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

• The .uplugin file is corrupted or contains unexpected binary data.
• The file was saved with the wrong encoding (UTF-16 or UTF-8 with BOM).
• The file is empty or missing the required JSON structure.

Unreal Engine expects .uplugin files to be plain UTF-8 JSON text starting with {.

✅ How to Solve

1. Open the .uplugin file in a text editor (VS Code, Notepad++, etc.).
2. Check the contents:
   • It should start with { and contain valid JSON.
   • Example minimal template:

   {
     "FileVersion": 3,
     "Version": 1,
     "VersionName": "1.0",
     "FriendlyName": "My Plugin",
     "Description": "Plugin description here",
     "Category": "Custom",
     "Modules": [
       {
         "Name": "MyPlugin",
         "Type": "Runtime",
         "LoadingPhase": "Default"
       }
     ]
   }

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3. Fix encoding:
   • Re-save the file as UTF-8 (without BOM).
   • Remove any stray characters before the {.
4. Validate JSON:
   • Use a JSON validator to confirm the file parses correctly.
5. Re-generate project files:
   • Run the Unreal Engine project file generation again.

💡 Tips

• Always keep plugin descriptor files under version control so you can restore them if corruption occurs.
• If you copy .uplugin files between systems, double-check encoding settings.
• A quick sanity check: open the file in a hex editor — the first byte should be { (0x7B), not 0xFF.

🚀 Conclusion

The “0xFF Not Valid JSON” error is not a compiler bug — it’s a file encoding or corruption issue in your plugin descriptor. By ensuring .uplugin files are valid UTF-8 JSON, you can avoid this problem and keep your Unreal Engine projects building smoothly.