Do the AC Bonus (Ex) class abilities of monks and swordsages stack?

Question

Monk and Swordsage both get the class feature AC Bonus (Ex).

Monk:

When unarmored and unencumbered, the monk adds her Wisdom bonus (if any) to her AC. In addition, a monk gains a +1 bonus to AC at 5th level. This bonus increases by 1 for every five monk levels thereafter (+2 at 10th, +3 at 15th, and +4 at 20th level). PHB, p. 40

Swordsage (ToB, p. 16):

Starting at 2nd level you can add your Wisdom modifier as a bonus to Armor Class, so long as you wear light armor, are unencumbered, and do not use a shield.

Do these bonuses stack?

Answer 1

Nope.

Strictly speaking, the Swordsage ability only works in light armor, and the Monk ability only works when he's in no armor at all.

Gotcha!

Answer 2

Out of the box mentioned abilities don't interact.

This is because by RAW swordsage needs light armor¹ for her AC Bonus to be active, while monk's AC Bonus works specifically when unarmored.

But with a little effort...

... we take two levels of argent fist prestige class (Faiths of Eberron, p. 70). This gives your character Holy Armor (Ex) ability:

[...] you retain the AC bonus granted by your monk levels (including Wisdom bonus to AC), even when wearing armor or carrying a shield or a medium or heavy load [...]

Thus we still need to know, do mentioned abilities stack or not.

To the conclusion

It is hard to be 100% sure on this issue, but there are some (not too strong) evidence that different abilities named ‘AC bonus’ stack unless one of the abilities specifies otherwise. For example, fist of the forest prestige class (Complete Champion, p. 80) has:

AC Bonus (Ex): While unarmored, you gain a bonus to your Armor Class equal to your Constitution bonus (if any). See the monk class feature (PH 40).

and shows in the sample stat block:

AC 21 [...] (+3 Con, +5 Dex, +2 Wis, +1 class).

On the other hand, AC Bonus feature of a ninja class (Complete Adventurer, p. 5) specifically states it doesn't stack² with AC Bonus of a monk:

[...] This ability does not stack with the monk’s AC bonus ability [...]

so there is at least a precedence of the rules saying the two don't stack when rules' authors don't need them to stack.

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1. There are many posts on the net stating that swordsage's AC Bonus should work in light armor at most, thus unarmored being a valid circumstance. In this case you don't need argent fist to make AC Bonuses of a monk and a swordsage interact - they simply interact all the time. While this has it's own justifications (unarmed swordsage) and is reasonable to some extent, this is not what RAW says.

2. This again may be argued in a way. Some people think that the reason why they don't stack actually is the same stat:

   [...] (a ninja with levels of monk does not add the bonus twice) [...]

   So you could imagine ninja's AC Bonus actually stacking with monk's AC Bonus supposing a monk have taken Kung Fu Genius feat (Dragon Compendium, p. 101), for example. Also, then ‘AC bonus’ class features shouldn't stack if they are keyed of the same stat in general and not for monk and ninja only. But again, this is not what RAW says.

Answer 3

Like Eikre's answer says, normally it doesn't matter if a creature can stack both a monk's extraordinary ability AC bonus and a swordsage's extraordinary ability AC bonus because a monk's AC bonus only applies when a creature wears no armor and a swordsage's AC bonus only applies when a creature wears light armor. So far as I'm aware, there's no way for a creature to count as wearing no armor and actually wear light armor or vice versa.

Nonetheless, in the Ask Wizards Web column "09/04/2006" the Sage weighed in anyway:

Does the AC bonus of the swordsage (from Tome of Battle) stack with the AC bonus of the monk, even though they are both based on Wisdom modifier?
No. The swordsage’s AC bonus, allowing the swordsage to apply her Wisdom bonus to her armor class, mimics the ability of the monk’s AC bonus. Since these abilities share the same name and have the same effect, they will not stack.

As always, keep in mind that the Sage usually offers opinions not errata.

(As an aside, if porting the swordsage to Patfinder, the two special abilities' effects wouldn't stack, Pathfinder's rules for stacking ability score modifiers being different from the D&D 3.5's.)

Answer 4

"Stacking" bonuses usually refers to having them all active at once. By that definition, then no, it is not possible to stack the Monk and Swordsage ability - by RAW the condition to activate either one also deactivates the other.

A Monk only gains their bonus when not wearing any armor or shield. If they put on light armor (or heavier) their AC bonus no longer applies.

A Swordsage only gains their bonus when wearing exactly light armor (see note below). If they do not wear armor, or if they wear medium (or heavier) armor, their bonus deactivates.

Thus these abilities cannot be used at the same time, and do not stack.

However, if a multiclass character took levels in both Swordsage and Monk, they would gain both of these abilities, and whenever they were in light or no armor, the appropriate ability would be active.

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Note: There has been a great deal of debate regarding the Swordsage's requirement to only allow the ability to work in light armor, and whether it was meant to be "in light armor or less". The only 100% official source (officially released Errata) does not discuss the swordsage's Wis to AC ability at all, so in RAW it remains in light armor only.

If you were to house-rule that the Swordsage ability should work when unarmored, then the abilities should stack fully. However, especially at low level, this has the potential to make for unreasonably high AC which will allow characters to breeze through combat. (Literally: all their enemies just keep fanning them off as they try to strike!)

Answer 5

Overlap

(Sort of)

Strictly read, each ability just says "you may add your wisdom bonus to AC." Neither says you can add it as an additional bonus or similarly indicates that it's providing a new AC bonus (typed or typeless); it's just a change to how you calculate AC. So, exactly like taking two classes doesn't allow you to add your dexterity bonus twice, you don't get to add your wisdom bonus twice.

(Not strictly "overlapping," since there is no second bonus to overlap with).

However, even if those abilities had been worded differently (e.g. "Add your wisdom modifier to AC as an untyped bonus"), it probably would not stack anyway, because both sources are your wisdom modifier. It would have to say something convoluted and explicit to actually get 2xWIS (like "[class] gain a dodge bonus to AC equal to their wisdom modifier").

Answer 6

There is also the adaptability section in Tome of Battle for the Swordsage that mentions an unarmored variant that loses proficiency with light armor, but not the bonus to ac that begins at level 2.

If you were to take that variant than they would stack because both classes then require no armor to add the wisdom modifier to ac.

Adaptation section, page 20 Tome of Battle