release cycle
Releases (12mo)
Commit activity

Resources

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