Fixing Unreal Engine "0xFF Not Valid JSON" Plugin Error

Solve the JsonReaderException '0xFF' invalid start of a value error when generating Unreal Engine project files.

📌 Overview

When generating project files in Unreal Engine, you may encounter an error like:

text

JsonReaderException: '0xFF' is an invalid start of a value.
LineNumber: 0 | BytePositionInLine: 0.

This error typically points to a plugin descriptor file (.uplugin) that UnrealBuildTool cannot parse as valid JSON.

Incorrect file encoding (UTF-16 with BOM instead of UTF-8)

File corruption or binary data at the start of the file

Editing the .uplugin file with a non-standard text editor

Copy-paste errors that introduce invisible characters

Locate the problematic .uplugin file

Open it in a proper text editor (VS Code, Notepad++)

Verify the file starts with { (not invisible characters)

Save with UTF-8 encoding (no BOM)

Regenerate project files

json

{
  // Your plugin configuration
}

Always use UTF-8 encoding for .uplugin and .uproject files

Avoid editing with Windows Notepad (use VS Code instead)

Run UnrealBuildTool with -verbose to see which plugin fails