Check whether JSON is valid, see the exact line and column of any error, and get warnings about duplicate keys, oversized numbers and other valid-but-risky JSON -- entirely in your browser. Fast, free, and built for simple browser-based workflows.
Validation happens entirely in this browser tab. The JSON you paste, drop or open here -- an API response, a config file, a webhook payload -- is never sent anywhere.
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JSON Validator is a free online tool that checks whether JSON is syntactically correct and shows exactly where it breaks if it isn't. Paste JSON, open a .json file, or drop one onto the page, and it returns a clear pass or fail verdict with the line and column of the first error -- never a vague "invalid JSON" message.
It earns its keep whenever you need a firm yes-or-no before trusting a file: a config file that silently breaks a build, an API response that keeps throwing a parse error with no explanation, a webhook payload you're about to write code against, or a file you want checked automatically before it's committed. Beyond pass or fail, it flags valid-but-risky details most validators skip entirely -- duplicate keys that most parsers silently resolve by keeping only one value, numbers too large or too precise for JavaScript to represent exactly, and nesting deep enough to trip up some parsers -- so a file that is technically valid doesn't go on to cause a completely different bug once something else reads it.
Everything runs inside your browser, so the JSON you paste, drop or open here never reaches this site's servers. That matters when you're checking a webhook payload with real customer data, an API key sitting in a config file, or anything else from production you'd rather not upload to a site you don't control. It also means validation is just as fast on a 50 MB export as it is on a short snippet -- there's no upload step to wait through first.
Paste JSON into the box, click Open file to choose a .json file, or drag one onto the box. Click "Try an example" to see a sample with a duplicate key and an oversized number.
Leave Strict JSON (RFC 8259) selected for standard files, or switch to JSONC if the file allows comments and trailing commas, like tsconfig.json.
Press Validate JSON, or Ctrl+Enter from anywhere on the page. A green panel means valid JSON; a red one shows the exact line, column and a snippet, plus a Go to error link.
Valid JSON still gets checked for duplicate keys, oversized numbers and deep nesting, each one listed with its exact location. Document stats such as size, line count and object/key counts appear below.
Click Copy Report to put the verdict, warnings and stats on your clipboard as plain text. Clear resets everything, including any error state.
Yes! All our tools are 100% free to use with no hidden costs, no registration required, and no credit card needed.
Absolutely. All tools run entirely in your browser (client-side). Your data never leaves your device and is not sent to our servers.
Yes, once the page is loaded, the tool will continue to work even if you lose internet connection since it runs in your browser.
There is no strict character limit, but very large amounts of text (100,000+ characters) may slow down your browser.
Yes! All our tools are free for both personal and commercial use.
JSON Formatter rewrites your JSON into readable text; this tool gives you a pass-or-fail verdict, the exact error location, and warnings about valid-but-risky JSON such as duplicate keys, oversized numbers and deep nesting, plus document stats. Use Formatter when you want readable output, and this page when you specifically need to know whether a file is valid.
No -- it stops at the first error, the same way every JSON parser does. Reliably finding a second error would mean guessing how the first one should have been fixed, which is not something a validator can do safely, so this is a fix-one-run-again process, same as any parser.
Any object where the same key appears more than once, like {"a":1,"a":2}. The JSON is still technically valid -- most parsers just keep the last value and silently drop the rest -- so the warning names the key and its path so you can decide whether that is actually what you meant.
A number like 9007199254740993 is valid JSON, but JavaScript and many other parsers can only represent integers exactly up to 2^53-1; anything bigger silently rounds. The warning names the exact value and its path so you know which fields need a string or a BigInt-aware parser downstream, without blocking you with a false error.
No. JSONC mode here only relaxes two things: // and /* */ comments, and trailing commas -- the same subset VS Code and tsconfig.json use. JSON5 also allows single quotes, unquoted keys and hexadecimal numbers, none of which this tool accepts in either mode.
No, this checks only that the JSON itself is well-formed -- it does not know your schema's required fields, types or formats. A dedicated JSON Schema validator is a different, heavier tool; this page is for catching syntax problems and risky-but-valid data before you get that far.
It means your JSON is nested more than 500 levels deep, such as arrays inside arrays inside arrays. That is valid JSON, but some parsers refuse anything past depth 512 or 1000, so extremely deep structures -- often produced by a bug rather than typed by hand -- are flagged before they break something downstream.
Yes. Validation runs in your browser with no upload step, so a large file is limited only by your own device's memory, not a server timeout. Very large input shows a brief "this may take a few seconds" notice before the result appears.
Plain text: the pass-or-fail verdict, every warning with its path, and the document stats, in that order, with no HTML or formatting. It is meant to be pasted straight into a bug report, a chat message or a code review comment.
Not as a single pass -- NDJSON is multiple JSON values back to back, and this tool validates one JSON document at a time. Pasting NDJSON produces a clear error naming the extra data after the first value, rather than a confusing generic failure.