How to Fix JSON from ChatGPT, Claude & Gemini
Last updated: October 2026 · 5 min read
Large language models like ChatGPT, Claude, and Gemini frequently generate JSON that looks correct but fails to parse. This guide explains the 5 most common LLM JSON errors and how to fix them — automatically or manually.
Fix it automatically — free
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The 5 Most Common LLM JSON Errors
1. Trailing Commas
JSON does not allow a comma after the last item in an object or array. JavaScript allows it — LLMs trained on JS code frequently include them.
❌ Invalid
{"name": "Alice", "age": 30,}✓ Valid
{"name": "Alice", "age": 30}2. Single Quotes Instead of Double Quotes
JSON requires double quotes for all strings and keys. Single quotes are valid in Python and JavaScript, so models sometimes output them.
❌ Invalid
{'name': 'Alice'}✓ Valid
{"name": "Alice"}3. Python Boolean & Null Values
Python uses True, False, and None (capitalised). JSON uses lowercase true, false, and null. LLMs trained on Python produce this constantly. See our in-depth reference on fixing Python True, False, and None in JSON.
❌ Invalid
{"active": True, "data": None}✓ Valid
{"active": true, "data": null}4. Unquoted Object Keys
JavaScript allows unquoted keys in object literals. JSON requires every key to be a double-quoted string.
❌ Invalid
{name: "Alice", age: 30}✓ Valid
{"name": "Alice", "age": 30}5. Markdown Code Fences
ChatGPT and Claude wrap JSON in markdown code blocks for display. When copy-pasted, the backtick fences break parsing.
❌ Invalid
```json
{"name": "Alice"}
```✓ Valid
{"name": "Alice"}How to Fix It — 3 Methods
Method 1 — Use QuietJSON Repair Tool (Fastest)
Paste your broken JSON at quietjson.com/json-repair/ and click Repair. Fixes all 5 error types automatically. 100% client-side — your data never leaves your browser.
Method 2 — Fix in Your Prompt
Add to your prompt: "Respond with only valid RFC 8259 JSON. No markdown, no code fences, no trailing commas, no single quotes."
Method 3 — Fix Manually
Find & Replace in your editor: swap single quotes for double quotes, remove trailing commas before } and ], replace True/False/None with true/false/null.
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