Hi Victor, Thank you for looking into these issues. They are very important to HPy too! HPy currently only supports head types for similar reasons to why they are important to sub-interpreters -- their lifecycle can be managed by the Python interpreter and they are not tied to the memory and life cycle of the dynamic library containing the C extension. E.g. with heap types the interpreter can control when a type is created and destroyed and when it can be accessed. We've run into some minor issues with the limitations in PyType_Slot (https://docs.python.org/3/c-api/type.html#c.PyType_Slot.PyType_Slot.slot) but we are working around them for the moment. It would be useful to have some sense of where PyType_FromSpec is headed -- e.g. is it a goal to have it support all of the features of static types in the future -- so that we can perhaps help suggest / implement small changes that head in the right direction and also ensure that HPy is aligned with the immediate future of the C API. Yours sincerely, Simon Cross