JSON Validator
Paste JSON and find out straight away whether it's valid — and if not, exactly where and why.
- Your data never leaves your device
- 100% free
- No sign-up
- Works offline after first load
How to validate JSON
- Paste JSON or open a file.
- Read the report — every problem is explained, with the line marked in the editor.
- Fix it automatically or by hand, then copy the formatted JSON.
About JSON Validator
Check whether JSON is valid and see exactly where it breaks.
Everything works in your browser, so it is instant and private, and your settings are remembered on this device.
Good for
- Checking API payloads before sending them
- Finding errors in config files
- Validating exported data and logs
Tip: Leave “Report duplicate keys” on — duplicates are valid syntax but almost always a mistake.
Frequently asked questions
How do I know if my JSON is valid?
Paste it or open the file — the badge shows Valid or the error straight away. A report lists every check: syntax, duplicate keys, number precision, encoding and the root value.
Why is my JSON invalid?
The most common causes are a trailing comma, single quotes, keys without quotes, comments and missing commas or brackets. The error message points to the exact place and explains how to fix it.
What are duplicate keys?
JSON objects may technically repeat a key, but most parsers silently keep only the last value — a frequent source of bugs. The validator lists every duplicate with its path.
Can it validate JSON Lines (NDJSON)?
Yes — turn on “JSON Lines” to check each line separately and list every invalid line.
Can it check against a JSON Schema?
Use the JSON Schema Validator to check structure, types and required fields against a schema.
Is my data uploaded?
No. Everything is processed by JavaScript in your browser — nothing you paste or open is sent to a server, so it is safe for API responses, tokens, configs and customer data. It also works offline once the page has loaded.