JSON Validator
Validate JSON syntax and get detailed line-by-line error messages.
Validate JSON before an API or config file fails
Paste JSON and check whether it can be parsed as strict JSON. This catches the mistakes that usually break API request bodies, package files, environment config, webhook payloads, and exported data: trailing commas, missing quotes, mismatched braces, bad escape sequences, and control characters inside strings.
The fastest workflow is to validate first, then format or convert once the payload is clean. That keeps a syntax problem from turning into a confusing app error later.
Common JSON syntax errors to check
The most frequent mistake is copying a JavaScript object and treating it as JSON. JSON requires double-quoted keys, double-quoted strings, no comments, no trailing commas, and no values such as undefined, NaN, or Infinity.
Other issues include missing commas between array items, extra commas after the last field, unescaped backslashes in Windows paths, accidental smart quotes, and newline characters pasted inside strings.
Private browser-side validation
Validation runs locally in your browser with JavaScript. FastDevTool does not upload the payload, log your keys, or send the error result to a backend.
That makes the page useful for internal API examples, staging configs, support snippets, and other data that should stay on your machine. Remove secrets before copying validation results into tickets or chat.
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Frequently Asked Questions
What makes JSON invalid?
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Common JSON errors include: trailing commas after the last element in arrays or objects, single quotes instead of double quotes for strings, missing quotes around object keys, unescaped special characters in strings, and mismatched brackets or braces.
Does this validator show the exact error location?
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Yes. When JSON is invalid, the validator reports the character position and attempts to identify the line and column number where the error occurred.
Is my data kept private?
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All validation runs entirely in your browser. No data is ever sent to any server.
Can I validate large JSON files?
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Yes. This tool processes JSON using your browser's native JSON.parse, which can handle files up to hundreds of megabytes depending on your device's memory.
Why does JavaScript accept data that this JSON validator rejects?
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JavaScript object literals can allow patterns that JSON forbids, such as unquoted keys, single-quoted strings, trailing commas, undefined, or comments. This validator checks strict JSON, which is what API request bodies, package files, and many config files expect.