No call attaches or welds an object to a gripper: friction is the only grasp

What is wrong. A robot holds an object by contact and friction only. There is no call that attaches or welds an object to a gripper while a simulation runs, and so none that releases one. A script or policy that closes the fingers and then fixes the object to the hand has no equivalent here: if the grip does not hold by friction, the object slips or stays behind.

The number. There is no error to measure: the call does not exist. What the two ways to hold an object did in trials (docs/guide/robots.md, Grasping): by friction, the default 2 mm cloth slipped out of a fingertip pinch (0 of 64 worlds carried an edge); a cup built into the hand's body in the robot's MJCF poured 96-99 % into a second container in all 64 worlds.

Why. Not built. The public API has no call that fixes an object to a robot's hand, or frees it, while a simulation runs.

Workaround. Two ways, both in docs/guide/robots.md, Grasping: - A friction grasp. Set the friction with the object's material or friction=, or with scene.set_friction(a, b, mu) for the finger-object pair. Give the fingers enough actuator force to squeeze (MJCF kp times the target error), close them onto the object, then ramp the lift. Judge the grasp by the object: record its .pos and check that it follows the gripper. - A body of the hand. To carry something rigidly, put its geoms in a <body> under the hand body of the robot's MJCF, with no joint between them. It then moves exactly with the hand, and it pours from a held container. The cost: the object cannot be put down.

Planned fix. None planned.