Best Free Online Developer Tools (No Sign-Up)

Format and validate

JSON formatter

Use it when an API returns one 40 KB line of JSON and you need to find a field. It also helps when a config file won't load and you can't see why.

The JSON formatter pretty-prints with 2 or 4 spaces, minifies, and validates with a plain-language error that gives the line and column. Its auto-fix repairs the usual suspects: trailing commas, single quotes and unquoted keys. You can upload a .json file and download the result.

XML formatter

Use it when you're debugging a sitemap, an RSS feed, a SOAP response or an e-invoice export.

The XML formatter indents, minifies and checks that the document is well-formed. Its tree view collapses deep nesting so you can see the structure at a glance, and it converts to JSON in one click.

YAML check

Use it when a Docker Compose or GitHub Actions file fails with an indentation error.

There's no separate YAML formatter, but the YAML to JSON converter doubles as a validator. Broken YAML gets the exact line and column; valid YAML shows how each value was read, which exposes surprises like version: 1.10 becoming 1.1.

Tip: For the common errors in each format and how to fix them, see our guide on formatting JSON, XML and YAML.

Convert data between formats

CSV is flat, so JSON to CSV works best with an array of simple objects. Reach for JSON to Excel when records contain nested objects or lists.

Encode and decode

Base64 encode and decode

Use it when you're building an HTTP Basic Auth header by hand, or checking what's inside a Base64 string in a config file.

The Base64 encoder works on UTF-8, so non-English text survives the round trip. For example, Hello, world becomes SGVsbG8sIHdvcmxk, and नमस्ते becomes 4KSo4KSu4KS44KWN4KSk4KWH. The Base64 decoder is forgiving: it ignores whitespace and a data: prefix, and accepts URL-safe characters and missing padding.

Warning: Base64 is an encoding, not encryption. Anyone can decode it in a second, so never treat a Base64 string as a way to hide a password.

URL and HTML encoding

• URL encode when a query value contains spaces or symbols: chai & samosa?city=Pune becomes chai%20%26%20samosa%3Fcity%3DPune.
• HTML encode when you need to show code on a web page: <b>Tom & Jerry</b> becomes <b>Tom & Jerry</b>.
• Image to Base64 when you want to inline a small icon in CSS or an email template.

Minify code

Use it when you're pasting a snippet into a CMS field with a size limit. It's also a quick way to see how small a script gets before you set up a build step.

The JavaScript minifier uses Terser, the same minifier many bundlers use. It removes comments and whitespace, shortens local variable names and simplifies expressions. A 142-character GST helper:

// Add GST to a price
function addGst(price, rate) {
  const tax = price * rate / 100;
  return price + tax;
}
console.log(addGst(1000, 18));

comes out as 66 characters:

function addGst(d,n){return d+d*n/100}console.log(addGst(1e3,18));

There are matching minifiers for CSS, HTML and JSON. For a production site, let your build tool minify automatically; these are for quick, one-off jobs.

Compare and clean text

Diff checker

Use it when a config works on staging but not on production, or a reviewer says "I only changed a line or two".

The diff checker compares two texts or files (up to 20 MB) side by side or inline. It highlights changed words or characters and can ignore whitespace, case and blank lines. It labels moved lines and exports a standard unified .diff you can apply with patch or git apply. Everything runs in a background thread in your browser. Our guide on comparing two texts explains how to read the output.

Duplicate line remover and word counter

• The duplicate line remover cleans lists of emails, SKUs or log lines. It can keep the first or last copy, or show only duplicates or only unique lines. It can also ignore case and sort A → Z or in natural order (1, 2, 10).
• The word counter counts words, characters and sentences and estimates reading time. It's handy for README limits, commit messages and app store descriptions.

Generate IDs, passwords and hashes

UUID generator

Use it when you need IDs for seed data, test fixtures or a database migration.

The UUID generator makes version 4 (random) or version 7 (time-ordered) UUIDs, from 1 up to 10,000 at a time. Version 7 starts with a timestamp, so new IDs sort in creation order, which keeps database indexes tidy. Output can be uppercase, without hyphens, in braces, quoted, comma-separated or a JSON array. You can also paste any UUID in to decode its version and timestamp.

Password generator and hashes

• The password generator creates random passwords or passphrases with your browser's cryptographic random source, and shows entropy in bits. Use it for test accounts, database users and service credentials.
• The SHA-256 hash generator hashes text with the browser's Web Crypto API, useful for checking a checksum or building a test fixture.

SEO helpers for developers

Developers often own the <head> of a site. These tools cover the technical parts:

• Meta tag generator: title, description, robots and canonical tags, with length warnings.
• Schema markup generator: JSON-LD for products, articles, local businesses, events, jobs and more, with required fields flagged.
• Robots.txt generator and XML sitemap generator: valid files that flag common mistakes such as relative URLs.
• Canonical tag checker: normalises a URL and strips tracking parameters such as utm_source and gclid.
• URL slug generator and UTM builder: clean slugs for new pages and consistent campaign links.

What to look for in an online tool

Not every free tool is worth trusting with your data. Before you paste anything, check these five things:

1. Where the work happens. Good tools process data in your browser. A quick test: load the page, switch off Wi-Fi, and try it. If it still works, nothing is being sent to a server for processing. (Some tools load extra code the first time you use a feature, so try each feature once while online.)
2. No sign-up for basic jobs. Formatting JSON shouldn't need an email address. An account wall often means your data or email is the product.
3. Clear error messages. "Invalid input" wastes your time. Line and column numbers save it.
4. Standard output. A unified diff, an RFC-style CSV or a real .xlsx file works with other tools. A proprietary format traps you.
5. No surprises with secrets. Strip API keys, tokens and customer data before pasting into any website, including this one. Use sample data where you can, and rotate anything you've shared by mistake.

Bookmark the JSON formatter and the diff checker. Between them, they cover a surprising share of everyday debugging.