I'm interested to hear other DMs views on Forcecage, the wording around it and how you'd use it at your table. I hope I can get some constructive conversation / views here, rather than being told I'm DMing wrong, or asking unnecessarily detailed questions.
I have two specific points of enquiry:
1) Size changing once trapped: If a creature is trapped within a Forcecage (let's say the box, as its smaller) that initially fits within the 10x10 cube, but then that creature grows beyond that size limitation (polymorph, etc.), what would you do? For example, would you:
Impose some kind of force-based crushing damage (perhaps like water pressure, taken from deep-sea adventure rules?). Does it start to ooze out of the sides like sausage meat in a meat grinder?
Interpret the wording of the spell itself to suggest that once it's too big to be held, it 'pops out'? "Creatures only partially within the area, or those too large to fit inside the area, are pushed away from the center of the area until they are completely outside the area." (I've been 'told' that this is specifically to do with when the spell is cast, but I'm opening this up for discussion, rather than to be 'told' to do it).
...rule / treat it in another way?
2) Size vs. 'space': An issue I'm stumped by is the extremely tenuous 'creature size' in D&D. For example, an adult blue dragon is 'huge', and therefore in grid/squares terms, is 15x15. Since that fits within a 20x20 Forcecage cage, I am well aware that I can simply rule quick and dirty and say 'Yes, based on base size alone, it fits. Cage the dragon!'
However, a dragon has a very long tail and an exceptionally broad wingspan. This might be overcomplicating it a bit, but first off, in the Player's Handbook (Page 191), it states:
“A creature's space is the area it effectively controls in combat, not an expression of its physical size. A typical medium creature isn't 5 feet wide.”
Incorporate flashing miniatures into that (especially if you have extremely visual players), and you can see the basic problem:
Again, I'm happy to go with a basic ruling of 'base size dominates, and therefore the dragon would fit. The creature is crumpled up to fit inside, even if its appendages were outside of that cube when the spell was cast'. However, it's one of those typical 'If players can do it, monsters can too' situations, where should I try to cast Forcecage on a player (say, as a polymorphed beast), they would/could argue that they are splayed out larger than the cage's base size - and yes, I'm the DM, I can rule how I see fit, and remind them that it's the same for the monsters as it is for them as player characters - but I'd rather be clear on this before it comes up!
In the 3.5E Draconomicon, and adult blue dragon is said to have an overall length of 55ft., with a 20ft. body, 14 ft. neck and 21ft. tail. Now, this may not be the same in 5E, but it raises some interesting visual ideas when talking about the dimensions of a creature.
[Edit: I’ve removed my comment about gargantuan creatures as the PHB states they can be 20x20 ‘or larger’ and therefore I won’t fret about them in this line of enquiry].
--------------------------------------------
I have had the default (and rather rude/blunt) 'just rule on base size and don't worry about the rest of it' response to this query, previously, and I can fall back on that if necessary - but I'd like to open this up for discussion to see what other DM's think (so please discuss, don't just aggressively tell me what to do!). I have found this discussed elsewhere on the web and it gets mixed responses, but I am hoping the D&D Beyond user base is going to be a safe space to ask this...!
(In short, I have a power gamer mixed in with my table, and so I like to be prepared for any/all table-debates as best as I can. And no, I'm not looking for advice on how to handle power gamers, lol.)
Combat space in 5e only very roughly corresponds to physical size -- a 26' storm giant has a 15' space. 5e seems to be using the same spaces as 3e, in which
Medium: 5' space, 4-8' tall/long.
Large: 10' space, 8-16' tall/long.
Huge: 15' space, 16-32' tall/long
Gargantuan: 20' space, 32-64' tall/long.
Colossal: 30' space, 64+' (not present in 5e).
As for your question, I'm not aware of any edition of D&D which actually explained what it means to fit, though the fact that they didn't just say "Can contain creatures of size X or smaller" implies that the DM is expected to apply judgment of some sort.
