How does multiclassing work for an artificer?

Question

I am making a character for a campaign that is starting soon. The character is going to start out as a cleric (War Domain) and will multiclass into artificer (Battle Smith).

I have looked at the multiclassing rules, but I am having trouble making heads or tails of it for a multi-class involving a 2/3 caster (the Artificer) and a full caster (the Cleric).

That leads me to my question: what would a level 15 War Domain cleric/level 5 Battle Smith artificer look like? How many spells per level? How many cantrips? Does the artificer's ability to replace cantrips still apply?

Answer 1

Eberron: Rising from the Last War has rules for multiclassing as an artificer on page 54:

Optional Rule: Multiclassing

If your group uses the optional rule on multiclassing in the Player’s Handbook, here’s what you need to know if you choose artificer as one of your classes.

Ability Score Minimum. As a multiclass character, you must have at least an Intelligence score of 13 to take a level in this class, or to take a level in another class if you are already an artificer.

Proficiencies Gained. If artificer isn’t your initial class, here are the proficiencies you gain when you take your first level as an artificer: light armor, medium armor, shields, thieves’ tools, tinker’s tools.

Spell Slots. Add half your levels (rounded up) in the artificer class to the appropriate levels from other classes to determine your available spell slots.

^{(Eberron: Rising from the Last War pg. 54)}

(For the record, these are the same rules that the last Unearthed Arcana artificer had prior to the publication of Eberron: Rising from the Last War.)

As you see here, spell slots are handled by halving your artificer levels, rounded up, unlike most half-casters. Otherwise, it works just the same as for other spellcasters: after you halve your levels (rounded up), and add that your levels (scaled appropriately) from other classes, find that number on the multiclass spellcaster table and that’s your spells per day.

Multiclass spellcasters don’t do anything special with cantrips: you have your cantrips from the cleric class levels, and your cantrips from your artificer class levels, and your levels in the other class doesn’t affect either. So you never get more cleric cantrips by taking artificer levels, and you never get more artificer cantrips by taking cleric levels.

The artificer’s ability to swap cantrips applies only to artificer cantrips (“one of the artificer cantrips you know”), and only when you take a level of artificer (“When you gain a level in this class”) (Eberron: Rising from the Last War pg. 55). It never lets you modify your cleric cantrips, and you don’t get to do it when you take a cleric level, only when you take an artificer level.

(The last Unearthed Arcana artificer instead had the ability to change one cantrip after each short or long rest—that ability is no longer found in the Eberron: Rising from the Last War version of the artificer, but even in the Unearthed Arcana version it was still limited to only modifying artificer cantrips.)

Answer 2

The artificer is treated as a (slightly modified) 1/2 spellcaster

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How many spells per level?

Eberron: Rising from the Last War includes instructions on how to use the artificer class with the optional multiclassing rule on page 54:

Spell Slots. Add half your levels (rounded up) in the artificer class to the appropriate levels from other classes to determine your available spell slots.

Add half your artificer levels (rounded up) to your other spellcaster levels and use the Multiclass Spellcaster table to determine spell slots. For your War cleric(15)/Battlesmith artificer(5) example, you would be considered an 18th level multiclass spellcaster.

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How many cantrips?

This is covered in the Spellcasting section of the multiclassing optional rule:

Spells Known and Prepared. You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class.

As a War cleric(15)/Battlesmith artificer(5) you would know 2 artificer cantrips, as well as 5 cleric cantrips.

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Does the artificer's ability to replace cantrips still apply?

The artificer gets the following ability along with the spellcasting class feature (page 55 of Eberron: Rising from the Last War):

When you gain a level in this class, you can replace one of the artificer cantrips you know with another cantrip from the artificer spell list.

The ability does apply, but only to replace an artificer cantrip with another cantrip from the artificer spell list when gaining a level in the artificer class. It does not apply when gaining levels in the cleric class, nor can it be used to swap out cleric cantrips.

Answer 3

iirc, the Artificer's multiclassing caster level is rounded Up because, if you were to go Artificer 1 then Ranger 1, if you rounded down on both you'd actually lose the spell slots you gained from your level of Artificer, being treated as a level 0 spellcaster. Artificers fill a funky little role as a "two-thirds" caster in 5e, getting both their spells at 1st level as well as cantrips, that no other half-caster gets.