JSON Formatter, Validator, Diff & AI Fix

A JSON formatter, validator, beautifier, and minifier in one tab

Paste a raw API response, a minified config file, a log line, or a mock-data fixture and this free online tool beautifies it into readable, indented JSON while validating the syntax as you type. It is built for the everyday work of developers, QA testers, and anyone wrangling APIs, config files, mock data, or database exports: prettify with 2 or 4-space indent, minify back to one line for production, diff two JSON objects side by side, or paste malformed JSON and let AI Fix repair it.

Formatting, validating, minifying, and diffing all run locally in your browser — there is no upload and no signup. The one exception is the optional AI Fix tab, which sends text to Google Gemini only when you click Fix. Once your JSON is valid you can convert a JSON array to CSV, decode a Base64 payload back into JSON, or beautify the surrounding HTML, CSS, and JavaScript.

How the validator actually works (and what it won’t accept)

The formatter runs on your browser’s built-in JSON.parse() — the same strict parser your code uses at runtime. Every keystroke reparses the input: valid JSON turns the status bar green with a “Valid JSON” badge and re-emits a beautified copy in the output pane; a syntax error turns it red and prints the parser’s exact message and character position, so you can jump straight to the problem. The Indent toggle re-beautifies at 2 or 4 spaces, Minify strips all whitespace to a single line, and Copy and Clear do what they say.

Because it is the strict parser, it only accepts standard JSON. Comments, trailing commas, single-quoted strings, unquoted keys, and JavaScript values like NaN, Infinity, and undefined are all rejected — that is correct behaviour, not a bug. If you are holding a JavaScript object literal or a JSON5-style config, the AI Fix tab can convert it to strict JSON for you.

One honest caveat: the exact wording and position in an error message come from your browser’s JavaScript engine, so Chrome, Firefox, and Safari phrase the same error slightly differently. The message under the input is a starting point, not a contract.

A worked example: beautify, then minify

Say an API hands you this single minified line — 66 characters, impossible to scan:

{"user":"alice","roles":["admin","editor"],"active":true,"age":30}

Paste it into the Input pane and the validator beautifies it at 2-space indent — 96 characters across 9 lines, now easy to read and edit:

{
                  "user": "alice",
                  "roles": [
                    "admin",
                    "editor"
                  ],
                  "active": true,
                  "age": 30
                }

Switch the Indent toggle to 4 and the same object grows to 114 characters; that extra whitespace is purely for humans. When you are ready to ship it in a request body or a config file, click Minify and it collapses straight back to the 66-character single line — same data, smallest payload. Beautify to read and edit, minify to transmit.

Common JSON errors and how to fix them

When the status bar turns red, it is almost always one of these. Each row shows a representative message from a Chromium-based browser and the fix:

Exact wording and the reported position depend on your browser’s JavaScript engine, so treat the messages above as representative rather than word-for-word. Note the last row: duplicate keys are not a syntax error, so the validator shows “Valid JSON” while quietly keeping only the final value — check for them yourself.

JSON vs JSONC vs JSON5 vs NDJSON — which one are you holding?

“It looks like JSON” often means one of several relatives, and this tool validates the strict form. Knowing which variant you have explains why it fails here and where to take it instead:

This tool validates and beautifies strict JSON. JSONC and JSON5 win for hand-edited config because they allow comments and trailing commas — but you need a parser that understands them; VS Code and TypeScript, for instance, read JSONC natively. NDJSON is the right call for streaming and huge exports, where you process one object per line rather than loading a single giant array. To turn any of these into strict JSON, the AI Fix tab can often do the conversion.

Beyond formatting: diff two objects and AI-fix broken JSON

Two features set this apart from a plain beautifier.

JSON Diff — normalize, then compare. Paste JSON A (original) and JSON B (modified) into the two panels and the diff appears automatically. Both sides are first parsed and re-serialized to 2-space indent, so pure formatting differences never register as changes; then a Longest Common Subsequence line diff — the same core idea behind git diff — marks added lines green with a +, removed lines red with a −, and unchanged lines plain, with a running “+N added, −N removed, N unchanged” summary and an “identical” verdict when the two match. Because it compares normalized text line by line, reordering object keys or array elements shows up as changes even when the underlying set is the same, and each side is capped at roughly 800 lines — minify or split anything larger.

AI Fix — repair, then re-validate. Paste malformed JSON (up to 10,000 characters) — missing commas, single quotes, unquoted keys, trailing commas, or a whole JavaScript object literal — and click Fix Broken JSON. The tool parses locally first; if the input is already valid it simply reformats to 2-space indent with no AI call. Otherwise the broken text and the parser error go to Google Gemini with strict instructions to preserve every key and value and never invent, drop, rename, or reorder anything, and the server re-parses the model’s answer before returning it, so the output is guaranteed to be valid JSON. “Use as input” drops the repaired result straight into the Formatter.

When not to reach for AI Fix: it is the one feature that leaves your browser, so keep secrets, API keys, and customer data out of it — and it is rate limited to 10 requests per minute per IP. For a simple trailing comma or a missing quote, fixing it by hand from the validator’s error message is faster and keeps everything local.

Frequently asked questions