Blender Python: multi-object edit mode silently corrupts other meshes' UVs/normals

In a long-running Blender session (e.g. via the Blender MCP) where multiple asset objects exist in the scene, calling bpy.ops.object.mode_set(mode='EDIT') while more than one mesh is selected enters multi-object edit mode. Subsequent edit-mode operators — bpy.ops.uv.smart_project, bpy.ops.uv.cube_project, bpy.ops.mesh.normals_make_consistent, etc. — apply to ALL selected meshes, not just the active one. There is no warning, no exception. The other assets' UVs or normals get rewritten silently. The corruption only surfaces later when the broken asset is rendered.

Concrete symptom: authored a new prop, ran bpy.ops.uv.cube_project(cube_size=1.0) in edit mode without first deselecting. A previously finished asset (a dryer in our case) had its UVs re-projected at 1m tile size, causing its texture to tile ~2.4× across each face. The user spotted the broken texture; the script reported success. The default Blender selection state is sticky across operators. If you build asset A, leave it selected, then build asset B and obj.select_set(True); bpy.context.view_layer.objects.active = obj for the new one, both A and B are still selected — view_layer.objects.active only sets the active, not the selection set. Edit mode then operates on every mesh in context.selected_objects. The trap is that the active object is correct, so visual feedback (object name in the header, etc.) looks right, but the operator's internal walk uses the selection list. Always deselect everything before activating and entering edit mode on a new asset. Template:

bpy.ops.object.mode_set(mode='OBJECT')          # exit any stale edit mode
bpy.ops.object.select_all(action='DESELECT')    # critical: clear selection
obj.select_set(True)
bpy.context.view_layer.objects.active = obj
bpy.ops.object.mode_set(mode='EDIT')
bpy.ops.mesh.select_all(action='SELECT')        # selects edit-mesh of single active obj only
# ... UV / normal / mesh ops here ...
bpy.ops.object.mode_set(mode='OBJECT')

The same hazard applies to object.shade_flat(), object.transform_apply, and any operator that internally walks context.selected_objects. Wrap asset-authoring routines in a context manager that deselects on entry and exit. After adding the deselect step to the asset-build template, no further cross-asset UV corruption observed across dozens of subsequent prop builds in the same session. 3D Asset Pipeline