Common JSON Errors Developers Make

Introduction

JSON looks simple, but small syntax mistakes still break requests, fixtures, and configuration files every day. Many of these issues come from copying data between tools with slightly different syntax rules.

Missing commas

A missing comma between properties or array values is one of the fastest ways to turn a valid payload into a parser error. It often happens when editing a long object by hand during debugging.

Invalid quotation marks

Strict JSON requires double quotes around keys and string values. Copying JavaScript object notation or smart quotes from documents can create inputs that look close to valid but still fail.

Trailing commas

Trailing commas are accepted in some programming contexts, but strict JSON does not allow them. This makes them a common source of confusion when developers move data between code and raw JSON tools.

Broken nesting

Unmatched braces, brackets, or misplaced closing tokens become much harder to spot in a large single-line payload. Formatting often makes these problems visible before validation confirms them.

Duplicate keys

Duplicate keys can be particularly dangerous because some parsers accept them and silently keep the last value. That can hide errors during debugging and produce misleading downstream results.

Debugging workflow tips

• • Format first when you need visual clarity
• • Validate immediately after editing a payload
• • Use sanitized examples when sharing bad payloads with teammates

Conclusion

Most JSON errors are small, but they cause outsized debugging pain. ToolPilot’s JSON Validator and JSON Formatter make it easier to catch structural mistakes before they reach production code or automated workflows.