Remember: . The only difference is the name on the tin.
Do not compress the parent folder itself. You must compress the files inside the folder. convert zip to sb3
If you’ve ever tried to open a Scratch project only to find a folder full of JSON files and random assets instead of a playable game, you aren’t alone. Many users accidentally end up with a ZIP file when they meant to have an .sb3 file. Remember:
Assuming files are in folder scratch_project/ with project.json and assets/: convert zip to sb3