When can you use a bonus action?

Question

In the rules for bonus actions it states:

You choose when to take a bonus action during your turn, unless the bonus action's timing is specified ...

I have seen this quote used to justify some gratuitous rulings that I am extremely unsure about. I think there is an assumption that you can't take bonus actions during the resolution of mechanics, but I have seen that many people do not share this opinion.

Take the example of Misty Step. Misty Step is a bonus action spell that lets you teleport 30ft. Can you use Misty Step in the following situations (all examples take place on the player's turn);

1. When you are the target of an opportunity attack to avoid the attack?
2. Once an opportunity attack has hit you but damage has not been dealt to avoid damage?
3. When pushed off a cliff to avoid falling to the bottom?
4. When using a Necklace of Fireball on your own position, but teleporting away before the damage is dealt?
5. When stepping on to a pressure plate trap but before the trap activates?
6. Some other examples you can think of to illustrate the RAW limits to bonus actions

Answer 1

You can only take bonus actions when you could take an action on your turn

The PHB says this on page 189:

Bonus Actions Various class features, spells, and other abilities let you take an additional action on your turn called a bonus action. (...) anything that deprives you of your ability to take actions also prevents you from taking a bonus action.

Anything includes, well, anything, including timing restrictions. Can you take an action in response to being targeted or hit by an opportunity attack, by triggering a trap, or in reaction to your main action?

I don't think you can. This is an instant response to a trigger of some kind, and that is the very defintion of a reaction:

A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else’s.

Because you cannot take an action as an instant response, and you only can take a bonus action when you can take an action, you also cannot take a bonus action.

Answer 2

These scenarios are examples of when you'd use a Reaction, not a bonus action.

There are 4 types of actions and you get 1 of each per turn as explained in Phb Pg 189-190 (unless an ability says otherwise):

• Actions - Main action (Phb has a list of default actions, but many action options will come from class abilities as well)

• Bonus actions - Quick minor action (Can be performed at any time during your turn that you can perform an action, possibly while an action is also being performed see: Can you use a bonus action between the separate attacks of a spell?)

From PHB- 189 states the concerning lines, emphasis mine.

You choose when to take a bonus action during your turn, unless the bonus action's timing is specified, and anything that deprives you of your ability to take actions also prevents you from taking a bonus actions

• Reactions - Instant reaction to a trigger, so long as the trigger is met, it can occur during any point of anyone's turn.
• Free Actions - Mechanically instant, but limited options (talking, dropping something)

(some might include Readied Actions, which basically set a single action to become a reaction to a defined trigger the next turn)

Performing actions, including spell casting have specific action time requirements that must be used to perform them as defined in the ability or spell description.

Only item 4 would be an example where you are not being acted on by an external entity, provoking a reaction. In the case of 4, you'd use your bonus action to misty step first and then toss the pearl where you were previously standing and flavor the action as described. (See Bonus actions above as you can do both simultaneously).

The other examples would be times in which you can not perform an "action" in response to the situation.

• As a result of triggering traps, only after.
• As a result of an attack of opportunity, only after. (There are abilities that let you act during these times, but all of them AFAIK are reactions, requiring reactions to use)

There are reaction spells specifically made for these scenarios though.

• Shield is a reaction spell for 1 and 2
• Feather fall is a reaction spell for 3
• Absorb elements (among other reactions) is a reaction spell for 5

These are able to be used as reactions per description of the ability. Somewhat silly to some as that means you can't cast feather fall as an action, but requires a reaction to use. Similarly you can't use bonus actions as though they were reactions.

• There are plenty of martial reactions that may be the solution to these scenarios, but regardless, they would need to be reactions rather than bonus actions.

Answer 3

We simply don't have rules, so use bonus actions whenever your GM lets you

The bottom line here is that we simply don't have a lot of hard rules on the timing of bonus actions. We know they can only be used on your turn, but that's about it. We don't even know if reactions can be taken when targeted but before being hit. If your GM wants you to be taking bonus actions during or in the middle of other things that are going on, they can. If they don't want that happening, then they can do that as well.

As far as the rules are concerned, there's nothing specifying the timing of actions or bonus actions (which are actions too); reactions, however, by default, occur after their trigger. Lacking any specific rules on action-resolution, "stacks" (such as from Magic: The Gathering), and action-timing, these sorts of questions are left entirely to the GM.

Answer 4

They're Not "Bonus Reactions"

I have been a DM far more than I have played over the years, and in my experience, the short answer for all of the Misty Step side cases you have mentioned, is "no". That said, I have long been a proponent of the "rule of cool". If being able to misty step away from damage is what makes the game fun for you and doesn't put off the rest of your party (assuming your DM agrees), then by all means, make its casting time "a bonus action or reaction".

I say that because that is essentially what the people you speak of are confusing it with. If one of my players has interrupt me to say, "I _____ before the trap activates..." then in my experience, they are expending a reaction, not a bonus action.

The bottom line is that this "bonus action" feels like something else entirely, and that something is a reaction:

A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else’s. -PHB, CH 9. Combat

If a spell or ability takes a bonus action and you want to use it like a reaction, then it is time to talk to your DM about house-ruling casting/activation time, not arguing about when a bonus action can be used.

As for actually using a bonus action, bonus actions are just actions that are granted by a character feature, item, or spell. You can't take a bonus action unless something you have says you can, but they are still taken as you would any other action.

As always, wording is important. You will note that WOTC carefully neglected to call these "bonus reactions".