What this does

Checks whether your text is valid JSON as you type and, when it is not, tells you exactly where it breaks: the line, the column, a plain-language reason, and a short excerpt of the surrounding lines with a caret under the offending character.

The validator uses its own strict scanner built to the JSON grammar (RFC 8259), so the messages and positions are the same in every browser instead of depending on each engine's wording.

Mistakes it recognises

Common slips get a specific hint rather than a generic error:

Trailing commas after the last item in an array or object.
Single-quoted strings and unquoted property names.
Comments (//, /* */) and # notes.
NaN, Infinity, and undefined values.
Missing commas between items, leading zeros, bare decimal points, and bad escape sequences.
Raw line breaks inside strings, unterminated strings, and several top-level values in one document.

Duplicate keys

Valid JSON can still hide a bug: when an object repeats a key, parsers silently keep the last value. This validator lists every duplicate key with its path, line, and column so you can spot data that was quietly overwritten.

Limits

Nesting deeper than 1000 levels is rejected. Input over 150,000 characters is checked in a background worker so the page stays responsive, and anything over 25 million characters is refused.

If the document is broken beyond a simple fix, try the JSON Repair tool, which can fix many of the mistakes above automatically.

Example

Input:

{
  "name": "Ada"
  "age": 36
}

Result:

Invalid JSON: Unexpected '"', expected ',' or '}' after the property value — line 3, column 3
A comma may be missing between two properties.

Privacy

Everything runs in your browser. Your input is never uploaded, stored, or put in the page URL.

JSON Validator