Playwright MCP Tester

Playwright MCP Tester

Purpose

This skill enables iterative UI testing using the Playwright MCP server. Instead of manually copying/pasting screenshots, the agent can:

1. Launch a browser and navigate to the app
2. Take screenshots programmatically
3. Interact with UI elements (click, type, etc.)
4. Verify visual state and fix issues in a loop

Setup

1. Install Playwright MCP Server

# Install the official Playwright MCP server (in project)
bun add -d @playwright/mcp
# or globally
npm install -g @playwright/mcp

2. Configure MCP Server

Create a project-level .mcp.json in the workspace root:

{
  "servers": {
    "playwright": {
      "command": "npx",
      "args": ["@playwright/mcp@latest"]
    }
  }
}

Or add to Claude Desktop settings (~/.claude/settings.json):

{
  "mcpServers": {
    "playwright": {
      "command": "npx",
      "args": ["@playwright/mcp@latest"]
    }
  }
}

3. Restart Claude/IDE

After adding the MCP config, restart to load the server.

Prerequisites

• Playwright MCP server must be configured (see Setup above)
• Dev server must be running (e.g., bun dev on port 5173)

MCP Server Commands

Browser Management

playwright_navigate
  - url: string (required) - URL to navigate to
  
playwright_screenshot
  - name: string (required) - Name for the screenshot
  - selector?: string - CSS selector to screenshot (optional, defaults to full page)
  - fullPage?: boolean - Capture full scrollable page
  
playwright_click
  - selector: string (required) - CSS selector to click
  
playwright_fill
  - selector: string (required) - CSS selector of input
  - value: string (required) - Value to fill
  
playwright_select
  - selector: string (required) - CSS selector of select element
  - value: string (required) - Value to select
  
playwright_hover
  - selector: string (required) - CSS selector to hover
  
playwright_evaluate
  - script: string (required) - JavaScript to execute in browser context

Iterative Testing Workflow

┌─────────────────────────────────────────────────────────────┐
│              PLAYWRIGHT ITERATIVE TEST LOOP                  │
├─────────────────────────────────────────────────────────────┤
│                                                             │
│  1. START dev server (if not running)                       │
│     ↓                                                       │
│  2. NAVIGATE to app URL                                     │
│     playwright_navigate → http://localhost:5173             │
│     ↓                                                       │
│  3. SCREENSHOT current state                                │
│     playwright_screenshot → "initial-state"                 │
│     ↓                                                       │
│  4. ANALYZE screenshot for issues                           │
│     - Check if UI renders correctly                         │
│     - Check for error messages                              │
│     - Check if data displays                                │
│     ↓                                                       │
│  5. EVALUATE results:                                       │
│     ├── All good? → Test complete ✅                        │
│     └── Issues? → Fix code, goto step 3                     │
│                                                             │
└─────────────────────────────────────────────────────────────┘

Commands

"Test UI"

Runs the iterative test loop:

1. Starts dev server if needed
2. Navigates to app
3. Takes screenshot
4. Analyzes and reports issues

"Debug UI"

Interactive debugging:

1. Navigate to specific page
2. Click elements
3. Fill forms
4. Take targeted screenshots

"Visual regression"

Compare current state to expected:

1. Take screenshot
2. Compare to baseline
3. Report differences

Example Usage

Basic UI Test

User: "Test UI"

Agent:
1. Run: bun dev (background)
2. Wait for server ready
3. playwright_navigate → http://localhost:5173
4. playwright_screenshot → "app-loaded"
5. Analyze: "Chart area is empty, no candlesticks visible"
6. Fix: Edit CandlestickChart.svelte
7. playwright_screenshot → "after-fix-1"
8. Analyze: "Charts now display data correctly"
9. Report: ✅ UI test passed

Interactive Element Test

User: "Test crypto selector"

Agent:
1. playwright_navigate → http://localhost:5173
2. playwright_screenshot → "before-click"
3. playwright_click → "button:has-text('ETH')"
4. Wait 2 seconds for data load
5. playwright_screenshot → "after-eth-selected"
6. Analyze: "ETH data loaded, chart updated"
7. Report: ✅ Crypto selector works

Console Error Check

User: "Check for JS errors"

Agent:
1. playwright_navigate → http://localhost:5173
2. playwright_evaluate → "window.__errors = []; window.onerror = (m) => __errors.push(m)"
3. Wait for app to load
4. playwright_evaluate → "return window.__errors"
5. Report errors found (if any)

Integration with Assembler

The Playwright tester integrates with the Assembler iteration loop:

┌─────────────────────────────────────────────────────────────┐
│           ASSEMBLER + PLAYWRIGHT INTEGRATION                 │
├─────────────────────────────────────────────────────────────┤
│                                                             │
│  FOR each Work Order:                                       │
│    1. IMPLEMENT code changes                                │
│    2. RUN Playwright test:                                  │
│       - Navigate to app                                     │
│       - Screenshot                                          │
│       - Check for errors                                    │
│       - Verify acceptance criteria                          │
│    3. EVALUATE:                                             │
│       ├── Pass? → Next WO                                   │
│       └── Fail? → Fix and re-test                           │
│                                                             │
└─────────────────────────────────────────────────────────────┘

Selectors Reference

Common Selectors for crypto-viz

// Navigation
'nav button'                    // Nav buttons
'button:has-text("BTC")'        // Crypto selector buttons
'button:has-text("ETH")'

// Charts
'.bg-surface'                   // Chart containers
'[style*="height: 400px"]'      // Main chart
'[style*="height: 150px"]'      // Indicator charts

// Settings
'.text-sm:has-text("Settings")' // Settings panel
'input[type="range"]'           // Sliders
'input[type="checkbox"]'        // Toggles

// Events
'.text-sm:has-text("Detected Events")' // Event list

Error Handling

Common Issues

Timeout Handling

If page doesn't load in 10 seconds:
1. Check if dev server is running
2. Check port availability
3. Restart server and retry

Best Practices

1. Always screenshot before and after changes
2. Wait for data to load before taking screenshots (2-3 seconds)
3. Check console for errors using playwright_evaluate
4. Use specific selectors to target elements
5. Name screenshots descriptively (e.g., "chart-after-eth-click")

Related Skills

• assembler-agent/ITERATION.md — Iteration loop integration
• validator-agent/SKILL.md — Validation report generation