ValidateThis

AI Output JSON Validator

Related tools

Format and repair utilities that pair with validating LLM JSON output.

Check that LLM output is valid JSON. Uses jsonrepair for small syntax issues, then JSON.parse. Does not validate a JSON Schema or OpenAI response_format — use those in your stack when you need strict contracts.

This page is tuned for text that is supposed to be JSON after a model answers you — not for auditing a whole codebase file. You paste output, we try a light repair pass, then parse. That is a different job from a full editor with URL fetch, diff, or schema enforcement (see JSONLint for a classic “messy JSON blob” workflow).

For strict shape and enums, pair this with JSON Schema or your provider’s structured output feature. OpenAI covers JSON mode and structured outputs in their API guides. Here we only answer: “after repair, does this parse as JSON?”

Typical problems in LLM output

  • Markdown fences (```json) copied along with the payload.
  • Trailing commas, half-fixed quotes, or a single object where your app expects an array.
  • Prologue or epilogue sentences wrapped around a JSON block — the model “explained” the answer instead of returning raw data.

Validation runs in your browser on ValidateThis; nothing is sent to our servers for this check unless a tool explicitly says otherwise.

How to use this tool

  1. Paste the model output: raw JSON, or JSON inside markdown code fences if you copied a chat block.
  2. Click Validate JSON. The tool runs a repair pass for common LLM glitches, then parses.
  3. If it still fails, tighten the prompt, strip fences in your app, or fix the text manually.

What this check helps you catch

  • Trailing commas and minor bracket/quote issues (repair pass)
  • Accidental markdown fences or prose mixed with JSON
  • Strings and brackets that make JSON.parse fail after repair
  • Not a substitute for JSON Schema validation or Chat Completions envelope checks

FAQ

Does this enforce OpenAI JSON mode or response_format?
No. It only checks that the pasted text is valid JSON after optional repair. For Chat Completions shapes (choices, message, error), use OpenAI Response Format Checker. For strict schemas, validate with JSON Schema in your pipeline.
What does jsonrepair fix?
Small syntax problems such as trailing commas and some bracket issues. It cannot turn free-form prose into JSON or fix a completely wrong structure.
Why does validation fail after repair?
The model may have returned non-JSON, multiple objects without an array wrapper, or invalid escapes. Extract only the JSON block in your application or adjust the prompt.
Where does validation run?
In your browser on ValidateThis, like other client-side tools here. See the site privacy policy for retention and logging.