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

Question

The Guardian model described in the Arcane Armor feature of the Armorer Artificer includes "Thunder Gauntlets":

Each of the armor’s gauntlets counts as a simple melee weapon while you aren’t holding anything in it...

If the Artificer holds tools for a material component of a spell, the description above doesn't apply. However, if the armor itself is used as the focus (assuming it's an Infused Item), the Artificer is effectively holding a weapon in each hand.

To deal with this, must the Artificer have taken the Warcaster feat (to allow performing the somatic component of a spell while holding a weapon)? Or can the Thunder Gauntlets be considered "sheathed" when not attacking with them?

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Note 1: All Artificer spellcasting requires a material component, so performing a somatic-only spell is not a concern.

Note 2: I had missed the part where the "plain" Arcane Armor can be used as a focus, so the parenthetical above assuming it was infused is unnecessary.

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Closing Thought

My key takeaway from the answers: Although the Thunder Gauntlets "count" as simple weapons, the Artificer doesn't hold them. Therefore the Artificer's hands remain free to use for somatic components and the Warcaster feat isn't necessary.

Answer 1

Your armor's gauntlets don't occupy your hands while worn

While in use, armor is worn, not carried. So the gauntlets that are part of the armor you are wearing don't occupy your hands, even while they "count as simple melee weapons". This means that even though you have a weapon available on each hand, your hands are still free to perform the somatic components of spells or perform any other action. (This versatility is one of the benefits of the Arcane Armor feature.)

You can use your arcane armor as a focus while wearing it

Normally, you need to (be able to) hold the material component of a spell in your hand in order to cast it:

A spellcaster must have a hand free to access a spell's material components -- or to hold a spellcasting focus -- but it can be the same hand that he or she uses to perform somatic components.

So you might be worried that you have to take off your armor and carry it in order to cast spells with it. However, the Arcane Armor feature specifically overrides this general rule, because the ability to use it as a spellcasting focus appears in the list of benefits you gain while wearing the armor, not carrying it:

You gain the following benefits while wearing this armor:

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

In fact, since you gain these benefits only while wearing the armor, this means that you actually cannot use your armor as a spellcasting focus while holding it. Contrast this with infusing a suit of armor with Enhanced Defense, which would technically allow the artificer to use the armor as a focus while holding it rather than wearing it. (Of course, if you infuse a suit of armor with Enhanced Defense and then also use your action to turn it into your arcane armor, then you would be able to use the armor as a spellcasting focus while either holding or wearing it.)

Answer 2

Yes, the armorer artificer can use their armor as a focus.

As a broader rule, an artificer can use any item that they have infused as a spellcasting focus for their artificer spells, be it a weapon, an object, or any other gear. This effectively allows an artificer to bypass the need to pick the War Caster feat if they want to have a weapon (or shield) in hand to perform the somatic and material componets of spells, as long as that item has one of their infusions and the material component has no cost and / or is not consumed.

This is stated in their spellcasting trait, under Tools Required, and in the PH under the functioning of spellcasting focus.

After you gain the Infused Item feature at 2nd level, you can also use any item bearing one of your infusions as a spellcasting focus.

A spellcaster must have a hand free to access a spell's material components - or to hold a spellcasting focus - but it can be the same hand that he or she uses to perform somatic components.

Hence, if the artificer wanted to use a weapon as their spellcasting focus, they can as long as that weapon bears one of their ifusions.

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The armorer specialization however allows an artificer to use their Arcane Armor as a spellcasting focus, as noted in the benefits of the Arcane Armor feature while wearing the modified armor.

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

This is allowed regardless of whether the armor, or any of its special weapons granted with the Armor Model armorer feature, are infused or not.

This in turn means that the Thunder Gauntlets of the Guardian armor model, which are the armor's gauntlets and so part of the Arcane Armor, can be used by the armorer as a spellcasting focus for their artificer spells whilst also counting as held weapons, allowing the artificer to use somatic components without any issue. Still, the artificer cannot use this special property if they are holding something that would prevent them from performing the somatic components of their spells in both hands even while wearing the Arcane Armor, such as holding anything that is not one of their infused items or a tool with which they are proficient.