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The summon spells introduced in Tasha's Cauldron of Everything include an armor class calculation, but it's unclear from the description if the AC should include the dexterity modifier or not.

For example the Undead Spirit from Summon Undead states the following:

Armor Class 11 + the level of the spell (natural armor)

But the creature has a Dexterity of 16 (+3).

So if I use Summon Undead as a 3rd level spell, is the creatures AC 14 or 17?

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Does what it says on the tin.

The Monster Manual rules for AC state:

a monster’s AC is based on its Dexterity modifier and natural armor, if any. If a monster has natural armor, wears armor, or carries a shield, this is noted in parentheses after its AC value.

This indicates that the listed AC on a monster's stat block accounts for the dexterity modifier. You should never have to add the dexterity modifier to the printed AC to figure out the actual AC of the monster. Something as fundamental as a monster's armor class should not be ambiguous or difficult to determine.

Think about it like this. Monster AC is Natural Armor base plus Dexterity modifier. So while the AC of summon undead is written as

11 + the level of the spell

The real under the hood calculation for the monster's AC is:

11 + each level above 3rd + Dexterity Modifier

So that if you cast at 4th level, the monsters AC is 11 + 1 + 3 = 11 + 4 = 15. Upcasting the spell is increasing the Natural Armor base AC.

And for what it's worth, this is consistent with the armor classes of the options for summoning 1 beast with the spell conjure animals. The AC of the CR 2 options is 12-14. 17 would be exceptionally resilient for a creature summoned with a 3rd level spell.

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    \$\begingroup\$ I'd actually write the 'under the hood calculation' as 8 + DEX mod + level of the spell. Upcasting is not what the OP confused about. It is whether DEX is applied in addition or not. \$\endgroup\$ – Vylix Apr 15 at 8:11
  • \$\begingroup\$ Different from Vylix or the above answer, I would argue that the "under the hood" calculation is AC = Base + Natural Armor + Dexterity + AC Boost = 10 + 1 + 3 (DEX mod) + number of levels upcast (i.e. number of spell levels above 3). Mathematically, of course, all three versions are equivalent. \$\endgroup\$ – gto Apr 15 at 15:15

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