Why is Soulknife Completely Incompatible With Extra-Attack?

The Soulknife subclass of rogue has the "psychic blades" ability, which says this:

You can manifest your psionic power as shimmering blades of psychic energy. Whenever you take the Attack action, you can manifest a psychic blade from your free hand and make the attack with that blade. This magic blade is a simple melee weapon with the finesse and thrown properties. It has a normal range of 60 feet and no long range, and on a hit, it deals psychic damage equal to 1d6 plus the ability modifier you used for the attack roll. The blade vanishes immediately after it hits or misses its target, and it leaves no mark on its target if it deals damage.

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.

Why is this ability entirely incompatible with extra-attack? For instance, if I multi-classed into fighter, taking five levels, why can I not make a psychic blade for both my attacks? Why do I have to use a bonus action to make another one? Does it make sense to just say I can make another one with my second attack but it only does 1d4? 

It's perfectly compatible with Extra Attack. You just can't use it in your extra attacks. You have to use another weapon but your Extra Attack feature works just fine. Plus, you get a pseudo light weapon attack as a Bonus Action.

The rest of your question is a house rule question because attacking with it multiple times in the attack action is not RAW.

Why did the designers make this design choice? Probably to restrict the follow up abilities relating to attacking with your psychic blades.

I also think it's right, but as discussed in Rogue/Ranger multiclass: How to use Ranger fighting styles with Soul Blades, just note that it doesn't work for dual-wielding light weapons, since it isn't a weapon with the Light property.

Oh, also, one more thing if it helps somehow. The feature has been updated in the 2024 PHB. Now it can be used with Opportunity Attacks.

Level 3: Psychic Blades

You can manifest shimmering blades of psychic energy. Whenever you take the Attack action or make an Opportunity Attack, you can manifest a Psychic Blade in your free hand and make the attack with that blade. The magic blade has the following traits:
[...]
The blade vanishes immediately after it hits or misses its target, and it leaves no mark if it deals damage.

After you attack with the blade on your turn, you can make a melee or ranged attack with a second psychic blade as a Bonus Action on the same turn if your other hand is free to create it. The damage die of this bonus attack is 1d4 instead of 1d6.

Okay, do you think it is reasonable to allow it to work with extra-attack?

Quote from cardboardboxofdirt >>

Okay, do you think it is reasonable to allow it to work with extra-attack?

 Sure! It's legit per the rules as SmiteMakesRight_3_5 said. Maybe you need to draw a weapon before the extra attack, but I'd say that's all.

EDIT: sorry, I didn't read the question properly. If your question is about attacking twice or more with the same Psychic Blade, that's DM territory.

But if it's fun for you and allowed by your DM, go for it :)

Give people multiple melee and or ranged attacks and they are still not happy.

I really don't see a problem between Extra Attack feature and Psychic Blades.

Attack Action:

When you take the Attack action, you can make one attack roll with a weapon or an Unarmed Strike.

Level 5: Extra Attack

You can attack twice instead of once whenever you take the Attack action on your turn.

It should just let you attack twice with the main blade (or repeat twice the attack if you will). Since psychic blades is not a replacement effect of the attack action or is it considered that?

To each his ownI guess, and talk to your DM about it.

Quote from Jkrentzien >>

I really don't see a problem between Extra Attack feature and Psychic Blades.

The psychic blades disappear after they are used to attack so they can't be used as the only weapon to attack multiple times as part of the attack action. They can be used as one of the weapons just fine.

You can attack with psychic blade and bonus action with a light weapon (which the psychic blade is not). If you have a physical weapon with light property in your hand, that off hand can attack with a bonus action. Now, if you attack with a psychic blade and have a scimitar available, you can you the 'nick' property to attack a second time in same attack. No dmg bonus to 2nd attack. Adding dual wielder feat allows you to attack with an off hand weapon, and apply the dmg bonus. Two weapon fighting allows you to attack with off hand with a weapon that doesn't have the two handed property.  

This means...a blade, created out of thin air, weighing...nothing, is not LIGHT. the creators made this choice to not over power the rogue. But, all it did was pose questions, without viable answers other than....because dnd said so. Infuriating

Quote from EzbinAlMakik>>

You can attack with psychic blade and bonus action with a light weapon (which the psychic blade is not). If you have a physical weapon with light property in your hand, that off hand can attack with a bonus action. Now, if you attack with a psychic blade and have a scimitar available, you can you the 'nick' property to attack a second time in same attack. No dmg bonus to 2nd attack. [...]

 The Light property and Nick doesn't interact with Psychic Blades in any case, since they aren't Light.

[...] Adding dual wielder feat allows you to attack with an off hand weapon, and apply the dmg bonus. Two weapon fighting allows you to attack with off hand with a weapon that doesn't have the two handed property.  [...]

 Not exactly.

