Simple RAW for 5e 4: Smackdown v. RAW

[Man_Over_Game]

Q 165.

Can someone, with citations, please explain what spells a Warlock/Wizard multiclass can use what slots on?

For the sake of example, can a Warlock/Wizard multiclass use a Warlock slot on Magic Missile, and/or a Wizard slot on Hex?

A 165

If you have both the Spellcasting class feature and the Pact Magic class feature from the warlock class, you can use the spell slots you gain from the Pact Magic feature to cast spells you know or have prepared from classes with the spellcasting class feature, and you can use the spell slots you gain from the Spellcasting class feature to cast warlock spells you know.

Spell slots can be used interchangeably. The difference is, Warlocks do not have varying spell slot levels, so when you use a Warlock Spell Slot, it always defaults to the Warlock Spell Slot level.

So in this case, if your Warlock's spell slot level is 3, and you use a Warlock Spell Slot for Magic Missile, it has to be cast at level 3.

However, there are cases where the difference between class spell slots may be relevant, just not with spellcasting. For example, the Eldritch Smite invocation (Xanathar's Guide to Everything) specifically says "Warlock Spell Slots", so you cannot use Eldritch Smite using Wizard spell slots.