JSONFix.ai — LLM Developer Tools

LLM JSON Validator & Auto-Repair Tool

Traditional JSON validators only tell you what's wrong. JSONFix.ai fixes it — and explains why your LLM produced broken JSON in the first place.

⚕ Validate & Fix LLM JSON

Why Standard Validators Fail on LLM Output

JSONLint, JSONFormatter, and other legacy validators parse JSON strictly — which is correct. But they only report the error. They cannot repair it, and they were built before LLMs existed.

LLMs produce a specific set of predictable errors (see below). JSONFix.ai knows these patterns and repairs them automatically, without you manually hunting down each offending character.

LLM-Specific JSON Failure Modes

All LLMsTrailing commas
{ "a": 1, "b": 2, }

LLMs mirror JavaScript training data where trailing commas are legal.

ChatGPT / ClaudeMarkdown code fences
```json
{ "key": "value" }
```

LLMs wrap output in markdown by default — you need raw JSON.

All LLMsTruncated output
{ "items": [{"id": 1}, {"id": 2

LLMs hit token limits mid-response, leaving unclosed structures.

ChatGPTundefined values
{ "result": undefined }

ChatGPT conflates JS and JSON; undefined is not a JSON value.

Use JSONFix.ai in Your LLM Pipeline

For programmatic use, call the repair API directly in your pipeline:

const res = await fetch("https://jsonfix.ai/api/repair", {
  method: "POST",
  headers: { "Content-Type": "application/json" },
  body: JSON.stringify({ json: llmOutput }),
});
const { repaired, changes } = await res.json();
const data = JSON.parse(repaired);

API access available on Enterprise plan.

Validate Your LLM JSON Output — Free

Works with ChatGPT, Claude, Gemini, Llama, and any other LLM.

Open JSONFix.ai →