Sims 4 Studio is a free and versatile tool for making custom content for Sims 4. Using this tool you can make your own custom hair, hats, and accessories, you can modify EA clothing and shoe meshes, and you can recolor objects, CAS items (clothes, accessories, hair, etc), walls, floors, and terrain paint. You can also make animations and poses to play in CAS or in the main game. Additionally, Sims 4 Studio is able to edit resources within a .package using the Warehouse tab (resource editor tab) and it can merge multiple .packages into a single .package.
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