I might suggest that polymorph wouldn't be able to effect a target inside the 10' solid wall cube as the cube would block all spells - as described by the spell.
That would leave the cage version open for this to happen. Which, if the creature were able to fit in the cage to begin with, it might be exceedingly difficult to make them big enough to abuse the spell interaction that you describe.
As to the space vs. size discussion that you have, kindly remember that a creature can also squeeze into a space that can accommodate a size category smaller than it. So, for example a Gargantuan creature could squeeze into a 15' cube, a Huge creature could squeeze into a 10' cube and so on.
For this to work, the caster of forcecage would have to create the cage version, then target the creature inside with 4th level polymorph or 9th level true polymorph and choose the Brontosaurus or Roc (both Gargantuan beasts) and this is where the DM has to come in. On the squeeze play. The caster then has to cast enlarge/reduce with the enlarge option which increases the size of the creature by one category. Couple little DM adjudications need to be made here. Polymorph and enlarge/reduce are both concentration spells, which implies the use of a second caster or some type of spell-like ability to handle one of the size altering spells. Then we have the squeeze. A Gargantuan should be able to be increased by one size category and still squeeze into a space that is one category smaller. So, add a third caster and enlarge again. You now have 3 casters concentrating on one target. If you get this far, I would suggest bouncing back to something like 10' of falling damage as the creature is slowly crushed to death (barely) every round until they drop to 0 hit points and revert to their original form. With the hit points they had, minus any that carried over past death. Have to re polymorph and enlarge again...
And P.S. what about spell components?
At this point in the plan, I might suggest that the DM have a quick aside with the party about trying to cheese the game. Failing that, I am an advocate of the monsters can do what the PCs can do. I also believe that anyone that has to go through that many firey hoops to win D&D might need some time off. This is nothing short of overly expensive torture of an NPC.
“Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime.” - Mark Twain - Innocents Abroad
Forcecage actually has different wording than Wall of Force
Forcecage:
"A prison in the shape of a cage can be up to 20 feet on a side and is made from 1/2-inch diameter bars spaced 1/2 inch apart. A prison in the shape of a box can be up to 10 feet on a side, creating a solid barrier that prevents any matter from passing through it and blocking any spells cast into or out from the area. When you cast the spell, any creature that is completely inside the cage's area is trapped. Creatures only partially within the area, or those too large to fit inside the area, are pushed away from the center of the area until they are completely outside the area."
Forcecage is either a 10' or 20' cube. Any creature that is completely within the area is trapped and anything too large or only partially within the area (which at DMs discretion could easily include the wingspan and tail of a dragon) to fit is pushed away from the center until it is completely outside. Based on this, the actual size and shape of the creature matters when looking at the effects of Forcecage - not the area controlled by the creature on a grid - which is specifically described in the rules as the creature's "space".
However, the wording of Wall of Force is different.
Wall of Force:
"An invisible wall of force springs into existence at a point you choose within range. The wall appears in any orientation you choose, as a horizontal or vertical barrier or at an angle. It can be free floating or resting on a solid surface. You can form it into a hemispherical dome or a sphere with a radius of up to 10 feet, or you can shape a flat surface made up of ten 10-foot-by-10-foot panels. Each panel must be contiguous with another panel. In any form, the wall is 1/4 inch thick. It lasts for the duration. If the wall cuts through a creature's spacewhen it appears, the creature is pushed to one side of the wall (your choice which side)."
With Wall of Force, the text specifically references the creature's space and if the spell crosses the creature's space the creature can be forced to the caster's side of the wall which includes the inside or outside of the spherical form.
Keep in mind that this is just looking at RAW. In this case, a dragon could be forced into a spherical Wall of Force but would not fit inside a Forcecage since one spell references the creatures space and gives the caster the option of which side to place the creature while Forcecage references the creature's actual size and if the size is too large the spell description specifically moves it outside the area of effect of the spell.
