Agent skill

before-and-after

Captures before/after screenshots of web pages or elements for visual comparison. Use when user says "take before and after", "screenshot comparison", "visual diff", "PR screenshots", "compare old and new", or needs to document UI changes. Accepts two URLs (file://, http://, https://) or two image paths.

View SKILL.md on GitHub Repository
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Install this agent skill to your Project

npx add-skill https://github.com/JoaquinCampo/Skills/tree/main/before-and-after

SKILL.md

Before-After Screenshot Skill

Package: @vercel/before-and-after Never use before-and-after (wrong package).

Agent Behavior Rules

DO NOT:

  • Switch git branches, stash changes, start dev servers, or assume what "before" is
  • Use --full unless user explicitly asks for full page / full scroll capture

DO:

  • Use --markdown when user wants PR integration or markdown output
  • Use --mobile / --tablet if user mentions phone, mobile, tablet, responsive, etc.
  • Assume current state is After
  • If user provides only one URL or says "PR screenshots" without URLs, ASK: "What URL should I use for the 'before' state? (production URL, preview deployment, or another local port)"

Execution Order (MUST follow)

  1. Pre-flight — which before-and-after || npm install -g @vercel/before-and-after
  2. Protection check — if .vercel.app URL: curl -s -o /dev/null -w "%{http_code}" "<url>" (401/403 = protected)
  3. Capture — before-and-after "<before-url>" "<after-url>"
  4. Upload — ./scripts/upload-and-copy.sh <before.png> <after.png> --markdown
  5. PR integration — optionally gh pr edit to append markdown

Never skip steps 1-2.

Quick Reference

bash
# Basic usage
before-and-after <before-url> <after-url>

# With selector
before-and-after url1 url2 ".hero-section"

# Different selectors for each
before-and-after url1 url2 ".old-card" ".new-card"

# Viewports
before-and-after url1 url2 --mobile    # 375x812
before-and-after url1 url2 --tablet    # 768x1024
before-and-after url1 url2 --full      # full scroll

# From existing images
before-and-after before.png after.png --markdown

# Via npx (use full package name!)
npx @vercel/before-and-after url1 url2
Flag Description
-m, --mobile Mobile viewport (375x812)
-t, --tablet Tablet viewport (768x1024)
--size <WxH> Custom viewport
-f, --full Full scrollable page
-s, --selector CSS selector to capture
-o, --output Output directory (default: ~/Downloads)
--markdown Upload images & output markdown table
--upload-url <url> Custom upload endpoint (default: 0x0.st)

Image Upload

bash
# Default (0x0.st - no signup needed)
./scripts/upload-and-copy.sh before.png after.png --markdown

# GitHub Gist
IMAGE_ADAPTER=gist ./scripts/upload-and-copy.sh before.png after.png --markdown

Vercel Deployment Protection

If .vercel.app URL returns 401/403:

  1. Check Vercel CLI: which vercel && vercel whoami
  2. If available: vercel inspect <url> to get bypass token
  3. If not: Tell user to provide bypass token, take manual screenshots, or disable protection

PR Integration

bash
# Check for gh CLI
which gh

# Get current PR
gh pr view --json number,body

# Append screenshots to PR body
gh pr edit <number> --body "<existing-body>

## Before and After
<generated-markdown>"

If no gh CLI: output markdown and tell user to paste manually.

Error Reference

Error Fix
command not found npm install -g @vercel/before-and-after
could not determine executable Use npx @vercel/before-and-after (full name)
401/403 on .vercel.app See Vercel protection section
Element not found Verify selector exists on page

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