Fix JSON Online: Repair, Validate & Format Invalid JSON

JSON.parse() is unforgiving: one misplaced comma, one unquoted key, one True instead of true, and your entire app stops. Online JSON fixers let you paste broken JSON, automatically detect and repair the errors, and copy back clean, valid JSON — without touching your codebase or installing anything. This guide explains how they work, what errors they catch, and when to use one.

What Is an Online JSON Fixer?

An online JSON fixer is a browser-based tool that takes invalid JSON as input, applies a set of repair heuristics, and returns valid JSON as output. The best fixers also format (pretty-print) the result so you can read the structure clearly.

Unlike a simple validator that only tells you that something is wrong, a fixer tells you what was wrong and corrects it. You get actionable output, not just an error message.

The Most Common JSON Errors That Online Fixers Repair

Trailing commas

The single most common JSON error. A comma after the last item in an object or array is valid JavaScript but illegal in JSON.

// Invalid
{ "name": "Alice", "score": 98, }

// Fixed
{ "name": "Alice", "score": 98 }

Single quotes

JSON requires double quotes for strings and keys. Single-quoted strings are a JavaScript-ism that many developers accidentally carry over.

// Invalid
{ 'name': 'Alice' }

// Fixed
{ "name": "Alice" }

Unquoted keys

JavaScript object literals allow bare (unquoted) keys. JSON does not.

// Invalid
{ name: "Alice", age: 30 }

// Fixed
{ "name": "Alice", "age": 30 }

JavaScript comments

JSON has no comment syntax. Comments copied from JavaScript source files or added to config files will break parsing.

// Invalid
{
  "debug": true,  // enable logging
  /* remove before production */
  "verbose": false
}

// Fixed
{
  "debug": true,
  "verbose": false
}

Python literals

Python uses True, False, and None for its boolean and null types. JSON requires lowercase true, false, and null.

// Invalid (Python-style)
{ "active": True, "deleted": False, "nickname": None }

// Fixed
{ "active": true, "deleted": false, "nickname": null }

Markdown code fences

When you ask an LLM (ChatGPT, Claude, Gemini) to output JSON, it often wraps the result in a markdown code block. The backticks and language tag aren't part of the JSON.

// Invalid (with markdown fence)
```json
{ "name": "Alice", "age": 30 }
```

// Fixed
{ "name": "Alice", "age": 30 }

Unclosed brackets or missing values

Truncated JSON — from a cut-off API response or a partial copy-paste — has unclosed objects or arrays. A fixer can auto-close them to produce syntactically valid output.

// Invalid (truncated)
{ "users": [{ "id": 1, "name": "Alice"

// Fixed (auto-closed)
{ "users": [{ "id": 1, "name": "Alice" }] }

UTF-8 BOM at the start of the document

Files written by some Windows tools start with a UTF-8 BOM (0xEF 0xBB 0xBF). RFC 8259 prohibits a BOM at the start of JSON, and JSON.parse rejects it with a position-0 error that looks like invalid first character. A repair pass strips it; if you're parsing by hand, do text.replace(/^/, '') first.

AI-generated JSON cleanup (LLM output)

LLM responses are the most reliable source of "almost-JSON" today. The patterns are consistent enough to repair mechanically:

• ```json … ``` fence wrappers around the payload
• Trailing commas inside arrays and objects
• Python-style literals (True, None) bleeding in from training data
• Unbalanced brackets when the model is cut off by a token limit
• Smart quotes (“ ”) from helpful "prettification"

Strip the fence, run the repair, then validate. For a full walkthrough specifically targeted at LLM output see the repair LLM JSON output guide.

How Online JSON Repair Works

Simple fixers use regular expressions — for example, replacing ,} with } to strip trailing commas. This works for simple cases but fails when the same character sequence appears inside a string value.

Better fixers use a repair parser: a JSON parser that, instead of throwing an error when it hits an unexpected token, tries to recover and continue. The parser walks the input character by character and applies heuristics contextually:

• When it sees a single quote where a double quote is expected, it switches quote modes
• When it sees a bare word where a quoted key is expected, it adds quotes
• When it sees True where true is expected, it lowercases it
• When it reaches end-of-input inside an open structure, it closes all open brackets

Because the repair is grammar-aware, it only modifies structural positions — it never corrupts string contents.

When to Use an Online JSON Fixer

Privacy: Does the JSON Leave Your Browser?

This is the most important question to ask about any online JSON tool. Many online formatters and fixers send your input to a server — where it may be logged, cached by a CDN, or indexed by a search engine.

JSON Fix runs entirely in your browser. When you paste JSON and click Repair, the processing happens in JavaScript on your device — nothing is sent to a server. You can verify this by opening the browser's Network tab: no request is made when you click the button.

This makes it safe to use with sensitive data like API responses that contain tokens, database exports with PII, or internal configuration files. For a full explanation of why this matters, see Why You Shouldn't Paste Sensitive JSON Into Online Tools.

Frequently Asked Questions

How do I fix JSON online?

Paste the broken JSON into a browser-based fixer like JSON Fix. It detects and repairs trailing commas, single quotes, unquoted keys, Python literals, comments, and markdown fences, then pretty-prints valid JSON you can copy back — no install and no signup.

Is it safe to paste sensitive JSON into an online fixer?

Only if the tool runs entirely in your browser. Many online formatters POST your input to a server where it can be logged or cached. JSON Fix processes everything client-side — open the Network tab to confirm no request is sent. See why this matters.

What's the difference between a JSON fixer and a validator?

A validator only tells you that the JSON is invalid and where; a fixer also repairs it and returns valid output. For a deeper comparison, see JSON formatter vs JSON repair.

Can an online fixer repair AI-generated JSON?

Yes — LLM output frequently includes markdown code fences and trailing commas, which fixers strip automatically. The targeted guide is repair LLM JSON output.

Fix JSON Online — Right Now

Paste your broken JSON into JSON Fix. The tool:

• Identifies the error type
• Repairs trailing commas, single quotes, unquoted keys, Python literals, comments, and markdown fences
• Pretty-prints the result with consistent indentation
• Shows a diff of what changed so you can review the fix
• Runs 100% in your browser — no data sent anywhere

Other tools in the suite:

• How to Handle Broken JSON in JavaScript — add JSON repair to your own code with try/catch and repair parsers
• Fix "[object Object]" and other JSON syntax errors — the complete reference for every common JSON error
• JSON Formatter vs JSON Repair — when to validate, format, or repair
• JSON Diff — compare two JSON documents to find differences
• YAML to JSON — convert YAML to valid JSON
• Base64 Decoder — decode Base64-encoded JSON payloads (like JWT claims)