P.S. As to doing compression damage, I am not sure when that would kick in. All creatures can squeeze into a space that is one size class smaller. They can move as if it is difficult terrain and make attacks at disadvantage but otherwise have no issues. This is likely because the size of the creature is typically smaller than the space controlled in the sense that the creature does not fill the volume.
If I was coming up with a houserule for it, I would not impose any damage on a creature until the creature was squeezed into a space at least 2 size categories smaller than it. So a creature that normally controls a 15'x15' area can easily squeeze legally into a 10'x10' area and might either be unable to move or attack if squeezed to 5'x5' or I might possibly impose either damage/turn or something else as a additional side effects for being compressed by 2 space categories depending on the creature involved.
In your example, a huge 15'x15' creature - if it fitted in the 10' cube would need to become at least gargantuan or larger before it might take damage.
P.P.S. If your power gamer is looking for tricks to kill things then the wall of force+sickening radiance combination (requires two casters) works well on almost any dragon that can't teleport.
David42 shone light on a portion that I missed and completely agree with. Forcecage isn't required to be set to it's maximum size when cast. But in order for it to initially contain the creature, it would need to be large enough to completely contain the creature within.
When you cast the spell, any creature that is completely inside the cage's area is trapped. Creatures only partially within the area, or those too large to fit inside the area, are pushed away from the center of the area until they are completely outside the area.
There really is no way for the spell interaction that you are describing to work. Either the creature is too large to be contained to begin with, or you can trap them, but then cannot make them effectively large enough to do significant amounts of damage to them once the polymorph drops from the creature reaching 0 hp.
Also the inverse of the attempt would likely falter as well. Say you attempt to Polymorph your Adult Blue Dragon into a Rat. Why would your blue friend fail that spell save? If your powergamer is a spellcaster with a save DC of 17 or 18 the monster has a ~40% chance to pass that save, without eating a Legendary Resistance. If you were to give your blue friend a Hidecarved Scale - Magic Resistance, it would give them advantage on that save and almost guarantee a pass.
If your powergamer and party are of high enough level to pull this off (looking at 13th level for 7th level spell slot), they might have easily defeated an Adult Blue Dragon about 2-3 levels ago.
I might strongly suggest that your powergamer be coaxed to reason and work within the constraints of the game instead of trying to test your willingness to say "I don't think that will work the way you expect it to, but you're more than welcome to try." And of course there's the old tried and true: "Are you sure you want to do that?"
The other end of the spectrum for this might be that you really do want this spell interaction to happen. In which case, I might suggest that there would be a deliberate decision to go against the intended outcome for this spell interaction and go off on your own. You are well within your rights and abilities to do so, but I would caution against it. It may open the door to more "cheesing the game" type shennanigans from your powergamer.
I would really appreciate your feedback on any of the responses that you've received thus far. It might be very insightful.
“Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime.” - Mark Twain - Innocents Abroad
@David42 - Thanks for the input. Yes, you err on the side of 'size is not space', which is where I began, and your comments on re-sizing within the cage or box are valid. Thanks :) For simplicity, I'm going to standardise my approach as follows:
To streamline situations, I'll rule on grid rather than size. Thus, anything huge or smaller can be Forcecage'd. However, the typical 'If that's how the spell works when players cast it, then that's how it works when monsters cast it' logic applies, so my powergamer will get a slapped wrist if they later on try to say that they are 'taller than a forcecage box's 10ft maximum height' (for example)
Since a Forcecage cage can hold a huge creature, if for any reason the creature gets BIGGER (gargantuan), I will default to the spell's wording and have the creature exit the cage. My description/perspective is that the 'power of the spell simply cannot contain a creature of that size'.
