Before-After Screenshot Skill

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

Agent Behavior Rules

DO NOT:

• Switch branches or stash another task's changes. Use an isolated worktree for any baseline server already authorized by the task.
• 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.
• Reuse before captures, baseline URLs, and revision information already established in the task. Confirm that they show the relevant behavior and match the after capture's viewport and starting state.
• Treat the current state as after only when the task context establishes it. If the before state remains ambiguous after checking available context, ask for the missing baseline. Do not ask again for a URL when a valid image pair or earlier baseline is already available.

Capture and delivery

1. Use an installed before-and-after CLI or npx @vercel/before-and-after.
2. For remote URLs, check access when needed. A 401/403 indicates protection. Existing image pairs do not require a URL or deployment access check.
3. Capture missing images with before-and-after "<before-url>" "<after-url>".
4. When the task calls for uploaded evidence or PR markdown, run before-and-after before.png after.png --markdown once. For local capture requests, deliver the files without uploading them.
5. Append the markdown to the PR when PR integration is part of the task.

Resolve bundled script paths relative to this skill's directory, not the project working directory. Reuse earlier evidence instead of recapturing it.

Quick Reference

# 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

# Default (0x0.st - no signup needed)
./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

# Check for gh CLI
which gh

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

# Write the existing body plus the comparison to a temporary file, then:
gh pr edit <number> --body-file /path/to/pr-body.md

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