{}JSON FYI
Topic

JSON validation

Tools and guides for verifying JSON conforms to the spec.

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.

JSON Diff

Compare two JSON documents structurally.

JSON Schema Validator

Validate JSON data against a JSON Schema — powered by Ajv.

Guides

Common JSON errors

The mistakes that cause 90% of parse failures.

How to validate JSON in code

JS, Python, Bash recipes.

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.

JSON arrays guide

Syntax rules, nesting, accessing elements, and common mistakes.

JSON Schema guide

Types, required fields, patterns, $ref, and Ajv validation examples.

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.

JSON objects guide

Key rules, nesting, accessing values, and common mistakes.

Expecting property name enclosed in double quotes

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