I tried installing the HACD library as suggested on the MuJoCo overview. I used the newer V-HACD 2.0 binaries the same developer released. The binaries are able to perform the decomposition into a .stl file, but MuJoCo cannot load them because they are in ASCII format (at least that's what the error message tells me). So I tried using Google SketchUp and added some plugins to perform CDs and export into binary .stl format. MuJoCo can render objects defined by the .stls generated by this method, but there is still no concavity. For example, I added a ring I made using SketchUp and its convex decomposition into the simulation. I placed a cylinder into the simulation inside of the ring. MuJoCo renders the objects just as I would expect and I can see faint lines in the ring where the convex decomposition broke apart the ring. However, once I press play, the ring shoots upwards as if the hole in the middle is solid. Have you found and confirmed a reliable method to generate concave meshes in MuJoCo? Have you tried using the HACD library with any success?