Suppose I'm not currently holding a sword, but my ally is going to toss one to me on their next turn. Can I ready an action to attack with the sword that I'm not yet in possession of, with a trigger of "when I acquire a sword"?

A similar situation arises with spells that grant the ability to perform new actions, such as dragon's breath. Can I ask my wizard ally to cast this spell on me and then ready an action to breathe fire with a trigger of "when I have dragon's breath cast on me"?

In general, the question is: if I am not currently capable of taking a certain action, can I still ready that action if I expect to become capable of it some time in the next round?

Yes you can Ready an action you currently cannot do

The rules for Readying say:

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.

Readying doesn't involve doing anything at all at the time that you Ready. All you have to do is choose "the action you will take". Decide a trigger and decide a future action, there are no other requirements other than that.¹

Or, as @RyanThompson says in a comment below:

The key idea is that the ready action means you're doing nothing now and maybe doing something later, and you can always do nothing right now.

Thus, there is no rules or logical reason why you cannot ready an action "to attack with a sword when a receive it from my ally" or "breath fire once my ally casts dragon's breath on me".

Of course if the thing or ability your action depends on fails to get to you before your next turn you have wasted your action, so this is even riskier than a normal Readied action.

Exception: You cannot Ready a spell you cannot cast

Readying a spell requires you to start casting that spell as part of the Ready action.

When you ready a spell, you cast it as normal but hold its energy...

If you do not meet the requirement to start casting spells (or that spell in particular) then you cannot Ready a spell. It is good to note that you don't have to choose a target or even have one in range to start casting the spell.

^{1 - You can describe the Ready mechanic narratively as taking some preparatory action (holding a sword above your head ready to strike, getting into a sprinter position to get ready to move, etc.) in your game, but that is not dictated or even implied by the rules. The rules simply state the mechanics of taking the action and let the DM and players find a narrative that work with it. Following the rules and creating a compelling logical narrative can sometimes be at odds with each other and nothing in the game says that they have to get along all the time.
It is up to the DM/table to decide where to draw that line and where and how much to tweak rules they see as crossing it. Regardless, the rules say what they say.}

Yes, you can ready actions you are not currently capable of performing*

(*Well, you can't ready a spell that you were not capable of casting, but I don't know how that would come up.)

What are the requirements for readying? (emphasis mine)

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.

and

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.

Examples include "If the cultist steps on the trapdoor, I'll pull the lever that opens it," and "If the goblin steps next to me, I move away."

It's simple enough to imagine a readied action to the effect of "If I receive a sword from my buddy Fighter McFighterson, then I attack the goblin." (Though, you may get some guff from your DM about interacting with an object off turn. Personally, I'd allow it.), or "If someone casts dragonbreath on me, I use it on the toothpick golems.".

Note 1 : Readying spells requires you to actually "cast" the spell and hold it until your trigger. That's why, if for some reason you received a spell slot, you couldn't then react to that by casting a spell.

Note 2 : Don't forget that if you use one of these triggers dependent on someone else, and the thing doesn't happen, you've wasted your action.

Yes. The player can state any action for the character to ready. It is incumbent on the DM to narrate what happens if the action is impossible.

TL;DR This comes back to: the player states what the character does; the GM narrates the result.

Ready an action:

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.

Having dragonbreath cast on you is a perceivable circumstance. Attempting to breathe fire on enemies is an action you will take in response to the trigger. This satisfies the player part of how adventuring works:

1. The DM describes the environment.




2. The players describe what they want to do.




3. The DM narrates the results of their actions.

Narrative Example

GM: "It's the rogue's turn"

Wizard Player: "My wizard exclaims to the rogue, 'Get ready to breathe fire!'"

Rogue Player: "My rogue nods knowingly to the wizard, and readies their action to breathe fire on the goblins as soon as the wizard casts dragon breath on him."

DM: "The party rogue takes a deep breath and glares angrily at the goblins as he begins to turn red in the face. It's the wizard's turn."

Wizard Player: "I cast dragon breath on the rogue."

DM: "You touch the rogue as you finish casting dragon breath. Our crimson faced rogue does his best dragon impression and breathes fire on the goblins. Roll for damage."