JSON Syntax Error: How to Find and Fix Every Type

A JSON syntax error means your data cannot be parsed by JSON.parse() or any standard JSON library. Even one stray character is enough to break an entire API response, config file, or LLM output. Here is a complete guide to every common cause and its fix.

1. Missing or Mismatched Quotes

JSON requires all string values and all keys to be enclosed in double quotes. Single quotes, backticks, or missing quotes are all invalid.

{ name: 'Alice', "age": 30 }   // ❌ Unquoted key, single-quote value

{ "name": "Alice", "age": 30 }  // ✅

2. Trailing Commas

Unlike JavaScript or Python, JSON forbids a comma after the last element in an object or array.

{ "a": 1, "b": 2, }   // ❌ trailing comma

{ "a": 1, "b": 2 }    // ✅

3. Comments Inside JSON

Standard JSON does not support comments in any form — no //, no /* */.

{
  // user record
  "id": 42
}

If you need comments in configuration files, switch to YAML or JSONC (.jsonc, supported by VS Code and TypeScript).

4. Unclosed Brackets or Braces

Every [ must be closed by ] and every { by }. This error is especially common in LLM-generated or truncated JSON.

5. Unescaped Special Characters in Strings

If a string value contains a double quote, newline, or backslash, it must be escaped.

{ "msg": "He said "hello"" }  // ❌

{ "msg": "He said \"hello\"" }  // ✅

How to Fix Syntax Errors Automatically

Paste your broken JSON into our JSON Formatter & Validator. It highlights the exact line of the error and, where possible, offers automatic repair for trailing commas, single quotes converted to double, and unquoted keys.