The Shield Master feat contains this:
If you aren't incapacitated, you can add your shield's AC bonus to any Dexterity saving throw you make against a spell or other harmful effect that targets only you.
The Enhanced Defense infusion of the Artificer contains this:
A creature gains a +1 bonus to Armor Class while wearing (armor) or wielding (shield) the infused item.
The Repulsion Shield infusion of the Artificer contains this:
A creature gains a +1 bonus to Armor Class while wielding this shield.
Assuming I have the Shield Master feat, and I have a regular shield with an AC bonus of +2. If I infuse the shield with either Enhanced Defense or Repulsion Shield, what will my DEX saving throw against spells that target only me be?
Will it be +3 because the infusion increased the shield's AC bonus?
Or will it be +2 because the infusion's AC bonus is the infusion's AC bonus and the shield's AC bonus is still +2?
Best Answer
Yes, the AC bonus is available for Shield Master
The infusion turns your shield into a magic item, and that magic item in this case grants an increased AC. This is explained on p. 12 of Tasha's Cauldron of Everything under Infuse Item:
The infusion is not a separate, independent thing, it is a property of the shield. As a result, you would benefit from the shield's improved AC for Shield Master.
The language of a "normal" magical shield +1, +2, or +3 is similar, it says "While holding this shield, you have a bonus to AC determined by the shield's rarity", it does not say the shield's AC bonus changes. So, unless you want to read this as meaning that a magical shield would not add more to your Dex save than a normal one, the infusion likewise will improve your Dex save. What counts for the feat is the overall bonus to AC delivered by the shield in either case.