AgentSkillsCN

before-and-after

捕捉网页或页面元素的前后对比截图，便于直观比对。当用户说“截取前后对比图”“进行截图对比”“可视化差异”“生成PR截图”“对比新旧版本”，或需要记录UI变更时，可使用此功能。支持输入两个URL（file://、http://、https://）或两条图片路径。

SKILL.md
--- frontmatter
name: before-and-after
description: 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.
allowed-tools:
  - Bash(npx @vercel/before-and-after *)
  - Bash(before-and-after *)
  - Bash(which before-and-after)
  - Bash(npm install -g @vercel/before-and-after)
  - Bash(*/upload-and-copy.sh *)
  - Bash(curl -s -o /dev/null -w *)
  - Bash(gh pr view *)
  - Bash(gh pr edit *)
  - Bash(vercel inspect *)
  - Bash(vercel whoami)
  - Bash(which vercel)
  - Bash(which gh)

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
FlagDescription
-m, --mobileMobile viewport (375x812)
-t, --tabletTablet viewport (768x1024)
--size <WxH>Custom viewport
-f, --fullFull scrollable page
-s, --selectorCSS selector to capture
-o, --outputOutput directory (default: ~/Downloads)
--markdownUpload 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

ErrorFix
command not foundnpm install -g @vercel/before-and-after
could not determine executableUse npx @vercel/before-and-after (full name)
401/403 on .vercel.appSee Vercel protection section
Element not foundVerify selector exists on page