Squeezing mechanics work when going one size smaller. Thus, the following scenario is fine:
Huge creature is polymorphed down to large
Now-large creature is Forcecage-box'd into a 10x10
Polymorph is dropped
The creature is now huge, in a large space - this is squeezing
Thanks for sharing your thought process - it was nice to see it's not just me going mad having to think up solutions to these crazy scenarios in advance and wanting to better understand the wording of Forcecage (which I don't think the designers ever thought would/could be gamed in such a manner!)
@Kaavel - Thanks for your response. I found it really useful, and I liked how you were factual and helpful :)
Now that I'm happy with squeezing and a go-to for 'if it transforms back into a size too big for the Forcecage, it exists, as the spell cannot contain it', it's given me a solid foundation to look at the remaining few scenarios - and you're completely correct in what you've said: In this ultra extra circumstance, why would a dragon (in this scenario) ever let itself be polymorphed? Pop a legendary resistance and let them waste the spell slot. You don't even need to waste an (innate spellcasting) Counterspell on it. And yes, I've just this past fortnight started taking content from Fizban's and adjusting Dragons, and Hidecarved Wards - Magic Resistance was definitely on that list of solutions. It's also possible to draw that same boost from the Mutations in EBERRON: RISING FROM THE LAST WAR. Also: If I was that worried about it, the Hidecarved Ward I could apply (at least in regards to dragons) is the Immutability trait. Then polymorphing stops being a concern, and the entire insane scenario never has to be considered.
The group are a high level, as I'm running an on-going campaign world now on four years, and the current party is four characters ranging from 12-14 (Barbarian/Cleric, Warlock, Artificer and Druid) - so they're no short on magic! You are completely correct - at this level / power, they can easily eat a (non-lair controlling, non-minion-supported) adult dragon for breakfast (but probably not before it devastates the ship that they are aboard and the crew, which at this level is the real risk they fight to prevent). Therefore - sure, they can happily nerf a single encounter in such a manner. The reason I brought the scenario up is a fear of the aforementioned power gamer basically doing this same tactic for each fight of this kind - it's a cool use of the spell and it works, but it could make encounters rather dull. Someone elsewhere (Twitter) quite sensibly explained that with Warlocks, it's about resource management. Sure, they have this awesome 7th level spell they can use... but only once a day. So if they are having a single big encounter a day (at sea), then yes, they'll likely crack this spell out. But in a dungeon crawl or a longer day full of more encounters, no, this isn't going to be too repetitive. That's where I needed to get to with my understanding. This spell is new at the table and the previous/first use of it was incredibly awesome - but as the cliffhanger saw a blue dragon appear, the power gamer's instant reaction was 'well we'll just cage that too', and I panicked that this was going to be how all combats would be going forward! Fortunately, in this instance, the dragon knows Far Step and will easily be able to teleport out of the cage (because even if it fails the save, it can legendary-resistance it's way out).
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I'm interested to hear other DMs views on Forcecage, the wording around it and how you'd use it at your table. I hope I can get some constructive conversation / views here, rather than being told I'm DMing wrong, or asking unnecessarily detailed questions.
I have two specific points of enquiry:
1) Size changing once trapped: If a creature is trapped within a Forcecage (let's say the box, as its smaller) that initially fits within the 10x10 cube, but then that creature grows beyond that size limitation (polymorph, etc.), what would you do? For example, would you:
2) Size vs. 'space': An issue I'm stumped by is the extremely tenuous 'creature size' in D&D. For example, an adult blue dragon is 'huge', and therefore in grid/squares terms, is 15x15. Since that fits within a 20x20 Forcecage cage, I am well aware that I can simply rule quick and dirty and say 'Yes, based on base size alone, it fits. Cage the dragon!'
However, a dragon has a very long tail and an exceptionally broad wingspan. This might be overcomplicating it a bit, but first off, in the Player's Handbook (Page 191), it states:
“A creature's space is the area it effectively controls in combat, not an expression of its physical size. A typical medium creature isn't 5 feet wide.”
