31 free tools · Runs in browser · No data sent to server
Free Developer Tools Online
The data that goes through developer tools is often sensitive — production JWTs, .env files, database schemas, API responses. Everything here runs in JavaScript inside your browser tab. No server call, no upload, nothing logged. Paste your JSON, decode your JWT, build your CRON — all of it stays local.
Data Format Converters
CSV ↔ JSON Converter
Convert CSV to JSON or JSON to CSV with delimiter, header, flattening, and preview controls in your browser
JSON to YAML
Convert JSON to YAML format. All processing happens locally in your browser
YAML to JSON
Convert YAML to JSON format. All processing happens locally in your browser
JSON to XML
Convert JSON to XML safely in your browser
XML to JSON
Convert XML to JSON instantly in your browser
JSON & Code Tools
JSON Formatter & Validator
Format, validate, and inspect JSON with a tree view
SQL Formatter & Beautifier
Format and beautify SQL with keyword casing, configurable indentation, and dialect support for MySQL, PostgreSQL, SQLite, and MSSQL
Markdown to HTML Converter
Split-pane live Markdown editor with GitHub Flavored Markdown support — preview rendered HTML or toggle to view the raw HTML output with a copy button
Auth & Security
JWT Decoder
Decode JSON Web Tokens without sending them to a server
Hash Generator
Generate MD5, SHA-1, SHA-256, and SHA-512 hashes from text or any file in your browser — all processing is local, nothing sent to a server
Base64 Encoder / Decoder
Encode or decode Base64 text in your browser — drag-and-drop any file to get its Base64 data URI, with URL-safe mode toggle
Web & CSS Utilities
Color Palette Generator
Generate harmonious color palettes with one click. Copy HEX, RGB, or HSL values and export as CSS, SCSS, Tailwind, or JSON
Color Contrast Checker
Check foreground/background color contrast against WCAG 2.1 AA and AAA standards — get pass/fail for all levels and a nearest-passing color suggestion
CSS Unit Converter
Convert between px, rem, em, vw, vh, and % instantly — configure base font size and viewport dimensions, get all conversions in a single table
OG / Meta Tag Previewer
Preview how your Open Graph meta tags look on Twitter/X, LinkedIn, and Facebook before publishing — live preview cards on every keystroke
Favicon Generator
Upload a PNG/SVG or enter an emoji to generate favicons at 16, 32, 48, 180, and 512 pixels — preview every size and download with the HTML snippet
DevOps & Infrastructure
Cron Expression Builder
Build cron expressions visually with dropdowns for minute, hour, day, month, and weekday — see a plain-English explanation and next 5 run times live
chmod Calculator
Click checkboxes for read/write/execute permissions across owner, group, and other — instantly see the numeric (755) and symbolic (rwxr-xr-x) notation
IP Subnet Calculator
Enter an IPv4 address and drag a CIDR slider to instantly calculate network address, broadcast, usable range, subnet mask, and host count
Unix Timestamp Converter
Convert between Unix timestamps and human-readable dates — see UTC, local, ISO 8601, and relative time all at once
Testing & Debugging
Regex Tester & Explainer
Test regular expressions with live match highlighting — see every match, captured group, and a token-by-token plain-English breakdown of the pattern
Text Diff Checker
Compare two texts, code snippets, or JSON files online and highlight every change instantly
HTTP Header Analyzer
Paste raw HTTP response headers to parse them into a table with plain-English explanations — flags missing security headers and CORS issues
API Ping Tester
Send HTTP requests to any endpoint from your browser and measure response time, status, and headers — test REST APIs without leaving the tab
Data & AI Utilities
Mock JSON Generator
Define a schema with field names and types — string, number, boolean, date, email, UUID, name, address — and generate realistic fake JSON data in one click
.env File Parser & Validator
Paste a .env file to parse it into a clean KEY=VALUE table — highlights duplicate keys in red, flags empty values in amber, and exports as JSON or shell exports
LLM Token Counter
Estimate token count and API cost for any text across GPT-4o, Claude, Gemini, and more — paste your prompt and see character count, token estimate, and price
Number Base Converter
Convert numbers between binary, octal, decimal, and hexadecimal instantly — see binary in 4-bit nibbles and toggle two's complement mode
Low-Level & Misc
WASM Analyzer
Upload a .wasm binary to decode its magic bytes, version, and section table — inspect imports, exports, and function signatures without tooling
Text to ASCII Art
Convert any text into large ASCII art banners using classic FIGlet-style fonts — copy the output straight into READMEs, terminals, or code comments
URL Parser & Builder
Paste a URL to parse it into protocol, hostname, port, path, query params, and fragment — edit any field to rebuild the full URL live
Why these tools run in your browser, not on a server
Most online developer tools work by sending your input to a server, processing it there, and returning the result. That's fine for a random string. It's less fine for a JWT from your production auth system, an .env file with database credentials, or a SQL schema you'd rather not copy to a third-party server. Every tool here runs as JavaScript in your browser tab. Open the Network panel — you won't see your data leave.
