Layout
Paste JSON and it is laid out as you type, with 2 or 4 spaces or a tab for each level, or squeezed onto one line with Minify for a config value or an API request. Sort keys puts every object's keys in A to Z order at every level, which makes two versions of the same file easy to compare line by line.
Finding what is wrong
If the JSON is not valid, the line and column of the first problem are shown, with the line itself and a caret under the spot, and what was expected there: a missing comma between two entries, a key without double quotes, a comma before a closing bracket, a single-quoted string or a comment. When it is valid, the line under the box says what it is, for example an object, 12 values, 3 levels deep, and warns when a key appears twice in one object.
Numbers and order kept
The JSON is read by a parser of its own rather than the browser's, so numbers are copied across exactly as written: 12345678901234567890 and 1.50 are not rounded or trimmed the way a JavaScript round trip would. Keys stay in the order they were written unless you sort them.
Questions
Why does my JSON fail with a comma error?
JSON does not allow a comma after the last item in an object or list, which JavaScript does. The error line points at the comma so you can delete it.
Can it read JSON with comments, or JSON5?
No. Comments, single quotes and unquoted keys are not JSON, and this checks against the JSON standard, so it names each one as an error. Take them out and the rest is laid out.
Is what I paste sent anywhere?
No. It is read and laid out in this page on your device, which matters for API responses with customer data in them. Nothing is kept once you close the tab.