Incorporate flashing miniatures into that (especially if you have extremely visual players), and you can see the basic problem:
Again, I'm happy to go with a basic ruling of 'base size dominates, and therefore the dragon would fit. The creature is crumpled up to fit inside, even if its appendages were outside of that cube when the spell was cast'. However, it's one of those typical 'If players can do it, monsters can too' situations, where should I try to cast Forcecage on a player (say, as a polymorphed beast), they would/could argue that they are splayed out larger than the cage's base size - and yes, I'm the DM, I can rule how I see fit, and remind them that it's the same for the monsters as it is for them as player characters - but I'd rather be clear on this before it comes up!
In the 3.5E Draconomicon, and adult blue dragon is said to have an overall length of 55ft., with a 20ft. body, 14 ft. neck and 21ft. tail. Now, this may not be the same in 5E, but it raises some interesting visual ideas when talking about the dimensions of a creature.
[Edit: I’ve removed my comment about gargantuan creatures as the PHB states they can be 20x20 ‘or larger’ and therefore I won’t fret about them in this line of enquiry].
--------------------------------------------
I have had the default (and rather rude/blunt) 'just rule on base size and don't worry about the rest of it' response to this query, previously, and I can fall back on that if necessary - but I'd like to open this up for discussion to see what other DM's think (so please discuss, don't just aggressively tell me what to do!). I have found this discussed elsewhere on the web and it gets mixed responses, but I am hoping the D&D Beyond user base is going to be a safe space to ask this...!
(In short, I have a power gamer mixed in with my table, and so I like to be prepared for any/all table-debates as best as I can. And no, I'm not looking for advice on how to handle power gamers, lol.)
Combat space in 5e only very roughly corresponds to physical size -- a 26' storm giant has a 15' space. 5e seems to be using the same spaces as 3e, in which
As for your question, I'm not aware of any edition of D&D which actually explained what it means to fit, though the fact that they didn't just say "Can contain creatures of size X or smaller" implies that the DM is expected to apply judgment of some sort.
I might suggest that polymorph wouldn't be able to effect a target inside the 10' solid wall cube as the cube would block all spells - as described by the spell.
That would leave the cage version open for this to happen. Which, if the creature were able to fit in the cage to begin with, it might be exceedingly difficult to make them big enough to abuse the spell interaction that you describe.
As to the space vs. size discussion that you have, kindly remember that a creature can also squeeze into a space that can accommodate a size category smaller than it. So, for example a Gargantuan creature could squeeze into a 15' cube, a Huge creature could squeeze into a 10' cube and so on.
For this to work, the caster of forcecage would have to create the cage version, then target the creature inside with 4th level polymorph or 9th level true polymorph and choose the Brontosaurus or Roc (both Gargantuan beasts) and this is where the DM has to come in. On the squeeze play. The caster then has to cast enlarge/reduce with the enlarge option which increases the size of the creature by one category. Couple little DM adjudications need to be made here. Polymorph and enlarge/reduce are both concentration spells, which implies the use of a second caster or some type of spell-like ability to handle one of the size altering spells. Then we have the squeeze. A Gargantuan should be able to be increased by one size category and still squeeze into a space that is one category smaller. So, add a third caster and enlarge again. You now have 3 casters concentrating on one target. If you get this far, I would suggest bouncing back to something like 10' of falling damage as the creature is slowly crushed to death (barely) every round until they drop to 0 hit points and revert to their original form. With the hit points they had, minus any that carried over past death. Have to re polymorph and enlarge again...
And P.S. what about spell components?
At this point in the plan, I might suggest that the DM have a quick aside with the party about trying to cheese the game. Failing that, I am an advocate of the monsters can do what the PCs can do. I also believe that anyone that has to go through that many firey hoops to win D&D might need some time off. This is nothing short of overly expensive torture of an NPC.
“Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime.” - Mark Twain - Innocents Abroad
Forcecage actually has different wording than Wall of Force
Forcecage:
"A prison in the shape of a cage can be up to 20 feet on a side and is made from 1/2-inch diameter bars spaced 1/2 inch apart. A prison in the shape of a box can be up to 10 feet on
a side, creating a solid barrier that prevents any matter from passing through it and blocking any spells cast into or out from the area.
When you cast the spell, any creature that is completely inside the cage's area is trapped. Creatures only partially within the area, or those too large to fit inside the area, are pushed away from the center of the area until they are completely outside the area."
Forcecage is either a 10' or 20' cube. Any creature that is completely within the area is trapped and anything too large or only partially within the area (which at DMs discretion could easily include the wingspan and tail of a dragon) to fit is pushed away from the center until it is completely outside. Based on this, the actual size and shape of the creature matters when looking at the effects of Forcecage - not the area controlled by the creature on a grid - which is specifically described in the rules as the creature's "space".
However, the wording of Wall of Force is different.
Wall of Force:
"An invisible wall of force springs into existence at a point you choose within range. The wall appears in any orientation you choose, as a horizontal or vertical barrier or at an angle. It can be free floating or resting on a solid surface. You can form it into a hemispherical dome or a sphere with a radius of up to 10 feet, or you can shape a flat surface made up of ten 10-foot-by-10-foot panels. Each panel must be contiguous with another panel. In any form, the wall is 1/4 inch thick. It lasts for the duration. If the wall cuts through a creature's space when it appears, the creature is pushed to one side of the wall (your choice which side)."
With Wall of Force, the text specifically references the creature's space and if the spell crosses the creature's space the creature can be forced to the caster's side of the wall which includes the inside or outside of the spherical form.
Keep in mind that this is just looking at RAW. In this case, a dragon could be forced into a spherical Wall of Force but would not fit inside a Forcecage since one spell references the creatures space and gives the caster the option of which side to place the creature while Forcecage references the creature's actual size and if the size is too large the spell description specifically moves it outside the area of effect of the spell.
P.S. As to doing compression damage, I am not sure when that would kick in. All creatures can squeeze into a space that is one size class smaller. They can move as if it is difficult terrain and make attacks at disadvantage but otherwise have no issues. This is likely because the size of the creature is typically smaller than the space controlled in the sense that the creature does not fill the volume.
If I was coming up with a houserule for it, I would not impose any damage on a creature until the creature was squeezed into a space at least 2 size categories smaller than it. So a creature that normally controls a 15'x15' area can easily squeeze legally into a 10'x10' area and might either be unable to move or attack if squeezed to 5'x5' or I might possibly impose either damage/turn or something else as a additional side effects for being compressed by 2 space categories depending on the creature involved.
In your example, a huge 15'x15' creature - if it fitted in the 10' cube would need to become at least gargantuan or larger before it might take damage.
P.P.S. If your power gamer is looking for tricks to kill things then the wall of force+sickening radiance combination (requires two casters) works well on almost any dragon that can't teleport.
David42 shone light on a portion that I missed and completely agree with. Forcecage isn't required to be set to it's maximum size when cast. But in order for it to initially contain the creature, it would need to be large enough to completely contain the creature within.
There really is no way for the spell interaction that you are describing to work. Either the creature is too large to be contained to begin with, or you can trap them, but then cannot make them effectively large enough to do significant amounts of damage to them once the polymorph drops from the creature reaching 0 hp.
Also the inverse of the attempt would likely falter as well. Say you attempt to Polymorph your Adult Blue Dragon into a Rat. Why would your blue friend fail that spell save? If your powergamer is a spellcaster with a save DC of 17 or 18 the monster has a ~40% chance to pass that save, without eating a Legendary Resistance. If you were to give your blue friend a Hidecarved Scale - Magic Resistance, it would give them advantage on that save and almost guarantee a pass.
