Countering the Forcecage spell with reactions?

Question

I am trying to create a reaction that a creature can use in order to avoid the OP effect of the forcecage spell. Here is the reaction description:

When the monster sees a creature within 120 ft. of it casting a spell it moves up to its speed.

If a spellcaster casts forcecage, as a reaction the creature within the spell area can move outside the area before getting caged. My doubt is that a reaction necessarily follows the action, and so if the creature sees the spellcaster casting the spell, it is too late yet. Am I correct? Is there a way to counter force cage using a reaction?

Maybe I could rewrite the reaction as follows:

When a creature enters within a 100-foot radius sphere centered on the monster, it moves up to its speed.

But this seems simply an hilarious chasing to me.

Answer 1

Your phrasing is fine

For comparison, look at Counterspell, it has a reaction timing identical to the reaction timing you list, differing only in range:

when you see a creature within 60 feet of you casting a spell

Counterspell interrupts a spell--happening before the spell goes off. Now, why is this? Well...gotta rummage deeper into the English language a bit. The rule for Reactions as specified in the DMG (page 252) says:

If a reaction has no timing specified, or the timing is unclear, the reaction occurs after its trigger finishes, as in the Ready action.

But in this case, the timing is being specified by the word choice. Specifically, the tense being used and the fact that the trigger is you seeing something. If you look at most Reactions, the language used is in the Present Indicative. "Makes a Save" "Takes Damage" "Dies" "Succeeds on an Attack Roll or Saving Throw."

Present Indicative is used to refer to something that has occurred in the general present time frame, but is not appropriately used of something actively happening right now. For comparison, Counterspell uses the word "Casting" which is in the Present Progressive, which is used to specify things that are actively happening right now.

So, Counterspell's timing is specified by the phrasing used. Reactions occur after their trigger, and the trigger is when you see a spell being cast, not when the spell is finished casting, as would be the case if Counterspell said "When a creature you can see casts a spell."

Can we make it better?

If you want to avoid any confusion and not have to get this deep in the weeds in the English Language, I suggest just copying the second half of the Reaction Timing from the spell Temporal Shunt which says (emphasis mine):

taken when a creature you can see makes an attack roll or starts to cast a spell

It's basically saying the same exact thing, but in plainer terms.

A Brief Warning

As-written, this Reaction would allow your monster to dodge many spells, not just Forcecage, assuming the environment is amenable to it.

Depending on the creature's speed, they could either run away from where they're standing to try and get out of an AoE, or duck off to somewhere with Total Cover and render themselves invalid targets to a targeted spell (or out of the AoE of a spell that doesn't spread around corners).

Answer 2

Why are you doing this?

If the intent is to negate a player who has chosen to use one of their extremely rare 7th level spell slots on Forcecage then don’t. Targeted nerfing of a PC’s abilities is a dick move.

Rather give the creature ways of escaping the cage with their actions, that way the Forcecage will negate the monster for a round and still be useful.

For a creature larger than 10’ the ability to go gaseous, shape change to a creature or shrink small enough to pass through the bars will do.

For any size creature, the ability to teleport when paired with a high charisma or Legendary Saves will work.

Answer 3

That would work, but there are many problems with it

By RAW a reaction shifts control to a different character when it is taken. It can only be done when the conditions for the reaction have been met. In the example given, you could allow a creature to move up to their speed as a reaction to them seeing someone casting a spell.

But there are many problems with this. Firstly, reactions are meant to be a swift response to something happening - usually in the middle of combat. Moving up to your speed doesn't really match that description.

Secondly, according to this posting, the target for the spell is chosen when casting is finished. The range for forcecage is 100ft. In most cases, being able to move up to your speed will not get you out of range of that spell.

Thirdly, there are a number of ways to reduce a character's speed to 0 for a short period of time in order to prevent them from being able to move out of range.

If you want to create a counter for forcecage, consider given the creature access to counterspell and/or some means of short range teleportation.