A creature inside a Forcecage can't leave it by nonmagical means, which Incorporeal Movement is. It's a solid barrier that also prevents any matter from passing through, which may include a Dragon Breath.
Note that the cage box can be up to 10 feet on a side, so the ghost dragon would have to be a wyrmling, otherwise it would be pushed away being too large to fit.
Breath weapons as well as spell effects have no problem going through the "bars" of the cage version (but will be stopped by the box).
I don't know of any creature able to squeeze through 1/2 inch spaces (most are 1 inch), but I would also let those escape (the cage, not the box). Incorporeal creatures still get trapped by the magical force and can only attempt to teleport out.
Breath weapons as well as spell effects have no problem going through the "bars" of the cage version (but will be stopped by the box).
I don't know of any creature able to squeeze through 1/2 inch spaces (most are 1 inch), but I would also let those escape (the cage, not the box). Incorporeal creatures still get trapped by the magical force and can only attempt to teleport out.
Since the spell says that creatures inside the forcecage (either version) can't leave it by non-magical means "A creature inside the cage can’t leave it by nonmagical means.", I would tend to run it that even if a creature is small enough to fit between the bars it would be prevented from escaping unless it uses magic. For example, I wouldn't allow an air elemental in a forcecage to squeeze between the bars to escape (it can move through a 1" gap without squeezing - so should be able to fit through a 1/2" gap if it was squeezing). However, I wouldn't want to nerf the spell when it says creatures can't leave the cage explicitly.
Spells and other effects pass through the 1/2" bar version of the cage without being impeded (so the dragon breath is fine). It would be a DM call as to whether ranged weapons or similar items would fit through the 1/2" gap.
Also, keep in mind that Forcecage + sickening radiance is a combination capable of killing any creature quite quickly if they have no way to escape from the cage - dragons are especially vulnerable to this unless you use spellcasting variants and ensure that they at least have misty step if not other teleportation magic.
Squeezing in less than an inch probably won't come up anyway, but I based it on the box saying it "prevents any matter from passing through it" and thus the cage does not.
It would be a personal ruling (if it comes up), the RAW does indeed say creatures can't escape by non-magical means.
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Can a Ghost Dragon walk out of a Forcecage (Cage version, not box)? Or is its ability tied to its movement and movement is nonmagical?
On a related note, if it uses its breath weapon from inside the cage how would that be impacted since there's a 1/2 inch gap between the bars?
A creature inside a Forcecage can't leave it by nonmagical means, which Incorporeal Movement is. It's a solid barrier that also prevents any matter from passing through, which may include a Dragon Breath.
Note that the
cagebox can be up to 10 feet on a side, so the ghost dragon would have to be a wyrmling, otherwise it would be pushed away being too large to fit.10 feet is only for the solid box. With the cage version it can be up to 20 feet on a side with gaps between the bars as narrow as 1/2 inch.
"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."
Sorry i meant box not cage
Breath weapons as well as spell effects have no problem going through the "bars" of the cage version (but will be stopped by the box).
I don't know of any creature able to squeeze through 1/2 inch spaces (most are 1 inch), but I would also let those escape (the cage, not the box). Incorporeal creatures still get trapped by the magical force and can only attempt to teleport out.
Since the spell says that creatures inside the forcecage (either version) can't leave it by non-magical means "A creature inside the cage can’t leave it by nonmagical means.", I would tend to run it that even if a creature is small enough to fit between the bars it would be prevented from escaping unless it uses magic. For example, I wouldn't allow an air elemental in a forcecage to squeeze between the bars to escape (it can move through a 1" gap without squeezing - so should be able to fit through a 1/2" gap if it was squeezing). However, I wouldn't want to nerf the spell when it says creatures can't leave the cage explicitly.
Spells and other effects pass through the 1/2" bar version of the cage without being impeded (so the dragon breath is fine). It would be a DM call as to whether ranged weapons or similar items would fit through the 1/2" gap.
Also, keep in mind that Forcecage + sickening radiance is a combination capable of killing any creature quite quickly if they have no way to escape from the cage - dragons are especially vulnerable to this unless you use spellcasting variants and ensure that they at least have misty step if not other teleportation magic.
Personally, i would not let a creature squeeze out of it since it says a creature inside the cage can't leave it by nonmagical means.
Squeezing in less than an inch probably won't come up anyway, but I based it on the box saying it "prevents any matter from passing through it" and thus the cage does not.
It would be a personal ruling (if it comes up), the RAW does indeed say creatures can't escape by non-magical means.