Why ChatGPT Gives You Broken JSON (And How to Fix It)

If you are building an app that relies on LLM outputs, you have run into this exact error: `SyntaxError: Unexpected token } in JSON at position 145`.

You ask the AI for a JSON object, and it responds with something like this:

```json
{
  "name": "ToolDeck",
  "features": ["PDF", "Image", "Video"],
}
```
I hope this helps! Let me know if you need anything else.

If you try to parse this in JavaScript, your app crashes. The AI failed you in three different ways at once.

Why AI models struggle with JSON

LLMs are text predictors, not compilers. They do not inherently understand strict JSON syntax. They understand the *pattern* of what a JSON file looks like based on their training data.

• Markdown formatting. The model wants to be helpful and readable, so it wraps the JSON in markdown code fences (```json).
• Conversational text. It wants to introduce the output and conclude the response.
• Trailing commas. In JavaScript and Python, a trailing comma is fine. In strict JSON, it is a fatal syntax error. AI models constantly make this mistake.
• Missing quotes. The AI might forget to put quotes around a key, or it might use single quotes instead of double quotes.

The solution: Automated JSON repair

Instead of writing complex regex to strip the markdown and fix the commas yourself, you can use a tool that runs `jsonrepair` directly in your browser.

The JSON AI Fixer automatically extracts the JSON payload, strips the markdown, removes trailing commas, and formats it with 2-space indentation.

The next step: JSON to TypeScript

Once you have clean JSON, what do you do with it? If you are a TypeScript developer, you probably need to create an interface or type definition to use it in your app.

Manually writing out nested interfaces for a complex JSON payload is tedious and error-prone. Our JSON to Code tool parses the JSON and generates perfect TypeScript, Go, or Rust models instantly.

Prompt engineering for better JSON

You can also fix the problem at the source. When prompting the AI, use clear instructions:

Respond ONLY with a valid JSON object. Do not include markdown formatting, code fences, or any conversational text. Do not include a trailing comma after the final property.

Even with these instructions, it is always good practice to run the output through a validator before passing it to `JSON.parse()` in your production code.