10 Best Free Online Developer Tools in 2026

Paste raw JSON and instantly get a collapsible, syntax-highlighted tree view. Essential when debugging API responses or reading config files. The best ones also validate your JSON and show line numbers for errors.

Paste a JWT token and see the decoded header, payload, and expiration time in one click. Crucial for debugging auth flows. Runs fully client-side — your token never leaves your browser.

Encode text or binary data to Base64, or decode a Base64 string back to readable text. Used constantly in JWTs, email attachments, CSS data URIs, and API payloads.

Test HTTP endpoints directly in your browser — set custom headers, body, and method. Great for quickly testing REST APIs without spinning up Postman or Insomnia.

Minify JavaScript, CSS, and HTML to reduce file sizes for production. Quick alternative to build tools when you just need to compress a single file before deploying.

Paste any URL and get a breakdown of scheme, host, path, query parameters, and fragment — each labeled and clickable. Saves time when debugging query strings or building URLs programmatically.

Generate cryptographically strong secret keys for signing JWTs. Choose key length (256-bit, 512-bit) and format (hex, Base64, random string). Never hand-type secrets again.

Format minified or messy CSS into clean, readable code. Useful when inspecting third-party stylesheets or working with build output that needs to be edited by hand.

Preview any URL across multiple device sizes — mobile, tablet, laptop, desktop — simultaneously. Faster than toggling DevTools breakpoints when you want a quick visual check across viewports.

Analyze a page's performance metrics — load time, resource sizes, and bottlenecks — without leaving your browser. Good for a quick sanity check before sharing a staging link.