It would be beneficial to be able to change geom parameters in real time via the API. For example, we would like to change a specific geom's parameters from the API such as size, shape, or position. To my knowledge, this is only doable right now by going through the MuJoCo GUI and changing the the loaded .xml through the file menu where the new file has the desired geom parameters predefined. I see two ways of achieving this: 1) Be able to change the .xml in real time with a function such as mj_load_xml(filename) where all geom parameters are defined in filename.xml. This function doesn't need to be too complicated if it mandates that the number of motors, joints, geoms, bodies, etc are the same between the two .xmls. Alternatively, calling mj_load_xml would require subsequent API calls to redefine structure sizes to make the API cooperate with the .xml you would like to run. 2) Change a specific geom's parameters in real time via the API with a function such as mj_set_geom(geomid, geomstruct). geomstruct would have all of the same fields users would normally have to define in the .xml to create a geom of desired shape, position, solimp, solref, etc. Please let me know if I have overlooked API calls that already exist with this functionality or if this functionality can be added in future releases.