Dual Wielder feat:

[...] Enhanced Dual Wielding. When you take the Attack action on your turn and attack with a weapon that has the Light property, you can make one extra attack as a Bonus Action later on the same turn with a different weapon, which must be a Melee weapon that lacks the Two-Handed property. You don’t add your ability modifier to the extra attack’s damage unless that modifier is negative. [...]

Two-Weapon Fighting feat:

When you make an extra attack as a result of using a weapon that has the Light property, you can add your ability modifier to the damage of that attack if you aren’t already adding it to the damage.

Btw, some related threads:

- Rogue/Ranger multi class: How to use Ranger fighting styles with Soul Blades
- Can a 3rd Level Rogue Soulknife get 3 ranged attacks using Light Weapons and Nick Mastery?
- Weapon and Psychic Blade 

Since the trade off is less sneak attack I would just assume the blade reappears whenever you need it to.  This is not OP at all.  Its just a weapon basically.

Psionic blades end up being worse than magic weapons eventually.

Quote from packman767 >>

Since the trade off is less sneak attack I would just assume the blade reappears whenever you need it to.  This is not OP at all.  Its just a weapon basically.

Psionic blades end up being worse than magic weapons eventually.

 RAW, Psychic Blade manifests whenever you take the Attack action or make an Opportunity Attack:

You can manifest shimmering blades of psychic energy. Whenever you take the Attack action or make an Opportunity Attack, you can manifest a Psychic Blade in your free hand and make the attack with that blade. The magic blade has the following traits: [...]

EDIT: see Thezzaruz's reply for extra details.

I think it's designed as it is due to balancing. A psychic blade can be used both melee and ranged. Being able to extra attack, then bonus action attack, with only psychic blades, meaning you can make three ranged attacks with no ammo or loading, and can never be disarmed of your weapon without actually severing your hands. That would not be balanced, even with the 5-level multiclass needed. 

I think it is balanced well as it is. ¯\_(ツ)_/¯

Quote from TarodNet >>

Quote from packman767 >>

Since the trade off is less sneak attack I would just assume the blade reappears whenever you need it to.  This is not OP at all.  Its just a weapon basically.

Psionic blades end up being worse than magic weapons eventually.

 RAW, Psychic Blade manifests whenever you take the Attack action or make an Opportunity Attack:

You can manifest shimmering blades of psychic energy. Whenever you take the Attack action or make an Opportunity Attack, you can manifest a Psychic Blade in your free hand and make the attack with that blade. The magic blade has the following traits: [...]

 Do you visualize the blade appearing and then disappearing between attacks, or simply appearing and persisting for the entire turn or encounter?  What I visualize is a conjured weapon that sort of ignites like a lightsaber and remains until I extinguish it.  But if it is more like the cantrip Sword Burst where it only lasts for the attack and then is gone, I would think of it more like a spell than a weapon.

It’s an interesting mechanic, and one that I might be using incorrectly.  LOL

Quote from packman767 >>

 Do you visualize the blade appearing and then disappearing between attacks, or simply appearing and persisting for the entire turn or encounter?  What I visualize is a conjured weapon that sort of ignites like a lightsaber and remains until I extinguish it.  But if it is more like the cantrip Sword Burst where it only lasts for the attack and then is gone, I would think of it more like a spell than a weapon.

It’s an interesting mechanic, and one that I might be using incorrectly.  LOL

The feature explicitly say that it disappear.

The blade vanishes immediately after it hits or misses its target, and it leaves no mark if it deals damage.

One might also note that Rogues doesn't get Extra Attack from their class so the fact that it doesn't mention anything about "between attacks" is because there isn't expected to be any between to worry about (the designers aren't much for considering multiclassing effects in their descriptive language).

And the part of the feature that allows for a Bonus Action attack clearly says it is made with a different blade (and a different hand) so also no between possible.

Quote from packman767 >>

Since the trade off is less sneak attack I would just assume the blade reappears whenever you need it to.  This is not OP at all.  Its just a weapon basically.

How is it less sneak attack exactly?

The Psychic Blades qualifies for Sneak Attack (they are finesse weapons) normally and the ability effectively bundles a specialized Two-Weapon Fighting and Enhanced Dual Wielding. The only differences between having those feats are that the Bonus Action attack is a d4 + attribute modifier instead of d6 + attribute modifier and, of course, the Psychic Blade abilities are restricted to attacking with the psychic blades.

If advantage on the sneak attack isn't enough, homing blades at level 9 should almost guarantee a sneak attack hits. I don't anticipate that they will actually fall behind a magic weapon.

while i am not sure this is what they intended it can't be used with things like commanders strike, its only with the attack action and attacks of opportunity. Other off turn attacks wont work with it. 

They really should have made it summoned weapons like pact of the blade style. 100% easier to adjudicate, less balance issues. Allow them to turn magic weapons that fit a certain class like one banded light blades into psychic versions. 

And they do fall behind magic weapons from levels 3-8 which is a huge part of most games. Level 9 probably not, but they probably are not better.