Capture a full-page screenshot, long screenshot

Full Page Screenshot

Capture a full-page screenshot of any web page via Chrome DevTools Protocol. Produces a single PNG that includes all content — even portions that require scrolling. Zero external dependencies beyond Node.js 22+ and Chrome with remote debugging enabled.

Prerequisites

• Node.js 22+ (uses built-in WebSocket)
• Chrome/Chromium with remote debugging enabled

Check environment readiness:

node "${SKILL_DIR}/scripts/full-page-screenshot.mjs" --check

If Chrome check fails, instruct user to open chrome://inspect/#remote-debugging and enable "Allow remote debugging for this browser instance".

Workflow

Option A: Screenshot an already-open tab (recommended for authenticated pages)

1. List available tabs:

node "${SKILL_DIR}/scripts/full-page-screenshot.mjs" --list

2. Identify the target by title/URL, then capture:

node "${SKILL_DIR}/scripts/full-page-screenshot.mjs" <targetId> /tmp/screenshot.png --width 1200 --dpr 1

Option B: Screenshot a URL (opens a background tab, captures, closes)

node "${SKILL_DIR}/scripts/full-page-screenshot.mjs" --url "https://example.com" /tmp/screenshot.png --width 1200 --dpr 1 --wait 15000

Note: --url mode creates a background tab. Pages requiring authentication (SSO, login walls) should use Option A instead.

Parameters

Parameter	Description	Default
output	Output PNG file path	/tmp/screenshot.png
--width	Viewport width in CSS pixels (articles: 1200, dashboards: 1440-1920)	1200
--dpr	Device pixel ratio (2 = Retina, but 4x file size)	1
--wait	Page load timeout in ms (--url mode only)	15000
--css	Custom CSS to inject before capture (e.g., hide elements)	—

Verify Output

# macOS
sips -g pixelWidth -g pixelHeight /tmp/screenshot.png

# Linux
file /tmp/screenshot.png

Core Capabilities

1. SPA scroll container expansion — Detects overflow-y: auto/scroll containers, scrolls through them to trigger lazy-loading, then removes overflow constraints (including Tailwind h-[calc(...)]) so all content renders in a single pass.

2. DOM stability detection — After readyState=complete, monitors DOM element count until it stabilizes. This ensures SPA frameworks finish rendering dynamic content.

3. Lazy-load triggering — Scrolls the viewport incrementally to fire IntersectionObserver callbacks, then waits for all <img> elements to complete loading.

4. Tiled capture for very tall pages — Pages exceeding 16,000px are captured in 8,000px tiles and automatically stitched using Python PIL. Falls back to saving tiles separately if PIL is unavailable.

5. Auto-discovery of Chrome — Reads DevToolsActivePort file to find the debugging port. Falls back to probing ports 9222, 9229, 9333.

6. CDP Proxy fallback — When a CDP proxy holds the browser WebSocket, the script falls back to proxy API endpoints (/eval, /screenshot, /scroll) for capture.

How It Works

1. Discover Chrome debugging port
2. Connect via WebSocket (CDP)
3. Attach to target / create background tab
4. Set viewport width via Emulation domain
5. Wait: readyState + DOM stability
6. Detect & expand scroll containers
7. Scroll through page (trigger lazy-load)
8. Wait for images to complete
9. Measure final content height
10. Page.captureScreenshot (or tiled capture)
11. Stitch tiles if needed (PIL)
12. Restore viewport, detach, clean up

Anti-Patterns

Do NOT	Do instead
Use --dpr 2 on pages > 10,000px tall	Use --dpr 1 to avoid Chrome memory issues
Use --url for authenticated/SSO pages	Use --list + targetId on a tab where user is logged in
Set --wait below 5000 for SPAs	SPAs need time to fetch data and render; use 10000-15000
Capture without checking --check first	Always verify Chrome debugging is available
Hardcode viewport widths for all pages	Use 1200 for articles, 1440+ for dashboards/tables
Skip output verification	Always verify with sips or file command after capture

Troubleshooting

Symptom	Cause	Fix
"Cannot find Chrome debugging port"	Remote debugging not enabled	Open chrome://inspect/#remote-debugging, enable it
"WebSocket connection timeout"	CDP proxy holding the connection	Script auto-falls back to proxy API
Blank/white screenshot	Page not loaded yet	Increase --wait value
Truncated at bottom	Scroll container not expanded	Script handles this automatically; file an issue if it persists
Out of memory	Very tall page + high DPR	Reduce --dpr to 1 and/or reduce --width
"PIL not available for stitching"	Python Pillow not installed	Install with pip3 install Pillow or accept separate tile files

Cross-References

• engineering/browser-automation — General browser automation patterns via CDP/Playwright
• engineering/performance-profiler — Performance analysis that may complement visual captures