So a situation came up last night where a player with a monk/rogue was flanking (I use the optional flanking rules to grant advantage) an enemy. He was armed with a shortsword, but even with advantage, he missed the attack roll and thus did not get to use his sneak damage.
Since he had taken the Attack action with a monk weapon (shortsword), he could make an unarmed strike as a bonus action, as granted by the monk's Martial Arts feature. He still has advantage on that role since nobody had moved and the attack hit. But then the issue came up; is unarmed strike a finesse weapon? Is it eligible for sneak attack?
I ruled yes and allowed the sneak damage on this attack, but should I have allowed this or strictly should this not have worked?
Best Answer
Strictly by Rules as Written and Intended, no.
By RAW, no. What you ruled doesn't work. Let's look at the important passages from the skills involved.
Martial Arts:
Sneak Attack:
While unarmed strikes count as weapon attacks, they are not weapons. It's an important distinction precisely because of situations like this.
Also note how Martial Arts doesn't say that it grants finesse. It could have and it would have been simpler. The fact they felt the need to write it the way that they did suggests that their intent was to keep the Monk Martial Arts skill separate from things like Sneak Attack.
By Rules as Fun, yes.
There's solid logic to your ruling and it gives one of your players an extra little oomph and, more importantly, fun. A ruling that goes against RAW isn't exactly wrong, especially in 5e where the rules talk a lot about the power of the DM.
What to do going forward.
What you should do in the future depends on how your table is run. If you want to stick to RAW or RAI, then you should probably tell the group at the next meeting that after further research you've decided that was a one time thing.
But if you don't mind breaking RAW and RAI a little because it's fun for the table, then there's nothing wrong with this interaction. Just keep in mind that you will be making your PC a little stronger than he's intended to be. Nothing wrong with that though, sometimes that's really what makes a campaign fun!