Nothing leaves your tab
No POST request with your JSON. No server log entry with your JWT. No S3 bucket storing your .env. The browser's JavaScript engine does the processing and writes the result directly into the DOM.
Works on restricted networks
Once the page loads, the tools keep working with no internet. Useful on corporate networks that block external API calls, on VPNs, or anywhere the network is unreliable.
No waiting for a response
Local processing means no network round-trip. Output updates as you type. A 5 MB JSON file formats in under a second. The latency is your CPU, not a distant server.
What developers use these for
- Minified API response that's unreadable — format and check if it's valid JSON — JSON Formatter
- Converting a JSON config to YAML for a Kubernetes manifest or GitHub Actions workflow — JSON to YAML
- JWT from an auth log — check which claims it contains and when it expires — JWT Decoder
- SHA-256 or MD5 hash for a string, a secret, or a file checksum — Hash Generator
- CRON expression that's firing at unexpected times — see what it actually means in plain English — Cron Expression Builder
- Encode an image or binary file as Base64 for a data URI or JSON payload — Base64 Encoder / Decoder
- Regex that works in one language but not another — test it live with real input — Regex Tester
- Spot duplicate keys, wrong values, or syntax errors in an .env file before deploying — .env File Parser
- Prompt that's hitting token limits — check the count per model before the API call — LLM Token Counter
- API response missing HSTS, CSP, or X-Frame-Options — check what headers came back — HTTP Header Analyzer
Guides & Related Reading
Questions about the developer tools
I pasted a production JWT into the decoder. Was that a mistake?
No. The decoder runs entirely in your browser — it splits the token on the dots, calls atob() on each part, and displays the JSON. That's all JavaScript, local to your tab. Nothing is sent outbound. If you're sceptical, open DevTools → Network tab, then paste a token — you'll see zero requests fire.
How is this different from running these tools in a terminal?
For most tasks it isn't — jq, python -m json.tool, and openssl base64 all do the same thing. The difference is setup. When you're on a machine without the right toolchain, a browser tab with a working formatter is faster than installing dependencies or remembering which flag does what. These are convenience tools, not replacements for a real shell.
Why does the JSON Formatter validate as well as format?
Formatting requires parsing first. A JSON parser either succeeds or throws a syntax error with a specific position. Surfacing that error — with the line and column number — is just exposing what the parser already knows. You can't format invalid JSON, so the validation step is built in rather than being a separate pass.
The LLM Token Counter gives different counts for different models. Why?
Each model uses a different tokenization vocabulary. GPT-4 uses cl100k_base, Claude uses a different BPE vocabulary, and Gemini uses SentencePiece. The same sentence can tokenize differently depending on which model built the vocabulary. The cost estimate multiplies the model-specific token count by that model's per-token rate.
When would I use Text Diff Checker instead of git diff?
When you don't have files in a repository to diff. API responses you copied from Slack, two versions of a config pasted from separate sources, before/after for a database migration, prompt text you're iterating on — anything that lives in a clipboard rather than a tracked file. No git init, no terminal, works on any two text blobs.
Is there a size limit on what I can process?
No server-side limit — there's no server. The practical ceiling is your browser's memory. Files up to around 10–20 MB parse quickly in modern browsers. Very large JSON (50 MB+) will be slow to render with syntax highlighting, but it'll complete. If you're routinely processing huge files, a CLI tool will be faster, but for one-off tasks the browser is fine.