Spell Thief states:
[...] Immediately after a creature casts a spell that targets you or includes you in its area of effect, you can use your reaction to force the creature to make a saving throw with its spellcasting ability modifier. The DC equals your spell save DC. [...]
For a multiclassed spellcaster who is the target of this feature, what spellcasting ability do they use to make the save?
For example, let's say you have a Warlock 17/Wizard 3, targeted by this feature do they use Charisma or Intelligence for the check?