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

📌 Overview

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

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.

🔎 What Causes It

1. 1Incorrect file encoding (UTF-16 with BOM instead of UTF-8)
2. 2File corruption or binary data at the start of the file
3. 3Editing the .uplugin file with a non-standard text editor
4. 4Copy-paste errors that introduce invisible characters

✅ How to Solve

1. 1Locate the problematic .uplugin file
2. 2Open it in a proper text editor (VS Code, Notepad++)
3. 3Verify the file starts with { (not invisible characters)
4. 4Save with UTF-8 encoding (no BOM)
5. 5Regenerate project files

{
  // Your plugin configuration
}

💡 Tips

1. 1Always use UTF-8 encoding for .uplugin and .uproject files
2. 2Avoid editing with Windows Notepad (use VS Code instead)
3. 3Run UnrealBuildTool with -verbose to see which plugin fails