Staff of Defense total AC and Shield Stacking

My party is about to confront Glasstaff and potentially obtain the Staff of Defense.

This slender, hollow staff is made of glass yet is as strong as oak. It weighs 3 pounds. You must be attuned to the staff to gain its benefits and cast its spells.

While holding the staff, you have a +1 bonus to your Armor Class.

The staff has 10 charges, which are used to fuel the spells within it. With the staff in hand, you can use your action to cast one of the following spells from the staff if the spell is on your class’s spell list: mage armor (1 charge) or shield (2 charges). No components are required.

The staff regains 1d6+4 expended charges each day at dawn. If you expend the staff’s last charge, roll a d20. On a 1, the staff shatters and is destroyed.

This will be given to my Wizard and I want to ensure I calculate his new AC correctly based on a couple of circumstances. He has +2 DEX modifier, therefore his current AC is 12.

• With the staff equipped he automatically gains +1 AC to total 13.
• If he casts Mage Armour his base AC becomes 13, +2 DEX and +1 from the staff totaling 16?
• If he casts shield as an action with the staff, does this add +5 to his AC total from the previous circumstances giving him 21 AC?
• Can he then cast shield as a reaction spell when attacked by an enemy giving him a total of 26 AC?
• Commented Jul 10, 2018 at 14:43

Your AC calculations are correct, but shield can only benefit your AC once

As Szega's answer point out, mage armor and shield do stack, and that the passive +1 AC from the staff will also stack with these spells.

However, there is something else to point out based on your wording.

If he casts shield as an action with the staff, does this add +5 to his AC total from the previous circumstances giving him 21 AC?

The shield spell usually has a casting time of 1 Reaction, meaning that the spell cannot be case as an action. However, the item's description does say:

"With the staff in hand, you can use your action to cast one of the following spells from the staff if the spell is on your class’s spell list: mage armor (1 charge) or shield (2 charges)".

This is likely where the confusion is coming from. I think this might just be poor wording, since shield cannot be cast as an action ordinarily. On the other hand, it uses 2 charges, and usually with magic items that cast spells via charges, the number of charges matches with the spell's level (as it does for mage armor), so it might be that this was intentional, hence why this shield is more "expensive"...

I've also just found this answer to a different question specifically about this confusion, and it appears that Jeremy Crawford says you can cast shield as an action with this staff. I'll include their quote here as well:

"That staff does, indeed, let you cast shield as an action. A DM could also allow casting the spell as a reaction."

However, you can still only gain the benefits of the shield spell once, so multiple overlapping castings won't help you. Here's the quote that most of the other answers have already included:

Combining Magical Effects

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap.

• IMHO the Staff of Defence needs some errata to clarify the action use! (I would probably house-rule that the wielder uses their action or reaction as appropriate when casting Mage Armor or Shield from the staff).
– PJRZ
Commented Jul 10, 2018 at 13:17
• @PJRZ Quite, I did a double-take when I read that, and can only assume that the "you can use your action" was written only with mage armor in mind... but yes, it would be good for this to be officially cleared up. Commented Jul 10, 2018 at 13:21
• @PJRZ Whereas that is what the item says keep in mind that it was the "first" 5E module and it could be a typo. Seems weird to have that spell be an action for 2 charges when the overall convention is a charge per level of the spell. Barring the Conjure Elemental from Staff of the Magi which likely costs more because it reduces the 1 minute cast to one action. Commented Jul 10, 2018 at 13:38

You will find most answers in the paragraph titled "Combining magical effects":

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. (PHB 205)

Thus mage armor and shield stack, but two castings of shield do not.

The passive bonus of the staff always applies, there aren't any conflicts regarding that. Your DM might say you cannot wield two staves of defense at the same time, though (DMG 141, "Multiple items of the same kind").

Calculation

Your calculations are spot on, except that you can't benefit from Shield twice, because Chapter 10 of the PHB says:

Combining Magical Effects

The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect — such as the highest bonus — from those castings applies while their durations overlap.

Conflict

In fact, there's a potential conflict with that item and the Shield spell. The staff says you "use your action..." but the spell is a Reaction, "which you take when you are hit by an attack or targeted by the magic missile spell".

Lost Mines was written before the DMG was finalized, so some of the items in it are a bit wonky. For example, the Staff of Defense and the Spider Staff require the spells they make available to be on the wielder's list - staves from the DMG do not.

Conclusion

The latest errata I could find (version 2.2) doesn't address the conflict specifically. However, the Chapter 7 of the DMG, under Activating an Item says:

Spells

[...] The spell uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration. Certain items make exceptions to these rules, changing the casting time, duration, or other parts of a spell.

The Staff of Defense is a specific case that overrides the general case. The wielder must use it as described - casting Shield as an action and cannot cast it from the staff as a reaction.

AC 21 is Max.

TL;DR Cannot Benefit from Shield Twice.

Shield Casting and Effect Stacking

If he casts shield as an action with the staff, does this add +5 to his AC total from the previous circumstances giving him 21 AC? Can he then cast shield as a reaction spell when attacked by an enemy giving him a total of 26 AC?

The above situation is not valid on two counts.

1. First, The character can only benefit from the effects of shield once.

Combining Magical Effects:

The effects of different Spells add together while the durations of those Spells overlap. The effects of the same spell cast multiple times don’t combine, however. Instead, the most potent effect—such as the highest bonus—from those castings applies while their durations overlap.

For example, if two clerics cast bless on the same target, that character gains the spell’s benefit only once; he or she doesn’t get to roll two bonus dice.

2. Shield cannot be cast as an action. It can only be cast as a reaction with the following condition:

"when you are hit by an attack or targeted by the magic missile spell"

Shield can only be cast as a reaction.

Your AC calculation is mostly correct, but with one exception:

Shield has a casting time of "1 reaction", meaning it cannot be cast as an action.

Thus, your wizard can achieve an armor class of 21 (10 + 2(Dex Mod) + 1(Staff Bonus) + 3(From Mage Armor) + 5(Shield, only cast as a REACTION)), which is still very good, and will give your wizard the best AC of the party.

• Hi! Welcome to RPG.SE! Take the tour. This is a good answer, but the calculation at the end is slightly off; mage armor gives 13 + DEX, so it's not quite 10 +2 (from DEX) + 3 (from mage armor), but rather 13 (from mage armor) +2 (from DEX). Still adds up to the same value, but the math behind it is slightly different. Commented Jul 10, 2018 at 13:17
• The staff specifically says you can cast shield as an action though. Commented Jul 10, 2018 at 13:18
• @PurpleMonkey Whereas that is what the item says keep in mind that it was the "first" 5E module and it could be a typo. Seems weird to have that spell be an action for 2 charges when the overall convention is a charge per level of the spell. Barring the Conjure Elemental from Staff of the Magi which likely costs more because it reduces the 1 minute cast to one action. Commented Jul 10, 2018 at 13:36