Validate JSON and find the exact error

Stop guessing where your JSON breaks

A validator that only says “Invalid JSON” wastes your time. Indentioshows the exact character where parsing failed, names the cause — a trailing comma, a single quote, an unclosed bracket — and suggests how to fix it. Valid JSON gets a clear green status.

What “valid JSON” actually requires

JSON’s grammar is famously small — it fits on a business card — and a document is valid if and only if it obeys every one of these rules:

• Exactly one top-level value. Usually an object or array, but a bare string, number, true, false, or null is also a complete JSON document. Two values side by side is not.
• Strings use double quotes. Never single quotes, never backticks — for values and for object keys alike.
• Keys must be quoted strings. { name: 1 } is JavaScript, not JSON.
• Commas separate, never terminate. No comma after the final element of an array or the final property of an object.
• Numbers follow one strict format — optional minus, digits, optional fraction, optional exponent. No leading zeros (012), no leading plus, no trailing decimal point (1.), no hex, and no NaN or Infinity.
• Only three literals exist, all lowercase: true, false, null. Not True, None, or undefined.
• No comments. Neither // nor /* */ is part of JSON.
• Control characters must be escaped inside strings — a literal newline or tab between quotes is invalid and must be written \n or \t.

The full breakdown, with examples of each, is in theJSON syntax rules guide.

Reading the error message

Parser errors describe the moment the parser gave up, which is frequently one or more lines after the mistake itself. Knowing the translation saves a lot of scrolling:

Because Indentio validates live and marks the position in the editor rather than reporting an offset, you can usually skip this translation entirely — but it is useful when the message comes from your own build log or server.

Your file might not be plain JSON

A surprising share of “invalid JSON” is valid — as something else. These formats look like JSON and are deliberately not:

• JSONC / JSON with comments — what tsconfig.json, .eslintrc.json, and VS Code’s settings actually use. Comments and trailing commas are allowed because the tools reading them ship a lenient parser. Strict JSON parsers reject both.
• JSON5 — an explicit superset allowing unquoted keys, single quotes, trailing commas, hex numbers, and Infinity. Valid JSON5, invalid JSON.
• NDJSON / JSON Lines — one complete JSON value per line, no enclosing array, the standard shape for log and analytics exports. Each line validates on its own; the file as a whole does not.
• A Python dict or JavaScript object literal pasted from a console — single quotes, True, None. Use json.dumps() rather than print() and it comes out as real JSON.

If your document is one of these, Auto-fix will convert most of them into strict JSON in one click — comments stripped, quotes normalised, trailing commas removed.

Valid here, rejected by your API

Syntax is only the first gate, and three things pass a validator while still breaking a real consumer:

• Duplicate keys. { "id": 1, "id": 2 } is syntactically legal — the specification simply says the behaviour is undefined. Most parsers keep the last value, some keep the first, and a strict server may reject the document outright.
• Integers that exceed the safe range. JSON numbers have no size limit, but JavaScript parses them as 64-bit floats, so anything above 2⁵³ silently loses precision — a Twitter-style 64-bit id arrives subtly wrong. This is why large identifiers are so often transmitted as strings.
• A byte-order mark. Three invisible bytes at the start of a UTF-8 file. Some parsers skip it, many treat it as an unexpected token at position 0 on a document that looks flawless.

Beyond those, the API is applying its own schema: required fields, expected types, permitted values. That is a different question from “is this JSON?”, and a syntax validator cannot answer it.

Validate, then fix

• Live validation as you type, with a status of Valid or the first error’s location.
• Hover any red underline for the reason and a targeted hint.
• Click Auto-fix to repair common issues, or Format to beautify once valid.
• Everything runs locally, so credentials, tokens, and customer data in a document you are debugging never leave your machine.

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