Validate JSON: what to know
An API rejects your request, a config file will not load, or a colleague sends data that “should work”. Paste the JSON above and the validator reads it as you type. Valid JSON gets a green check and a short description of what it holds; invalid JSON gets the line and column of the first problem, a sentence saying what is wrong, and a Go to error button that puts the cursor on it. Everything runs in your browser, so tokens, customer records and private configs stay on your device.
Detailed guide
What the validator checks
The check follows the JSON standard (RFC 8259) exactly. Keys and text values need double quotes; numbers cannot start with a zero or a plus sign; true, false and null are written in lower case; and there must be exactly one value at the top level. Anything else, such as a trailing comma after the last item, a comment, or single quotes copied from JavaScript or Python, is reported with a message that names the fix rather than a generic “unexpected token”.
Duplicate keys are legal JSON but usually a mistake, because most programs keep only the last value. When the same key appears twice in one object, the validator lists it as a warning so you can decide which value you meant.
Fixing invalid JSON
For the common mistakes, press Repair. It removes comments and trailing commas, turns single quotes into double quotes, quotes bare keys, replaces Python’s True, False and None, closes brackets left open at the end, and wraps JSON Lines in an array. Review the repaired result before you use it: a repair makes the text valid, but only you know whether the data is what you intended.
If Repair cannot fix a problem, the message stays on screen. Go to error selects the character where reading stopped; the real mistake is often just before it, such as a missing comma at the end of the previous line.
Numbers and key order stay as written
Many formatters run JSON through the browser’s JSON.parse and back. That silently changes data: 12345678901234567890 becomes 12345678901234567000, 1.50 becomes 1.5, and keys that look like numbers jump to the front of each object. Toolkitta reads the text with its own parser that keeps every number, every escape sequence and every key in its original order, so a formatted or validated file only changes in its spacing unless you ask for sorted keys.
Questions about validating JSON
- Is my JSON sent to a server?
- No. Validation, formatting and repair run in your browser. Nothing is uploaded, stored or turned into a share link, and closing the tab clears it.
- Why does it say a comma is missing on the next line?
- Reading stops at the first character that cannot follow what came before. When a comma is missing at the end of a line, that character is the quote that starts the next key, so the position points there.
- Can it validate JSON Schema?
- No. It checks that the text is valid JSON, not that the data matches a schema. Use a schema validator in your project for that.
- How large a file can I check?
- Up to 10 MB opened from a file. Very large files are faster to inspect in the tree view than in the formatted text.