Shadowblade, BB/GFB and Two Weapon Fighting

You seem to be missing the update to Bladesinger in Tasha's, which allows them to cast a cantrip in lieu of one of their attacks when they take the Attack action.

Tucker Collins

Leaving out the Booming Blade question, and just attacking with a short sword and a shadow blade, I am looking at the Two Weapon Fighting as having 2 requirements: 1) take the Attack action (which you would if you just attack with the short sword and don't use the Cast a Spell) and 2) attack with a light melee weapon that you're holding in one hand (which you have done if you attack with the short sword.) If both those requirements are fulfilled, you can use the other hand for a bonus attack, which in our case is holding the Shadow blade. There is no statement that you can ONLY attack with that weapon and hand (even with multiple attacks), just that you have attacked with it (which is fulfilled if you do it once). If you were an Eldritch Knight and attacked once with the short sword, and 3 times with the Shadow Blade, you would still fulfill those two requirements, and qualify for the Bonus Attack in the opposite hand from the one you used with a light melee weapon.

I am very confused by this, and need to see your other video you mention, but it is not linked yet, and I couldn't find it on your Youtube page. The Extra Attack from Bladesinger and the Bonus action attack from Two Weapon Fighting both require you to use the Attack action. However, to use Booming Blade, you are not using the Attack action- you are using the Cast a Spell action, which then lets you make an attack as part of that Cast a Spell action- these are not the same things. Thus neither of the things triggered by the Attack action would occur. I must be missing something, but I am not sure what it is.

Similar to the complexities of shadow blade, the soulknife rogue's Psychic Blades manifest "whenever you take the Attack action", rather than "whenever you make a melee weapon attack", so RAW they cannot be used for attacks of opportunity, booming blade / green flame blade / steel wind strike...