Free tool
JSON Formatter & Validator
Pretty-print, minify and validate JSON in one click. Built for developers debugging structured LLM outputs — with precise error messages when parsing fails. No data leaves your browser.
Input
Output
Formatted JSON appears here…
What it is for
Structured output is where most language model integrations break. You ask for JSON, the model returns JSON wrapped in an explanation, or with a trailing comma, or with single quotes — and whatever consumes it fails with an unhelpful error.
This formats, validates and pretty-prints JSON, with error messages precise enough to find the problem quickly.
How it works
Paste JSON and it validates, indents and highlights structure. When parsing fails it names the position and the reason rather than reporting that something, somewhere, is malformed.
It runs in your browser. Nothing you paste is transmitted, which matters given how often the JSON people are debugging contains real data.
When to use it — and when not to
Use it when debugging model output that should have been valid JSON, when reading a large API response by hand, and when checking a payload before sending it.
It validates syntax, not meaning. JSON that parses cleanly can still have the wrong shape for your schema — for that you need schema validation, which is a different job.
Things worth knowing
If a model keeps returning JSON wrapped in prose or fenced code blocks, the fix is usually in the prompt rather than in post-processing: state that the entire response must be valid JSON with no surrounding text, and give an example of the exact shape.
Most providers now offer a structured-output or JSON mode that enforces this at the API level. If you are parsing model output with a regular expression to find the JSON inside it, check whether that mode exists first — it almost certainly does.
Frequently asked questions
Is my JSON data private?
Completely. Formatting and validation happen in your browser with the native JSON parser. Your data never leaves your device.
Why does my LLM JSON output fail to parse?
The most common causes are markdown code fences wrapped around the JSON, trailing commas, or explanatory text before or after the object. Ask the model to return "valid JSON only, no markdown" to fix it.