Hi, After some searching for a solution⦠that does not break backward-compatibility, I found a way to save classes that use slots. - if a class just uses ivar Slots, it saves it 100% backward compatible (e.g. loadable in other systems) - if a class uses a special slot or class var, it saves the definition of the slot as the variable name in the monticello model. - on load it checks if there is a space in a variable name, and if yes â> decode these as Slot/Global var definions. - the code path for non-Slot classes stays largely untouched. This is not beautiful but backward compatible⦠you can even load classes with Slots in Pharo3, it then creates invalid ivar names, which of course means the code does not run but it is readable. e.g. create a class like Object subclass: #A slots: { #iv => TestSlot } classVariables: { } category: 'Playgroundâ (TestSlot is a slot that does not use up space in the instance and instead saves the state in the Slot object, which means all instances share the state. Just a small example) Limitations -> no support for class variables yet -> class instance variables not yet done, either -> nut used much, for sure there will be bugs This is in 4.0 438 Marcus