What counts as valid JSON?

RFC 8259: double-quoted keys and strings, no trailing commas, no comments, only the literals true/false/null, finite numbers only, and a single root value.

Why is my number reported as invalid?

JSON forbids leading zeros (007), hex/octal/binary literals, NaN, Infinity and trailing dots (1. or .5). Use 7, 0.5 and 1.0 instead.

Can I validate against a JSON Schema?

Use our JSON Schema Validator tool. This tool only checks JSON syntax.

JSON Validator — Free Online Tool

What is the JSON Validator?

The JSON Validator is a free online json tools from Markvibe. Validate JSON and pinpoint errors. It runs entirely in your browser — your data never gets uploaded — and works on desktop, tablet and mobile with no sign-up.

Reviewed by The Markvibe Team · Last updated 2026-06-28

Example

A worked example so you know exactly what to expect.

Input

{"name":"Markvibe","tools":500,}

Output

❌ Invalid JSON — unexpected character '}' at position 31 (trailing comma).

How to use JSON Validator

1Paste JSON into the editor — it's validated as you type, no button needed.
2Valid JSON shows a green badge with the document size, depth and key count.
3Invalid JSON shows the exact line/column plus the error class — most common: trailing comma, missing quote, unescaped backslash, NaN/Infinity (not allowed in JSON).
4Hover any error to see a fix suggestion; click it to jump the cursor to the bad character.
5Once valid, click "Format & Copy" to get a prettified version ready to ship.