Can you use your infused Armor as your spellcasting focus?

Question

Artificers have to hold a tool or an infused item in hand to cast spells...

Tools Required

You must have a spellcasting focus—specifically thieves' tools or some kind of artisan's tool—in hand when you cast any spell with this Spellcasting feature (meaning the spell has an 'M' component when you cast it). You must be proficient with the tool to use it in this way. See chapter 5, "Equipment," in the Player's Handbook for descriptions of these tools. After you gain the Infuse Item feature at 2nd level, you can also use any item bearing one of your infusions as a spellcasting focus.

So an infused Armor (ie. armor with enhanced defense) shouldn't be suited, or is it?

But some armors (ie. Half-Plate) come with gauntlets and the whole armor counts as one infused item... so technically you constantly have infused items in both hands even if you hold nothing in it, am I right? Or do I get something very wrong?

Answer 1

You can't use worn infused armor as a focus.

Your analysis of the "Tools Required" section is correct. You must have an infused item in your hand if you want to use it as your arcane focus. If you're wearing infused armor, you're not holding it in your hand, so you can't use it as your focus.

Even if the infused armor includes gauntlets, you won't have all of the infused item in your hand. You'll only be holding part of it, so a strict reading of the rules will conclude that it's not eligible to be a focus.

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Your DM may make an exception for flavor.

One of the great things about being an artificer is figuring out how to re-flavor your spells, describing how exactly they manifest via your mastery of magical substances and devices. (See "The Magic of Artifice" sidebar on page 11 of Tasha's Cauldron of Everything for reference.)

Because of this class emphasis on creative re-skinning, your DM may be open to an especially fun idea about how to cast one or more spells through your gauntlet. Discuss it with them outside your play session. They may choose to allow it, or or the two of you may come up with an alternative that you'll both find acceptable.

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An Armorer's worn arcane armor can be used as a focus.

The Artificer's Armorer subclass can use worn armor as a spellcasting focus, but it can't be just any infused armor; it must be the arcane armor granted by the subclass's 3rd-level feature. While wearing it:

You can use the arcane armor as a spellcasting focus for your artificer spells.

Note that this armor can hold an infusion just like any other armor. Furthermore, at 9th level the Armorer gains the Armor Modifications feature which allows the arcane armor to hold multiple infusions. However, these infusions have no bearing on the arcane armor's ability to be used as a spellcasting focus when worn.

Related: Can an Armorer Artificer wearing Guardian-model Arcane Armor cast a somatic spell without holding tools or having the Warcaster feat?

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The cleric has a similar, though rarely used, ability.

It's interesting to note that, if a cleric's holy symbol is emblazoned on their armor, they too can use their worn armor as a spellcasting focus.

This almost never comes up, though, because most clerics use a shield with their holy symbol on it as their focus instead.

Related: Do you need a free hand for your holy symbol if you're wearing it visibly?

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Answer 2

Not if you're wearing it

When you remove the aside, the "Tools Required" section says the following:

You must have a spellcasting focus in hand when you cast any spell with this Spellcasting feature

Thus, in theory, if you were not wearing an infused suit of armor you could hold it in your hand and use it as a spellcasting focus. If you are wearing the armor, even if it includes gauntlets, then the armor as a whole is not "in hand" and can't be used as a spellcasting focus unless another feature allows it such as Arcane Armor from the Armorer subclass.

Answer 3

I think that you absolutely can use your worn infused items as a spellcasting focus, in the same way clerics can wear their spellcasting focus. But you'd still need a free hand for the somatic components.

Saying you can't use infused scale mail as a spellcasting focus by default is like saying you can't use your armorer scale mail as a spellcasting focus, because neither specify that you can use them without holding them anywhere.

You're never holding 'the entire staff', you're holding the handle (or just a random part of the staff, but that's neither here nor there). With armor that touches your hands, you're holding it enough for it to count. I would even argue that, similar to a wizard wearing a crystal around his neck, you could simply touch or grab it for it to be viably used as a spellcasting focus as long as your hand is free.