JSON validator, JSON linter, JSON checker: the same job, three names
People search for all three and mean one thing: tell me whether this JSON is valid, and if it is not, tell me exactly where. That is what this page does. Paste or drop a file, and you get either a clean parse or the precise line and column of the first thing the parser could not accept.
"Lint" is borrowed from C tooling and usually implies style opinions on top of correctness. There are none here, because JSON has no style to have opinions about. It either parses or it does not. Anything calling itself a JSON linter is doing what this does.
It is a JSON editor, not a one-way check
The panel is a real editor: codeMirror, with syntax colouring, folding, line numbers and bracket matching, so the usual loop of copying broken JSON somewhere else, fixing it, and pasting it back does not apply. Fix it in place and it revalidates as you type.
For the mistakes that are mechanical rather than interesting: a trailing comma, single quotes, an unquoted key. There is a repair button that applies the obvious fix. Try repair rewrites the document with the fix and lays it out with indentation. It never guesses at your data, only at your punctuation.
When the message itself is the confusing part
Parsers are terse and often point a line or two past the real mistake, because that is where they finally gave up rather than where you went wrong. If the message is what has you stuck, the JSON error reference explains the common ones (across JavaScript, Python, Java, PHP, Go and C#) with a broken sample and the fix beside it.
Common mistakes and the messages they cause:
New to the format itself? What is a JSON file covers what it is and the six types it has, and how to open a JSON file covers getting one open on any device.
Once it parses
The JSON formatter pretty-prints it, the viewer opens it as a tree you can search and copy paths out of, and JSON compare diffs two documents as data rather than as text, so reordered keys stop reading as changes.
Nothing you paste is uploaded
Validation runs in your own browser. There is no upload step and no server round trip for your content, which matters because the JSON people need to validate is usually a config file or an API response with credentials in it. The claim is asserted by tests that watch the network on this page, not just written here.