If your powergamer and party are of high enough level to pull this off (looking at 13th level for 7th level spell slot), they might have easily defeated an Adult Blue Dragon about 2-3 levels ago.
I might strongly suggest that your powergamer be coaxed to reason and work within the constraints of the game instead of trying to test your willingness to say "I don't think that will work the way you expect it to, but you're more than welcome to try." And of course there's the old tried and true: "Are you sure you want to do that?"
The other end of the spectrum for this might be that you really do want this spell interaction to happen. In which case, I might suggest that there would be a deliberate decision to go against the intended outcome for this spell interaction and go off on your own. You are well within your rights and abilities to do so, but I would caution against it. It may open the door to more "cheesing the game" type shennanigans from your powergamer.
I would really appreciate your feedback on any of the responses that you've received thus far. It might be very insightful.
“Travel is fatal to prejudice, bigotry, and narrow-mindedness, and many of our people need it sorely on these accounts. Broad, wholesome, charitable views of men and things cannot be acquired by vegetating in one little corner of the earth all one's lifetime.” - Mark Twain - Innocents Abroad
@David42 - Thanks for the input. Yes, you err on the side of 'size is not space', which is where I began, and your comments on re-sizing within the cage or box are valid. Thanks :) For simplicity, I'm going to standardise my approach as follows:
Thanks for sharing your thought process - it was nice to see it's not just me going mad having to think up solutions to these crazy scenarios in advance and wanting to better understand the wording of Forcecage (which I don't think the designers ever thought would/could be gamed in such a manner!)
@Kaavel - Thanks for your response. I found it really useful, and I liked how you were factual and helpful :)
Now that I'm happy with squeezing and a go-to for 'if it transforms back into a size too big for the Forcecage, it exists, as the spell cannot contain it', it's given me a solid foundation to look at the remaining few scenarios - and you're completely correct in what you've said: In this ultra extra circumstance, why would a dragon (in this scenario) ever let itself be polymorphed? Pop a legendary resistance and let them waste the spell slot. You don't even need to waste an (innate spellcasting) Counterspell on it. And yes, I've just this past fortnight started taking content from Fizban's and adjusting Dragons, and Hidecarved Wards - Magic Resistance was definitely on that list of solutions. It's also possible to draw that same boost from the Mutations in EBERRON: RISING FROM THE LAST WAR. Also: If I was that worried about it, the Hidecarved Ward I could apply (at least in regards to dragons) is the Immutability trait. Then polymorphing stops being a concern, and the entire insane scenario never has to be considered.
The group are a high level, as I'm running an on-going campaign world now on four years, and the current party is four characters ranging from 12-14 (Barbarian/Cleric, Warlock, Artificer and Druid) - so they're no short on magic! You are completely correct - at this level / power, they can easily eat a (non-lair controlling, non-minion-supported) adult dragon for breakfast (but probably not before it devastates the ship that they are aboard and the crew, which at this level is the real risk they fight to prevent). Therefore - sure, they can happily nerf a single encounter in such a manner. The reason I brought the scenario up is a fear of the aforementioned power gamer basically doing this same tactic for each fight of this kind - it's a cool use of the spell and it works, but it could make encounters rather dull. Someone elsewhere (Twitter) quite sensibly explained that with Warlocks, it's about resource management. Sure, they have this awesome 7th level spell they can use... but only once a day. So if they are having a single big encounter a day (at sea), then yes, they'll likely crack this spell out. But in a dungeon crawl or a longer day full of more encounters, no, this isn't going to be too repetitive. That's where I needed to get to with my understanding. This spell is new at the table and the previous/first use of it was incredibly awesome - but as the cliffhanger saw a blue dragon appear, the power gamer's instant reaction was 'well we'll just cage that too', and I panicked that this was going to be how all combats would be going forward! Fortunately, in this instance, the dragon knows Far Step and will easily be able to teleport out of the cage (because even if it fails the save, it can legendary-resistance it's way out).