summoning creatures to use as spell components

[Vixeryz]

like... using summon monster 1 to summon a cat so that i can shave it for its "pinch of cat fur" to cast cats grace
or when using a mount spell, using the hair from the summoned horses to recast it just before the previous spell ends

or summoning demons and then slaying them for their blood, hearts, claws, etc

[Pizza Lord]

Since you mention summon monster I will assume you are talking about (summoning) as opposed to (calling). With calling spells, a creature is actually there and such things as harvesting parts would work.

Otherwise:
The part about summoning demons and slaying them is easy enough to answer. They disappear when slain (technically at 0 hit points, not even slain, unless they have ferocity or something,) which would likely include any severed parts, blood, or organs.

As for summoning a creature to harvest something without killing it, like fur or hair, that would also vanish with the creature at the spell's expiration, just like any spells the creature cast will also end, even if they have a longer duration. While there is no mention of ongoing poison effects, these should probably have been addressed as well somewhere (damage already taken would remain.)

As for summoning a creature, harvesting a part of it, and then casting a spell before the creature vanishes... that's easy to answer but a bit harder to provide ruling for. I believe the intent for such things is that they are useless and not sufficient for providing spell components or other materials.

While the rules have been narrowed, edited, and stripped down over the years and from terrible oversight on cut-and-paste jobs, I think looking at the intent of other spells, such as wall of iron not being suitable for the use of creating other objects or even being sold as iron, despite being iron and in every way actually iron, that should be a good guideline, even though that's a creation spell with an instantaneous duration and actually makes iron.

I don't think that summoning a cat to grab some quick cat hair for another spell will ruin the game (gathering a cat's eye or tail or blood might just be a judgement call based on your personal feelings of such things.) I mean, if you're going to use the reason that torturing/butchering/mutilating a creature is okay because it's just going to reform and isn't a 'real' animal... then that's probably a suitable reason why it doesn't function as a 'real' spell component or ingredient. This is magic, not science, so that's a perfectly reasonable explanation. I don't think the intention of summoning spells is to allow easy reagent gathering.

[Yure]

Look at the spell Minor Creation. It specifies you can't use made objects as spell components... Well you can but the spell fails. I would consider it to be the same.

The Feat Eschew Materials fixes that problem though... To a certain amount anyways.

[Matthew Downie]

Yure wrote:

Look at the spell Minor Creation. It specifies you can't use made objects as spell components... Well you can but the spell fails. I would consider it to be the same.

That could be taken as an argument for the opposite ruling: if Minor Creation specifies that the components from the spell can't be used, that suggests that components can be used by default unless the spell calls it out as an exception.

It sounds like we're only talking about materials that are free and unlimited in any spell component pouch - and if you don't have one of those you probably couldn't cast the summoning either. Personally, I'd allow it, as long as you used the component within the duration of the summoning spell.

[Yure]

Yeah, I guess. I see what you mean. Seems like a waste of a spell just to get a component. But yeah, why not. It is already going to cost the caster a spell and at least a move action to gather that up.