Local Tools Online

JSON Formatter & Validator

Paste messy, minified, or broken JSON and get back a clean, indented, syntax-highlighted version — instantly, without sending your data anywhere. This runs entirely in your browser tab. If you closed your wifi right now, it would keep working.

Why use a local JSON formatter

Most “JSON formatter” tools you’ll find in search results are just a textarea that POSTs your input to a server before showing you the result. That’s a problem if your JSON contains API keys, customer records, internal config, or anything from a production database — which, in practice, it often does. Copy-pasting a live API response into a random web tool is one of the more common ways sensitive data ends up somewhere it shouldn’t.

This tool parses and formats JSON using your browser’s own JavaScript engine. Nothing you paste in is transmitted, logged, or cached on a server. You can verify this yourself: open your browser’s network tab before pasting — you’ll see zero outbound requests tied to your input.

How to use it

  1. Paste your raw JSON into the input box — minified, pretty-printed, or even JSON with trailing commas or comments (we’ll flag those separately from strict syntax errors).
  2. The formatter automatically indents and color-codes the output as you type or paste.
  3. If the JSON is invalid, you’ll get a specific error pointing to the line and character where parsing failed, rather than a generic “invalid JSON” message.
  4. Use the indent-width selector to switch between 2-space, 4-space, or tab indentation depending on your project’s style guide.
  5. Click copy to grab the formatted result, or download it as a .json file directly.
  6. There’s also a “minify” toggle if you need to go the other direction — collapsing formatted JSON back into a single compact line for production configs or API payloads.

Common situations this solves

  • Debugging an API response. You’ve got a wall of unformatted JSON from a curl command or browser dev tools console and need to actually read it.
  • Cleaning up a config file before committing it, so diffs in version control stay readable.
  • Validating hand-written JSON before you paste it into a package.json, tsconfig.json, or similar file where a single missing comma breaks the build.
  • Converting minified JSON from a bundler or API log into something you can actually scan for the field you’re looking for.
  • Checking third-party JSON payloads for shape and structure before writing a parser or schema against them.

Frequently asked questions

Does this work with JSON5 or JSONC (JSON with comments)? The strict validator follows the standard JSON spec (RFC 8259), so comments and trailing commas will be flagged as errors. If you’re working with JSONC (common in VS Code config files like .vscode/settings.json), toggle the “relaxed mode” option, which will format the file while tolerating comments and trailing commas.

What’s the maximum file size this can handle? Since parsing happens in your browser rather than being uploaded to a server, the practical limit is your device’s available memory rather than a server-imposed cap. Files up to several hundred MB should format without issue on most modern laptops and phones, though very large files may take a second or two longer to render with syntax highlighting.

Can I use this offline, like on a plane? Yes. Once the page has loaded once, the formatting logic runs entirely client-side with no server dependency. If you’ve visited the page before, it may even work without an internet connection at all depending on your browser’s caching.

Does it detect duplicate keys? Yes — technically valid per the JSON spec but usually a mistake, duplicate keys at the same object level are flagged with a warning even though the JSON will still format successfully.

Is there a size limit for what gets highlighted with syntax colors? Very large files (multi-MB) may switch to plain-text rendering automatically to keep the page responsive, while still formatting correctly — you’ll see a small notice if this happens.

Can I sort object keys alphabetically? Yes, there’s a “sort keys” toggle that reorders all object keys alphabetically at every nesting level, which is useful for generating consistent diffs when comparing two JSON files.