Web Application Testing

To test local web applications, write native Python Playwright scripts.

Use this skill when the task needs local server lifecycle management, custom Python assertions, or a repeatable test script. For ad-hoc browser navigation and interaction that does not need custom Python, use $playwright-cli.

Helper Scripts Available:

• scripts/with_server.py - Manages server lifecycle (supports multiple servers)

Always run helper scripts with --help first to see usage. DO NOT read the source until you try running the helper and find that a customized solution is absolutely necessary. These scripts can be very large and thus pollute your context window. They exist to be called directly as black-box scripts rather than ingested into your context window.

Dependency Preflight

Before writing a native Playwright script, verify the selected project environment:

uv run python -c "from importlib.metadata import version; print(version('playwright'))"

If the import fails:

1. For a one-off page inspection without custom assertions, use the declared $playwright-cli fallback.

2. For repeatable assertions or managed server testing, ask before installing dependencies. After approval, install into the project's development environment rather than adding a browser runtime to production dependencies:

   uv pip install --python <project>/.venv/bin/python playwright
   uv run python -m playwright install chromium
   

3. Re-run the import preflight, then continue with native Python Playwright.

If the project already declares a Playwright dev extra, prefer syncing that extra over an ad-hoc environment install. Never use bare pip or a machine-global Python environment.

Decision Tree: Choosing Your Approach

User task → Is it static HTML?
    ├─ Yes → Read HTML file directly to identify selectors
    │         ├─ Success → Write Playwright script using selectors
    │         └─ Fails/Incomplete → Treat as dynamic (below)
    │
    └─ No (dynamic webapp) → Is the server already running?
        ├─ No → Run: uv run python scripts/with_server.py --help
        │        Then use the helper + write simplified Playwright script
        │
        └─ Yes → Reconnaissance-then-action:
            1. Navigate and wait for networkidle
            2. Take screenshot or inspect DOM
            3. Identify selectors from rendered state
            4. Execute actions with discovered selectors

Example: Using with_server.py

To start a server, run --help first, then use the helper:

Single server:

uv run python scripts/with_server.py --server "npm run dev" --port 5173 -- uv run python your_automation.py

Multiple servers (e.g., backend + frontend):

uv run python scripts/with_server.py \
  --server "cd backend && uv run python server.py" --port 3000 \
  --server "cd frontend && npm run dev" --port 5173 \
  -- uv run python your_automation.py

To create an automation script, include only Playwright logic (servers are managed automatically):

from playwright.sync_api import sync_playwright

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True) # Always launch chromium in headless mode
    page = browser.new_page()
    page.goto('http://localhost:5173') # Server already running and ready
    page.wait_for_load_state('networkidle') # CRITICAL: Wait for JS to execute
    # ... your automation logic
    browser.close()

Reconnaissance-Then-Action Pattern

1. Inspect rendered DOM:

   page.screenshot(path='/tmp/inspect.png', full_page=True)
   content = page.content()
   page.locator('button').all()
   

2. Identify selectors from inspection results

3. Execute actions using discovered selectors

Common Pitfall

❌ Don't inspect the DOM before waiting for networkidle on dynamic apps ✅ Do wait for page.wait_for_load_state('networkidle') before inspection

Best Practices

• Use bundled scripts as black boxes - To accomplish a task, consider whether one of the scripts available in scripts/ can help. These scripts handle common, complex workflows reliably without cluttering the context window. Use --help to see usage, then invoke directly.
• Use sync_playwright() for synchronous scripts
• Always close the browser when done
• Use descriptive selectors: text=, role=, CSS selectors, or IDs
• Add appropriate waits: page.wait_for_selector() or page.wait_for_timeout()

Reference Files

• examples/ - Examples showing common patterns:
  • element_discovery.py - Discovering buttons, links, and inputs on a page
  • static_html_automation.py - Using file:// URLs for local HTML
  • console_logging.py - Capturing console logs during automation