Can you cast a spell at a lower caster level?

[Senko]

If you have a spell that has effects based on your caster level e.g Xd6 damage per level or at 3rd level the mount ignores difficult terrain are you able to deliberately cast it at a lower caster level? That is if you have an 8th level caster casting phantom steed it gets this ability . . .

8th Level: The mount can ride over sandy, muddy, or even swampy ground without difficulty or decrease in speed.

could the caster cast it as a 7th level caster with reduced duration and abilities so it looks more like a normal horse struggling through muddy ground or must a spell apply a full caster level when cast? I know there are metamagic to affect spell level but I can't find anything on caster level as your generally assumed to want the best effect you can get.

[Melkiador]

I am not aware of any official rule on doing this, but it makes sense that it'd be possible.

But I don't think a phantom steed could ever "pass" for a normal horse. Or at the least, it'd be a trivially easy knowledge(nature) check to notice the difference.

[Belafon]

CRB page 208 wrote:

You can cast a spell at a lower caster level than normal, but the caster level you choose must be high enough for you to cast the spell in question, and all level-dependent features must be based on the same caster level.

An 8th-level wizard could cast phantom steed at CL7. He could NOT cast it at CL4 (because that is too low for the wizard to cast it in the first place).

As Melkiador said, there’s not any advantage to doing it. It is still a recognizably phantom horse. You can always ride it slower if you want to pretend to be bogged down in the difficult terrain. Other than deliberately giving someone a stunted version of a spell (spellcasting service provider with tiered pricing options?) the main reason for doing this is to reduce the lethality of a level-based spell like fireball.

[Mysterious Stranger]

You conjure a Large, quasi-real, horselike creature (the exact coloration can be customized as you wish). It can be ridden only by you or by the one person for whom you specifically created the mount. A phantom steed has a black head and body, gray mane and tail, and smoke-colored, insubstantial hooves that make no sound. It has what seems to be a saddle, bit, and bridle. It does not fight, but animals shun it and refuse to attack it.

The description specifies a horselike creature not a horse. It also states it hoves are insubstantial and do not make any sound. The fact that the steed does not make any sound as it moves is going to be easy to spot especially in muddy ground. I have to agree with @ Melkiador that figuring out it is not a normal horse is not going to be difficult.

The spells description states you can customize the coloration, but latter states it has a black head and body and gray mane and tail. I am not sure what you can customize, but suspect it meant appearance. If so, you might be able to make it appear as a normal black and grey horse, but the hooves not making any sound will still tend to give it away.

[Heather 540]

I agree with Melkiador and Belafon. There's no rule saying you can't but you can't go below the lowest level that the spell requires to cast it in the first place.

[Senko]

It was just an example I used before going to bed, there are other spells where I might want to cast a spell at a lower level than my actual level. Especially with third party spells. Still seems like its reasonable but GM decision.

[Melkiador]

Belafon quoted the relevant text. You can cast the spell at a lower level as long as the level is the minimum for casting the spell for your class. A 10th level wizard could cast a 5th level fireball, since they get 3rd level spells at their 5th caster level. A 10th level sorcerer could only go down to level 6, because they get new spells a level later

[Senko]

Opps my bad I read Belafon's answer but not the quoted text as I thought it was something they were replying to. So its not even GM required you can just do it excellent.

[Pizza Lord]

As the others have mentioned, you can always cast at a lower level as long as the caster level is enough to actually cast the spell. There are very few instances where doing so is beneficial unless you're actively trying to do less damage or make an effect end faster than it normally would.

One such example would be wall of suppression where you would cast it at one CL lower than normal, that way it wouldn't interfere with your own casting but your opponents would be hindered (assuming you have a higher CL than your opponents' casters). Have to watch out for your magic items though, since those usually have a lower CL than you when you are able to actually cast this spell.

[Mightypion]

One example is casting holy word, without wanting to warcrime indoctrinated cultists who are no threat to you whatsorever and simply have been indoctrinated/brainwashed into being evil.