So I have found a few questions that are closely related, but I can't find exactly the answer I am looking for. Say you are multiclassed warlock and wizard. You can cast either class's spells through any of the spell slots. But which stat is used for the spell DCs? If you cast a warlock spell with a wizard spell slot, should the DC be calculated with Int or Cha? (or vice versa, if you cast a wizard spell with a warlock spell slot)
Essentially, is the ability used to calculate DC based on the class that the spell is originally from, or the slot it is being cast with?