Which spell save DC to use for a Staff of the Magi for a multiclass Eldritch Knight/Sorcerer?

Question

Ok, we are playing an intentionally strange and absurd campaign, and we have a lot of odd class combinations and have access to excessively powerful magic items. I have multi-classed my Eldritch Knight and taken one level of Sorcerer, and have access to a Staff of the Magi.

The description of the Spell Save DC to use when casting spells with the staff says this:

While holding the staff, you can use an action to expend some of its charges to cast one of the following spells from it, using your spell save DC and spellcasting ability (DMG, page 203)

I only took the level of sorcerer to be able to use this staff (and a few other minor perks), and therefore my Charisma is much lower than my Intelligence. Normally when casting spells I would use Charisma for spells obtained through sorcerer class, and Intelligence for Eldritch Knight spells. I would like to use Intelligence for the staff, but since it can only use it because I have a level in Sorcerer, does that mean I have to use Charisma for the DC, or can I use Intelligence?

Answer 1

You choose which spellcasting ability to use!

The Sage Advice Compendium (or here in D&D Beyond) addresses a similar question about the hat of disguise magic item (which lets you cast a spell from it, but doesn't specify a spellcasting ability or spell save DC):

For the hat of disguise, how do I set the spell save DC for disguise self? Does the wearer have to be a caster?

Use your spellcasting modifier to set the DC. If you don’t have a spellcasting modifier, use your proficiency bonus to set it (this builds on the rule on page 141 of the DMG, under “Spells”).

And per the "Spells" section on p. 141 of the DMG (or here in the basic rules):

A magic item, such as certain staffs, may require you to use your own spellcasting ability when you cast a spell from the item. If you have more than one spellcasting ability, you choose which one to use with the item. If you don't have a spellcasting ability—perhaps you're a rogue with the Use Magic Device feature—your spellcasting ability modifier is +0 for the item, and your proficiency bonus does apply.