Open that folder. You should see:
Critical Check: Is project.json inside a subfolder (like myproject/project.json)? If yes, move everything up one level so the folder looks like: convert zip to sb3 fix
your_folder/
├── project.json
└── assets/
If you are reading this, you have likely downloaded a Scratch project file, only to find it sitting on your computer as a .zip archive instead of the expected .sb3 file. You tried renaming it, dragging it into the Scratch editor, or uploading it to the Scratch website, but nothing worked. Maybe you saw an error message like: "Not a valid Scratch 3 project file" or "Upload failed. Corrupt file." Open that folder
You are not alone. This is one of the most common frustrations for teachers, students, and game developers in the Scratch ecosystem. Critical Check: Is project
In this guide, we will explain why ZIP files masquerade as SB3 files, the exact steps to convert them correctly, and most importantly—the fixes for when the simple conversion goes wrong.
Scratch is strict about two things inside the SB3 archive:
When you right-click a folder and choose "Compress," your ZIP often contains the folder itself, not just its contents. That is the #1 cause of the error.