What are the effects of Pact Magic and Spellcasting being stacked on the same character?

Question

I have a lvl. 4 Divination Wizard who, for RP purposes, will be multi-classing into a Hexblade Warlock. However, I'm unsure how their mutual magic abilities will work together afterwards. They're both spell casters with rules affecting their abilities in different ways, but how does it work when those abilities are both available simultaneously?

Answer 1

Combining the Spellcasting & Pact Magic class features

While it may seem confusing, since a Wizard and a Warlock can both cast magical spells, the class feature of Spellcasting (Wizard, Cleric, Paladin, Sorcerer, Druid, Ranger, Arcane Trickster, Eldritch Knight, Bard) is different from the class feature of Pact Magic (Warlock). If you look in each class description right after the table with levels 1-20, you find that the Warlock has Pact Magic while the others have Spellcasting as a class feature. (Barbarians and Monks don't have either). How both class features calculate a spell save DC or spell attack modifier is identical, using the spell casting ability of example Wizard (Intelligence) or Warlock (Charisma).

The key differences are:

Spellcasting

1. Regain all expended spell slots when you finish a long rest. (PHB, p. 101)
2. Slots combine with other Spellcasting classes (not the Pact Magic class) when multiclassing.
3. Accumulate many slots when advancing in level.
4. Spell slots have different levels. (In this case, 4 1st / 3 2nd)

Pact Magic

1. Regain all expended spell slots when you finish a short or long rest. (PHB, p. 107)
2. Only one class has it (Warlock)
3. Does not combine slots when multiclassing.
4. Has very few slots, relative to Spellcasting classes.
5. All Pact Magic spell slots are cast at the same level. For the current Warlock level(1) of this PC, this doesn't matter but it will matter at level 3 when all Pact Magic slots will be 2nd level.

How do these class features combine?

The Spellcaster slots and Pact Magic slots do not combine on the table in Chapter 6, Multiclassing. You treat the spell slots (for tracking and bookkeeping purposes) separately for your Wizard 4 / Warlock 1.

1. Level 1 Warlock: 1 slot, first level, refreshes on a short or long rest.

2. Level 4 Wizard: 4 level 1, 3 level 2, refresh on long rest¹.

3. Remember which class that you got the spell from.

   Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell. Similarly, a spellcasting focus, such as a holy symbol, can be used only for the spells from the class associated with that focus.

   You can carry two spell casting foci, or use a spell component pouch, to deal with material components.

How to mix and match the spells and the spell slots

You can expend the spell slots as you wish across classes, with a few caveats.

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. (PHB, Chapter 6)

An Example of Wizard 4 / Warlock 1 multiclassing

1. Cantrips:
   4 from Wizard List (Firebolt, Dancing Lights, Mage Hand, Shocking Grasp);
   2 from Warlock list (Eldritch Blast ,Chill Touch)
   Always available, cast with the spell casting ability per class, cast at 5th level (4 Wiz + 1 WAR = 5). Roll 2d10 for Firebolt, 2d10 for Eldritch blast, etc.

2. Seven spells prepared by the Wizard, spellcasting

   (1) Sleep, Magic Missile, Ice Knife; (2) Web, Hold Person, Shatter, Magic Weapon

3. 2 spells known by the Warlock, pact magic
   (1) Armor of Agythys, Charm Person

4. Using those spells

   You could cast Armor of Agythys (first level Warlock spell) with a second level Wizard slot. That spell slot recharges on a long rest. You could also cast the sleep spell (prepared as a Wizard) with a first level slot from the Warlock. That spell slot recharges on a short or long rest.

   • The tricky part

     If you use a 1st level Wizard spell slot to cast that Charm Person from Warlock's known spells, you still use Charisma for the spell casting DC save calculation.
     
     
     What you could not do is cast Hold Person (a second level spell) that you prepared as a Wizard with a 1st level Warlock slot. (HP needs a 2d level or higher slot).
     
     When you reach that 3rd level of warlock, the Pact Magic slots are now all second level so you could cast hold person with a Warlock slot then, but would still use Intelligence to calculate the spell save DC for the target.

For further reading

These two question/answers, here and there, also address some interactions of pact magic and spellcasting class features when multi-classing.

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¹ Granted, once per long rest "level divided by two" spell slots can be restored using the arcane recovery class feature during a short rest)

Once per day when you finish a short rest, you can choose expended spell slots to recover. For example, if you’re a 4th-­‐‑level wizard, you can recover up to two levels worth of spell slots. You can recover either a 2nd‑level spell slot or two 1st‑level spell slots. (PHB, Wizard)