What spell attack modifier does Eldritch Cannon use when multiclassing another casting class?

Question

The Artificer’s Eldritch Cannon features states, under Force Ballista:

Make a ranged spell attack, originating from the cannon, at one creature or object within 120 feet of it. On a hit, the target takes 2d8 force damage, and if the target is a creature, it is pushed up to 5 feet away from the cannon.

But what type of modifier do I use if I multiclass into Warlock? Do I use Intelligence since it is the Artificer's spell attack modifier and it's an Artificer feature, or can I also use Charisma from the warlock?

I would assume Intelligence for what I have mentioned above, but if it's unclear I can talk to my DM since I'm gonna focus on improving Charisma.

Answer 1

Use Intelligence.

The rules for attack rolls state:

The ability modifier used for a spell attack depends on the spellcasting ability of the spellcaster.

(The corresponding section of the PHB adds, "as described in chapter 10".)

The Sage Advice Compendium¹ further clarifies this for us in this rules question, in particular, the second footnote explaining "spellcasting ability mod":

How do I know which ability modifier to use with an attack roll and its damage roll?

The Player’s Handbook specifies which ability modifier to use with an attack roll (PHB "Attack Rolls") and which one to use with the corresponding damage roll (PHB "Damage Rolls"). Here’s a summary:

Attack Type	Attack Roll
Melee weapon attack	Strength mod. *
Ranged weapon attack	Dexterity mod. *
Spell attack	Spellcasting ability mod. **

* Add your proficiency bonus if you’re using a weapon with which you’re proficient.

** Add your proficiency bonus. Your spellcasting ability is determined by your class or whatever feature gave you the ability to make the spell attack.

In this case, since the feature that gave the ability to make a spell attack is an artificer feature, we use Intelligence.

It is also worth mentioning that this is consistent with the rules for ability modifiers used with spells by multiclassed spellcasters:

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.

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^{1 The Sage Advice Compendium is Wizards of the Coast's official compendium of rulings and rules interpretations, presented in question and answer format.}

Answer 2

You use your Intelligence modifier for the spell attack

The spellcasting feature of the artificer reads:

Spell attack modifier = your proficiency bonus + your Intelligence modifier

The Multiclassing rules on p 164 PHB state:

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.

And the Sage Advice Conpendium expands on this:

How do I know which ability modifier to use with an attack roll and its damage roll? The Player’s Handbook specifies which ability modifier to use with an attack roll (p. 194) and which one to use with the corresponding damage roll (p. 196). (…) Your spellcasting ability is determined by your class or whatever feature gave you the ability to make the spell attack.

Since the Force Ballista is a feature of the Artificer class, it uses the spellcasting ability of the Artificer class to make spell attacks.