Greetings D&D Beyond,

I am creating a Soulknife Rogue for a couple One-Shots that are coming up for our group, and I have a question on the Psychic Blades: Bonus Attack.

The Psychic Blades Bonus Action under the Soulknife Subclass states the following:
"After you attack with the blade, you can make a melee or ranged weapon attack with a second psychic blade as a bonus action on the same turn, provided your other hand is free to create it. The damage die of this bonus attack is 1d4, instead of 1d6."

Do any of you know why the 'Bonus Action' is only activated if the first attack is with a Psychic Blade?

I was hoping I could use my Rapier in one hand and then manifest a Psychic Blade in the other for the 2nd attack, but the way it is worded, I would not get the Bonus Action if I use my Rapier.

Thoughts?

I can't claim to be an expert but based on the wording the Psychic blades are created with an attack action. It could be just a wording issue but part of the appeal for that subclass is being unarmed but always armed. I have a party member who plays a Soulknife and the only real reason they have a physical weapon on them is to screw with people or to surrender them to guards if needed. You may be able to get away with switching and using the rapier in off hand.

I don't know if it was an oversight on the part of the designers, or if it was a deliberate attempt to make the class less exploitable. I'm sure there's some kind of crazy multiclass combo someone could figure out that would overly take advantage of the ability to take a full attack action with any weapon you want, and still get a free bonus action thrown dagger thing. But the way the Psychic Blades are designed seems very, very aggressively designed to only be useful for a Rogue. If you have Extra attack you basically can't combine the Psychic Blades with it, since the blades vanish after any attack roll, hit or miss. The fact that the bonus action blade reduces to a d4 kind of just makes it a slight update to keeping a dagger in your off-hand for Two-weapon Fighting, which means it's mostly useful for trying for a second sneak attack if your initial attack misses.

I'd treat them as simple weapons, i.e. the two weapon fighting rule. 

Their damage dice fit within the context, and the second die is in line with the "bonus action attack is lower damage". 

My guess is this is what they were going for anyhow.

The fact that this guy gets a cool magic mind-knife and still wants to attack with a rapier tells you all know need to know about rogue weapons in 5e. They should just change the class icon to a rapier already.

If you're mainly worried about being able to attack off-turn, you can hold a real weapon and swap it to the empty hand as you're attacking with the psychic blades. Whether that uses up your free Interaction with an Object for the turn is up to your DM.

This may be a bit of a tangent, but as Transmorpher stated, I think the designers were trying to future-proof the Psychic Blades from any possibility of exploitation and overshot the mark. Not only can they not be used for extra attack if the rogue is multiclassed, they can essentially never be used for an opportunity attack (if I understand correctly) rules as written.