JSON Formatter and Validator

JSON Formatter and Validator

Format, validate and minify JSON with exact error lines in your browser, nothing sent anywhere.

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Checked as you type. Nothing leaves this device. Ctrl/⌘ + Enter formats.

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How it works

      
      
    
        
  1. Paste, type or drop your JSON. Use the box on the left, the Paste button, or drop a .json file. A sample is loaded so you can try it straight away.
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  2. Read the verdict. The badge says whether the JSON is valid. If not, you get the line, the column and a plain-language reason, the line is highlighted, and common mistakes come with a fix you can accept in one click.
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  3. Format, minify, copy or save. Pick 2 spaces, 4 spaces or tabs, sort keys if you like, browse the tree view, then copy the result or download formatted.json or minified.json.
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Questions, answered

      
      

        Is my JSON uploaded anywhere?
        
No. JSON Formatter and Validator runs entirely in your browser. Whatever you paste or drop, including API responses with tokens or customer data, is checked and formatted with JavaScript on your device and never sent to a server. You can disconnect from the internet and it keeps working.

      

      

        How is the error location found?
        
The browser's own JSON parser decides whether the text is valid. Because every browser words its errors differently, a small tokenizer then walks the text and stops at the first character that breaks the rules, so you always get the same line, column and plain-English reason. Trailing commas, single quotes, unquoted keys, missing commas and comments get a suggested fix that is only applied when you click it.

      

      

        Will formatting change my numbers or text?
        
No. The output is printed from the tokens you wrote, not from re-encoded numbers, so a 20-digit ID or a price written as 4.50 stays exactly as it is. Escapes like \u00e9 are shown as readable characters unless you tick "Keep \u escapes". Duplicate keys are kept and flagged with a warning instead of being silently dropped.

      

      

        Is it really free?
        
Yes. It is open source under the MIT license, with no account, no limits and no ads. If it saves you a paid subscription, you can say thanks on Ko-fi.

      

      

        Does it work offline, and with big files?
        
Yes. After your first visit the page is cached by a service worker, so it loads without a connection, and you can install it as an app from your browser's menu. Files over about 5 MB are parsed in a background worker with a progress bar so the page stays responsive.

      

      

        Was this written by AI?
        
Yes. JSON Formatter and Validator was generated by Claude Code from a written specification inside an automated pipeline, with a test suite that runs in a real browser. The full source is on GitHub, so you can read every line before you trust it with your data.

      

      
    


    

      

        
Built by one person, given away for free

        
JSON Formatter and Validator is a free alternative to paid tools like JSONLint. If it earns a place in your bookmarks, a coffee keeps the updates coming.

        ☕ Support Or on Ko-fi