McFighterson has taken an Attack action and he's made an attack with a weapon that he's holding in one hand. Can you elaborate further why doesn't that qualify for the bonus action? Your argument isn't clear to me.

@Ruse Added another paragraph explaining vis a vis Extra Attack being a red herring.

It doesn't say the entire Attack action has to be spent making such attacks. Even one such attack within the Attack action should qualify. (I think Crawford has tweeted to say precisely that, but can't find it.)

@V2Blast Crawfords tweets are no longer any more significant than any other random musing on the internet. Once they get into a Sage Advice Compendium, they become a different story.

@NautArch How is it the same? Agile Parry works as long as the Kensei is holding the weapon. An L3 or L4 Kensei holding the weapon can't actually swing it if they want to get the AC bonus; at L5, they have Extra Attack and can use the weapon and make an unarmed attack. (Note that Martial Arts and/or Flurry of Blows do not trigger Agile Parry, because they are not the Attack action - they're bonus actions.)

I'd love to know why crawford's tweets are no longer arbitration. Also, your answer relies on the assumption that the "with a one handed weapon" clause applies to both the "take the attack action" and "make an attack".

@MilesBedinger The latest version of the SAC says his tweets are no longer official rulings, just previews of possible content in future SAC updates. It gives him room to change his mind.

@T.J.L.: They might not be "official rulings", whatever that counts for, but they're still guidance from the rules designer of 5e and are helpful to consider in one's answer where relevant (even if you disagree with the tweet). Relevant meta. In any case, it's irrelevant unless someone actually finds such a tweet and I'm not misremembering.

@V2Blast Sure, but I don't have to dispute him in my answer. If somebody else wants to use it to support their answer, that's their prerogative.

@NautArch Agile Parry has text that explicitly covers just holding the weapon. Two Weapon Fighting's text said you have to make the attack action with both weapons in hand; there's nothing that allows you to split it up. There's also no text so support Extra Attack changing the parameters of TWF.

Edited to add: "In any case, it's irrelevant unless someone actually finds such a tweet and I'm not misremembering." (If such a tweet exists, you don't have to dispute it in your answer - nor would someone else have to include it in their answer as support - but it would strengthen your answer to address such counterarguments and explain why you disagree with that ruling.)

@miles-bedinger you've got your answer and tjl has theirs. Let's let the votes and OP sort it out :)

I always inferred from the way the Attack action is set up (it lets you make an attack, or X attacks with class features, with movement between them in the latter case), that you take the Attack action, at which point you spend a period of time in which you are actively taking the action, during which you are making attacks and potentially moving.

I don't know how to illustrate this well, but I'm picturing something like Attack Action: (attack, move, attack, move, draw weapon with free object interaction, attack), where you start the action at the beginning, but everything within the parenthesis is still mid-attack-action, since you haven't finished making the attacks it allows you.

As such, the attack action would still be ongoing at the point where they switched to 2 light 1h weapons and attacked with one of them, meaning they still qualify for both A) taking the Attack action and B) attacking with a light melee weapon.

(Especially since you cannot insert bonus actions between attacks during the Attack action, and must resolve it fully before doing so)

I can understand the viewpoint of the latter, but the combat movement rules basically say "You can break up your movement between/around actions on your turn, and also, you can break up your movement between multiple attacks"- that phrasing indicates to me that multiple attacks are supposed to basically be clumped into/considered part of that one Attack action, as opposed to only that first attack at the beginning of the Attack action being part of it.