Can a rogue/Kensei monk use Sneak Attack with a Kensei weapon?

Question

The 3rd-level feature of the UA Kensei Monk subclass, Path of the Kensei, reads as follows:

• You gain proficiency with three martial weapons of your choice. A martial weapon is considered a kensei weapon for you if you’re proficient with it.

• Whenever you wield a kensei weapon, you choose whether to use Dexterity or Strength for the attack and damage rolls of the weapon, and you choose whether to use your Martial Arts damage die in place of the weapon’s damage die.

The way this is written leads me to interpret it as gaining the ability to turn any weapon into a finesse weapon, as long as it is a Kensei weapon. By this logic, the Kensei subclass would allow me to get a finesse longsword. However, it is not specified that the heavy or two-handed properties cannot be on a Kensei weapon.

Does this mean I can make a greatsword into my Kensei weapon and then multiclass into Rogue to sneak attack with it?

Answer 1

No, this is a converse error

Your logic is something like this:

1. Sneak Attack […] The attack must use a finesse […] weapon.

   ^{(Player’s Handbook, pg. 96)}

2. Finesse. When making an attack with a finesse weapon, you use your choice of your Strength or Dexterity modifier for the attack and damage rolls.

   ^{(Player’s Handbook, pg. 147)}

3. Kensai Weapons. […] Whenever you wield a kensei weapon, you choose whether to use Dexterity or Strength for the attack and damage rolls of the weapon,

   ^{(Xanathar’s Guide to Everything, pg. 34)}

4. ⇒ “kensai weapons are finesse weapons” (alleged implication of 2 & 3)

5. ⇒ “kensai weapons can be used for sneak attack.” (implication of 1 & 4)

The problem is in the jump from 3 to 4:

4. “kensai weapons can use Strength or Dexterity” ⇏ “kensai weapons are finesse weapons”

This logical implication is not supported by your evidence (and indeed, evidence supporting it is not in the rules at all).

What you have is a statement that

6. “X is a finesse weapon” ⇒ “X can use Strength or Dexterity” (equivalent to 2)

but what you want is the “converse,” that

7. “X can use Strength or Dexterity” ⇒ “X is a finesse weapon” (6 “in reverse”)

But you cannot logically reverse an implication like that. The reversed statement is known as the “converse,” and a statement and its converse are not logically equivalent. This kind of logical fallacy is known as a “converse error” (or “affirming the consequent” or “fallacy of the converse” or “confusion of necessity and sufficiency”). You cannot determine whether or not a converse is true from the original statement.

In other words, just because all finesse weapons can use Strength or Dexterity, does not mean that all weapons that can use Strength or Dexterity are finesse weapons.

As an example, consider this:

8. “X is a finesse weapon” ⇒ “X can be used for sneak attack” (equivalent to 1)

This is a true statement, a rewording of what I quoted from PHB pg. 96. Now consider its converse:

9. “X can be used for sneak attack” ⇒ “X is a finesse weapon” (converse of 8)

We know that this statement is not true. Because page 96 actually says “The attack must use a finesse or a ranged weapon,” so “X can be used for sneak attack” could mean X is a finesse weapon—or it could mean that X is a ranged weapon. Just knowing that X can be used for a sneak attack, and knowing the rule on page 96, is not enough information to decide whether or not X is a finesse weapon. We need another statement—like “X is not a ranged weapon,” or even something like “all weapons that can sneak attack are finesse weapons [yes even the ranged ones]” (which wouldn’t make a lot of sense in the context of the game but wouldn’t be any kind of logical problem), in order to answer whether or not X is finesse here.

Likewise, knowing that kensai weapons can use Strength or Dexterity is not enough information to say they are finesse weapons: you need an actual statement saying that they are finesse weapons, or that all weapons that can use Strength or Dexterity are finesse weapons, or something. Unfortunately for you, the rules don’t provide any such statement.

Answer 2

No.

The Rogue Sneak Attack REQUIRES the weapon to be ranged or finesse.

Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll. The attack must use a finesse or a ranged weapon.

While the kensai weapon bonus provides a similar benefit, it does not actually change the weapon to become a finesse weapon.

Compare the wording on Monk Weapons to the Revenant Blade Feat:

Monk Martial Arts Class Feature:

You can use Dexterity instead of Strength for the attack and damage rolls of your unarmed strikes and monk weapons.

Revenant Blade

A double-bladed scimitar has the finesse property when you wield it.

The feat specifically grants the finesse property, which is what is needed for the rogue sneak attack class feature, the martial arts class feature grants a similar benefit, but does not specifically impart the finesse property.

Related: Can unarmed strike work with a rogue's sneak attack?

Answer 3

No, a Kensei weapon doesn't gain finesse

While the Kensei allows you to use your Dexterity modifier this doesn't make it a finesse weapon. Finesse is a specific named property of weapons that just happens to have the same effect as the Kensei feature. Since it is stated nowhere in the Kensei feature that the finesse property is applied, those weapons don't have the finesse property.