/skills/browser-testing-with-devtools/SKILL.md

---
name: browser-testing-with-devtools
description: 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 via Chrome DevTools MCP.
---

# 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

```bash
# Add Chrome DevTools MCP server to your Claude Code config
# In your project's .mcp.json or Claude Code settings:
{
  "mcpServers": {
    "chrome-devtools": {
      "command": "npx",
      "args": ["@anthropic/chrome-devtools-mcp@latest"]
    }
  }
}
```

### 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 | Interactive debugging, state inspection |

## The DevTools Debugging Workflow

### For UI Bugs

```
1. REPRODUCE
   └── Navigate to the page, trigger the bug
       └── Take a screenshot to confirm visual state

2. INSPECT
   ├── Check console for errors or warnings
   ├── Inspect the DOM element in question
   ├── Read computed styles
   └── Check the accessibility tree

3. DIAGNOSE
   ├── Compare actual DOM vs expected structure
   ├── Compare actual styles vs expected styles
   ├── Check if the right data is reaching the component
   └── Identify the root cause (HTML? CSS? JS? Data?)

4. FIX
   └── Implement the fix in source code

5. VERIFY
   ├── Reload the page
   ├── Take a screenshot (compare with Step 1)
   ├── Confirm console is clean
   └── Run automated tests
```

### For Network Issues

```
1. CAPTURE
   └── Open network monitor, trigger the action

2. ANALYZE
   ├── Check request URL, method, and headers
   ├── Verify request payload matches expectations
   ├── Check response status code
   ├── Inspect response body
   └── Check timing (is it slow? is it timing out?)

3. DIAGNOSE
   ├── 4xx → Client is sending wrong data or wrong URL
   ├── 5xx → Server error (check server logs)
   ├── CORS → Check origin headers and server config
   ├── Timeout → Check server response time / payload size
   └── Missing request → Check if the code is actually sending it

4. FIX & VERIFY
   └── Fix the issue, replay the action, confirm the response
```

### For Performance Issues

```
1. BASELINE
   └── Record a performance trace of the current behavior

2. IDENTIFY
   ├── Check Largest Contentful Paint (LCP)
   ├── Check Cumulative Layout Shift (CLS)
   ├── Check Interaction to Next Paint (INP)
   ├── Identify long tasks (> 50ms)
   └── Check for unnecessary re-renders

3. FIX
   └── Address the specific bottleneck

4. MEASURE
   └── Record another trace, compare with baseline
```

## Writing Test Plans for Complex UI Bugs

For complex UI issues, write a structured test plan the agent can follow in the browser:

```markdown
## Test Plan: Task completion animation bug

### Setup
1. Navigate to http://localhost:3000/tasks
2. Ensure at least 3 tasks exist

### Steps
1. Click the checkbox on the first task
   - Expected: Task shows strikethrough animation, moves to "completed" section
   - Check: Console should have no errors
   - Check: Network should show PATCH /api/tasks/:id with { status: "completed" }

2. Click undo within 3 seconds
   - Expected: Task returns to active list with reverse animation
   - Check: Console should have no errors
   - Check: Network should show PATCH /api/tasks/:id with { status: "pending" }

3. Rapidly toggle the same task 5 times
   - Expected: No visual glitches, final state is consistent
   - Check: No console errors, no duplicate network requests
   - Check: DOM should show exactly one instance of the task

### Verification
- [ ] All steps completed without console errors
- [ ] Network requests are correct and not duplicated
- [ ] Visual state matches expected behavior
- [ ] Accessibility: task status changes are announced to screen readers
```

## Screenshot-Based Verification

Use screenshots for visual regression testing:

```
1. Take a "before" screenshot
2. Make the code change
3. Reload the page
4. Take an "after" screenshot
5. Compare: does the change look correct?
```

This is especially valuable for:
- CSS changes (layout, spacing, colors)
- Responsive design at different viewport sizes
- Loading states and transitions
- Empty states and error states

## Console Analysis Patterns

### What to Look For

```
ERROR level:
  ├── Uncaught exceptions → Bug in code
  ├── Failed network requests → API or CORS issue
  ├── React/Vue warnings → Component issues
  └── Security warnings → CSP, mixed content

WARN level:
  ├── Deprecation warnings → Future compatibility issues
  ├── Performance warnings → Potential bottleneck
  └── Accessibility warnings → a11y issues

LOG level:
  └── Debug output → Verify application state and flow
```

### Clean Console Standard

A production-quality page should have **zero** console errors and warnings. If the console isn't clean, fix the warnings before shipping.

## Accessibility Verification with DevTools

```
1. Read the accessibility tree
   └── Confirm all interactive elements have accessible names

2. Check heading hierarchy
   └── h1 → h2 → h3 (no skipped levels)

3. Check focus order
   └── Tab through the page, verify logical sequence

4. Check color contrast
   └── Verify text meets 4.5:1 minimum ratio

5. Check dynamic content
   └── Verify ARIA live regions announce changes
```

## Common Rationalizations

| Rationalization | Reality |
|---|---|
| "It looks right in my mental model" | Runtime behavior regularly differs from what code suggests. Verify with actual browser state. |
| "Console warnings are fine" | Warnings become errors. Clean consoles catch bugs early. |
| "I'll check the browser manually later" | DevTools MCP lets the agent verify now, in the same session, automatically. |
| "Performance profiling is overkill" | A 1-second performance trace catches issues that hours of code review miss. |
| "The DOM must be correct if the tests pass" | Unit tests don't test CSS, layout, or real browser rendering. DevTools does. |

## Red Flags

- Shipping UI changes without viewing them in a browser
- Console errors ignored as "known issues"
- Network failures not investigated
- Performance never measured, only assumed
- Accessibility tree never inspected
- Screenshots never compared before/after changes

## Verification

After any browser-facing change:

- [ ] Page loads without console errors or warnings
- [ ] Network requests return expected status codes and data
- [ ] Visual output matches the spec (screenshot verification)
- [ ] Accessibility tree shows correct structure and labels
- [ ] Performance metrics are within acceptable ranges
- [ ] All DevTools findings are addressed before marking complete