browser-testing-with-devtools

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---
name: browser-testing-with-devtools
description: Tests in real browsers via Chrome DevTools MCP. Use when building or debugging anything that runs in a browser. Use when you need to inspect the DOM, capture console errors, analyze network requests, profile performance, or verify visual output with real runtime data. Requires the chrome-devtools MCP server to be configured.
---

# Browser Testing with DevTools

## Overview

Use Chrome DevTools MCP to give your agent eyes into the browser. This bridges the gap between static code analysis and live browser execution — the agent can see what the user sees, inspect the DOM, read console logs, analyze network requests, and capture performance data. Instead of guessing what's happening at runtime, verify it.

## When to Use

- Building or modifying anything that renders in a browser
- Debugging UI issues (layout, styling, interaction)
- Diagnosing console errors or warnings
- Analyzing network requests and API responses
- Profiling performance (Core Web Vitals, paint timing, layout shifts)
- Verifying that a fix actually works in the browser
- Automated UI testing through the agent

**When NOT to use:** Backend-only changes, CLI tools, or code that doesn't run in a browser.

## Setting Up Chrome DevTools MCP

### Installation

Add the following to your project's `.mcp.json` or Claude Code settings:

```json
{
  "mcpServers": {
    "chrome-devtools": {
      "command": "npx",
      "args": ["-y", "chrome-devtools-mcp@latest", "--isolated"]
    }
  }
}
```

`-y` skips the npx install confirmation. By default the server launches Chrome with its own dedicated profile (under `~/.cache/chrome-devtools-mcp/`), separate from your personal browser; `--isolated` goes one step further and uses a temporary profile that is wiped when the browser closes. This is the right setup for most testing.

There is also `--autoConnect` (Chrome 144+, requires enabling remote debugging via `chrome://inspect/#remote-debugging`), which attaches the agent to your **running** Chrome instead. Only use it when the test genuinely needs your logged-in state — see Profile Isolation under Security Boundaries first.

### Available Tools

Chrome DevTools MCP provides these capabilities:

| Tool | What It Does | When to Use |
|------|-------------|-------------|
| **Screenshot** | Captures the current page state | Visual verification, before/after comparisons |
| **DOM Inspection** | Reads the live DOM tree | Verify component rendering, check structure |
| **Console Logs** | Retrieves console output (log, warn, error) | Diagnose errors, verify logging |
| **Network Monitor** | Captures network requests and responses | Verify API calls, check payloads |
| **Performance Trace** | Records performance timing data | Profile load time, identify bottlenecks |
| **Element Styles** | Reads computed styles for elements | Debug CSS issues, verify styling |
| **Accessibility Tree** | Reads the accessibility tree | Verify screen reader experience |
| **JavaScript Execution** | Runs JavaScript in the page context | Read-only state inspection and debugging (see Security Boundaries) |

## Security Boundaries

### Profile Isolation

The blast radius of every rule below depends on which browser the agent is attached to. With `--autoConnect`, the agent attaches to your running Chrome's default profile and — per the chrome-devtools-mcp docs — has access to **all open windows** of that profile: logged-in email, banking, GitHub sessions, saved cookies. (`--browser-url` is less exposed by design: Chrome requires a non-default user data directory to enable the remote debugging port — don't defeat that by pointing it at a copy of your real profile.) One page with injected instructions plus an agent holding your authenticated browser is the worst-case combination — the untrusted-data rules below become the only line of defense instead of one of two.

**Rules:**
- **Default to the dedicated profile** (no connect flags) or `--isolated`. Testing