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Free online developer tools
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Website diagnostics, data conversion, and test file delivery — 100% free, no signup required, all in one developer hub

❯devlab --start
🔍 Check a URL 🛠️ Use the tools
23
Check tools
40
Conversion tools
100+
Test files
$0
Free forever

★ Favorites

⏱ Recently Used

🔍 Diagnostics

Enter a URL and check security, SEO, LLMO, structured data, and more in one shot.

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🔍

URL Total Check

Security + SEO + structured data in one report
🤖

LLMO Check

llms.txt, AI crawler permissions, structured data
🛡️

Security Check

HTTPS, HSTS, CSP, XFO, mixed content
🖼️

Open Graph Validator

og:*, Twitter Card, image reachability, 1200×630
🔏

DNSSEC Check

AD flag / DS / DNSKEY verification
🍪

Cookie Inspector

Secure / HttpOnly / SameSite / __Host-
🌍

CORS Tester

Preflight + actual request
📧

Email Auth

SPF / DKIM / DMARC / MTA-STS
🔐

TLS / Cipher Analyzer

TLS 1.0 / 1.1 / 1.2 / 1.3
🌐

DNS Propagation

Cloudflare / Google / Quad9 / AdGuard

🛠️ Conversion & generation tools

Runs entirely in your browser. No data is sent to any server.

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🔧
JSON Formatter
Format, validate, minify
📄
YAML ⇔ JSON
Two-way conversion
🪪
JWT Decoder
Header & payload viewer
🔄
Base64
Encode / decode
🔗
URL encoder
Encode / decode
⏱️
Timestamp converter
Unix ⇔ ISO 8601 ⇔ local
🔍
Regex tester
Match highlighter
📝
Text diff
Line-by-line compare
🔒
Password generator
Cryptographic RNG
🎨
Color converter
HEX / RGB / HSL
🆔
UUID generator
Generate v4 in bulk
🔑
File hash
MD5 / SHA-1 / SHA-256
🔐
Text hash
SHA-256 / SHA-1
📏
Byte converter
MB / MiB / GB / GiB
🧬
JSON → Types
TS / PHP / Python / Go / Rust
🌀
cURL ⇄ fetch / axios
13 languages
📑
Markdown Preview
Real-time HTML
✍️
JWT Signer
HS256 / 384 / 512
⏰
Cron Parser
Next 10 runs
🎲
Test Data Generator
CSV / JSON / SQL
➕
All tools
All 40 tools

📁 Test files

Dummy files for upload feature testing, boundary-value tests, and error-handling QA.

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🖼️
Image files
PNG/JPG/WebP/GIF
📄
PDF
100KB – 100MB
📦
ZIP
1MB – 100MB
📊
CSV
Includes Japanese rows
📋
JSON
10KB – 100MB
📝
Text
Plain TXT
⚡
Threshold tests
9.9MB / 10MB / 10.1MB ...
💔
Broken files
For error-handling tests

📚 Reference

Upload limits cheatsheet MIME types reference Image format comparison HTTP status codes File size units Magic bytes table WordPress upload limit All references →

✨ Why DevLab

  • ✅ 100% free, no signup, instant access
  • ✅ Website diagnostics with just a URL
  • ✅ Browser-only data conversion tools
  • ✅ Pixel-perfect test file sizes
  • ✅ curl / wget / PowerShell examples included
  • ✅ MD5 / SHA-256 hashes published for every file
  • ✅ SEO + LLMO + security checks in one place
  • ✅ Dark mode supported