Developer Tools

JSON Validator – Check JSON Syntax & Errors Instantly

Strict JSON checking with exact line and column numbers. Your JSON is never changed.

Paste your JSON and press Validate to find syntax errors, with the problem marked in context. This free JSON checker works entirely in your browser, so nothing you paste is uploaded.

Your JSON never leaves your device

0 lines · 0 characters
Result
Your validation result will appear here...

What is JSON validation?

JSON validation checks that a piece of text follows the JSON format exactly: double-quoted strings, matching brackets, commas in the right places and only the allowed value types. A single misplaced character makes the whole document unreadable to a parser. This tool checks your text strictly, reports the first problem it finds with its line and column, and leaves your input untouched. To tidy or compress JSON instead, use the JSON Formatter.

Common JSON Errors

Missing commas

Two items next to each other need a comma between them. The error points at the first character after the gap, such as the quote that starts the next property.

Trailing commas

A comma after the last item in an object or array is valid in JavaScript but not in JSON. The tool points at the extra comma.

Wrong quotes

JSON strings and property names must use straight double quotes. Single quotes, curly quotes and unquoted keys are all errors.

Brackets and braces

Every opening { or [ needs a matching closer in the right order. When one is missing or mismatched, the message says which one is open and where it started.

Broken strings

Raw line breaks and tabs inside a string, unclosed quotes and invalid escapes such as \x all fail. Use \n, \t and \\ instead.

Values JSON doesn't allow

Comments, undefined, NaN, Infinity, True and False with capitals, numbers with leading zeros or a leading plus, and text that continues after the closing bracket.

Your JSON Stays on Your Device

Everything on this page runs in your browser. Your JSON is never uploaded, logged or stored, which makes it safe for API responses, tokens, customer records and private configuration. It's also fast, because there's no waiting on a server, and it keeps working even if your connection drops after the page has loaded.

Developer Uses

APIs and payloads

Check a request body or response before you send it or debug it, and see at once whether the problem is your JSON.

Config files

Validate package.json, tsconfig-style files and app settings before a deploy, where one stray comma can stop everything starting.

Logs and exports

Check a copied log entry, a database export or a webhook body to confirm it is a complete, well-formed document.

Test data and fixtures

Make sure hand-edited mock data and fixtures still parse. Duplicate keys are flagged, as they are valid but easy to miss.

Works on Mobile and Desktop

The tool works in any modern browser on phones, tablets and computers, with nothing to install. On a large screen your JSON and the result sit side by side. On a phone they stack, the input scrolls sideways for long lines, and every button is sized for touch.

Frequently Asked Questions

What does a JSON validator do?
It checks that your text follows the JSON rules exactly. If it does, you get a success message and a summary of what is inside. If not, you get the reason, the line and column, and the surrounding lines with the problem marked.
How is this different from a JSON formatter?
A formatter rewrites JSON to make it readable or compact. This validator only checks it, and never changes your text, spaces or line breaks. That makes it a good fit when you need the exact original text confirmed, or an error located precisely.
How do I find the error in my JSON?
Press Validate JSON. The result shows the line and column, and the nearby lines with a marker under the problem. Press Show in input to select that character in your text. Note that the real mistake is sometimes just before the reported spot, for example a missing comma on the previous line.
Why does my JSON fail when it works in JavaScript?
JSON is stricter than JavaScript object syntax. It does not allow single quotes, unquoted keys, trailing commas, comments or values such as undefined and NaN. A JavaScript object literal often needs a few edits before it is valid JSON.
Are duplicate keys an error?
The JSON standard doesn't forbid them, so the text is valid. However, parsers disagree about what to do with them and most keep only the last value, so the tool lists duplicate keys as a warning with their positions.
How large a JSON file can I check?
Up to 5 MB of text, which is checked in a fraction of a second on most devices. Deeply nested data is supported too. Very large files are better checked with a command-line tool.
Is my JSON uploaded anywhere?
No. The check runs in your browser, on your own device. Your JSON is never uploaded, logged or stored, so it is safe to use with API keys, customer data and private configuration.