If the issue is with the humanoid, just don’t the tool in the NPC model.
Make a folder in workspace called NPCProps or something like that and then in the folder add another folder for each NPC called (The NPC’s Name). Or you can just put the props directly into the folder and name them after the NPC. As you can see belew, I made a folder in workspace and added a folder for the NPC, with the props in it. Under the prop I added a WeldConstraint and it works perfectly.
Of course then you have to set the properties of the WeldConstraint. In case you don’t know how, simply set Part0 to the prop and then set Part1 to the hand of the NPC.
I hope I was able to help you! Your sword looks really nice, good job!