Flipped normals in a GLTF 3D model usually result from incorrect face orientation during modeling or export settings that didn’t preserve normal direction.
Normals define which side of a face is "front"—if flipped, the model appears inside-out because rendering engines use normals for lighting and visibility. Common causes include accidentally reversing faces in tools like Blender or missing export options to retain normal data.
To fix it, try recalculating normals in your modeling tool (e.g., Blender’s "Recalculate Normals" in Edit Mode) or check the GLTF exporter for unintended normal-flipping toggles (like "Flip Y/Z" axes) before exporting again.