Do you get your free interaction during a readied action?

Question

Example situation:
You're on the other side of a door. You have a spell slot available for Magic Missiles. Can you ready an action to, on the Rogue's signal, open the door and cast Magic Missiles at someone behind the door?

Two situations are possible:

• You can use your free interaction as part of your Readied Action to open the door before casting the spell.
• You need to spend a turn opening the door, wait 6 seconds, and then cast Magic Missiles on your following turn.

Which is it? Is it a third case?

More generally:
Do you get your free interaction during a readied action?

Answer 1

By RAW, the answer is No if not on your turn.

Ready Action states:

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. 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."

The key here is that unless the trigger occurs on your turn, then your Readied Action does not occur on your turn and you do not have access to on turn aspects of the rules. In addition, they have limited your Readied choices to Action or Movement and not both. The Readied Action is both limited in scope and dependent on a specific trigger.

This includes things like Extra Attack, but extends to Free Object Interactions, which can be found under:

Other Activity On Your Turn

You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack.

Note that the section is titled On Your Turn. If your Readied Action occurs on your turn, then you can interact with an object or feature for free, but if it does not, then this is not available. And for those wondering, Section Titles are important (emphasis is mine)!

In any piece of writing, context matters. If a rule has multiple sentences, they're meant to be read together. For example, the first sentence of Divine Sense is meant to be read with the rest of the feature's sentences, which explain that first sentence.

But does it really matter?

At first pass, I'd probably say no. Although the designers have gone out of their way to consistently rule what can and can't be done on a turn and the limitation of ready to an action OR movement (and not both.) They clearly have chosen to limit the extent of the Ready Action and give it a high cost (your reaction usage, less impact for an Attack action, etc.) I do not know the reasons, but allowing side cases seems against their intent.

However, should a DM decide to waive it, I don't see this being a cheese-concern situation, but your mileage may vary and be careful about a slippery slope of Ready actions.

Answer 2

Only if your readied action happens on your turn

That is the general answer.

More generally: Do you get your free interaction during a readied action?

Other Activity on Your Turn (Basic Rules, p. 70)

You can communicate however you are able, through brief utterances and gestures, as you take your turn. You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack.

Your interaction with an object takes place during your turn, not during someone else's turn. (Emphasis mine)

Regarding your specific scenario:

• If your spell casting is triggered on the door opening, someone else needs to open the door for you to use that readied action as your reaction (which is being taken on someone else's turn).

  If you do it on your turn, all you have to do is open the door (interact, free) and cast magic missile.

  If the rogue is an Arcane Trickster, mage hand legerdemain solves this, since him giving you the signal does not cost an action nor a bonus action on his turn, and he can use mage hand to open the door. (That's a specific case of a rogue, though ...)

Answer 3

The Ready Action simply allows you to move or take an Action in response to a trigger which you specify:

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.

Free object interactions happen as part of an action or movement:

You can also interact with one object or feature of the environment for free, during either your move or your action. For example, you could open a door during your move as you stride toward a foe, or you could draw your weapon as part of the same action you use to attack.

I see no reason why a free object interaction can't then happen as part of the action or movement you take as part of the Ready action.

Answer 4

Only if you are not in combat.

All the other answers state what happens during a combat. But from the context of your question, it seems more like an ambush scene than something that is happening while people fling steel, claw and magic at each other.

But if you are not in combat, there is no initiative. There are no limit to actions (to the reasonable point of not monopolizing the DM while nobody else gets to say what their characters are doing).

But now you need some ability tests for both concerned parties, and probably an initiative test.

This DM would call the ability tests upon opening the door, and then the initiative test right after.