Hello, Jonathan
I'm trying to upload the low-poly model to Sketchfab ,with no normal map just for testing but I'm having some trouble with auto smooth. If I set the entire model to Smooth Shading in Blender and then I set some edges to sharp everything looks fine: some parts are smooth some are sharp... in Blender. Once uploaded to Sketchfab everything is Smooth, even the sharpened edges.
In this article https://help.sketchfab.com/hc/en-us/articles/209143406-Vertex-Normals#blender they talk about "valid vertex normals"
Help please :)
Hey Marco, which file type are you using to upload to Sketchfab? So far I've been using FBX, which seems to keep the smooth and sharp edges. It'd also be good to double check that you're only exporting the selected objects, since it may be exporting the whole scene, and another object that looks like the low poly but doesn't have the auto smooth (like a cage) might be showing over top.
So fast your reply :D
I created a new .blend file with only the low-poly model but actually I uploaded the whole .blend file. I will try to export the single object as FBX, ok?