Most JSON failures follow repeatable patterns. Use this checklist to debug quickly without jumping between random assumptions.
1) Trailing commas
JSON does not allow trailing commas in objects or arrays. This is the most frequent mistake when copying from JavaScript objects.
2) Single quotes instead of double quotes
Keys and string values must use double quotes in JSON. Replace all single quotes introduced from manual editing or template literals.
3) Unescaped control characters
Newlines, tabs, and backslashes in strings need escaping. If errors appear near long text fields, inspect escape sequences first.
4) Hidden invalid characters
Copying payloads from docs or chats can introduce smart quotes or invisible characters. Re-typing the problematic line often resolves mysterious parser errors.
5) Incorrect root structure
If your API expects an object but receives an array (or vice versa), syntax can still be valid while integration fails. Confirm shape expectations with contracts and tests.
Validate syntax first with the JSON Validator, then verify structure and business rules in your application tests.