{}JSON FYI
Topic

Common JSON errors

Diagnose and fix parser failures, lint warnings, and gotchas.

Tools

JSON Validator & Linter

Validate, lint and format JSON in your browser.

JSON Validator

Strict RFC 8259 conformance check.

JSON Linter

Warnings for trailing commas, duplicate keys, comments.

Fix Invalid JSON

Repair JSON with a catalog of common errors.

Guides

Common JSON errors

The mistakes that cause 90% of parse failures.

JSON comments

Why they don't exist and what to do instead.

Trailing commas in JSON

Why every parser rejects them.

Unexpected token in JSON

Causes, positions, and fixes for this common error.

JSON parse error at position 0

Five root causes — empty string, BOM, HTML response, undefined.

Duplicate keys in JSON

What parsers do and how to find and remove duplicates.

SyntaxError: Unexpected token '<'

HTML returned instead of JSON — how to detect and fix it.

Unexpected end of JSON input

Truncated JSON — the four root causes and fixes.

Python JSONDecodeError

json.loads() error causes and fixes with code examples.

TypeError: Converting circular structure to JSON

Four patterns for serializing circular data safely.

JSON.stringify returns undefined

Why it happens and how to handle non-serializable values.

Postgres: invalid input syntax for type json

Single quotes, empty strings, NUL bytes, and double-encoded JSON — with fixes.

ValueError: No JSON object could be decoded

The Python/requests error that means the body wasn't JSON.

Expecting property name enclosed in double quotes

The Python JSONDecodeError from single quotes, trailing commas, or a dict — fixes for each.