Shadowcaster + reserve feats

[Yogibear41]

Is there any reason why a Shadowcaster cannot use reserve feats? Their highest level mysteries function as arcane spells, they have levels like spells, and they have schools/descriptors. I've been reading through the tome of magic to try and find a solid answer on this but I haven't seen anything that really tells me one way or another. It seems it would be very beneficial if they could, and help alleviate one of their biggest weaknesses of their extremely limited resources per day.

[The Viscount]

There isn't explicit text allowing it, but we can use shadow magic PrC interactions as a guide. Because they require only the ability to cast spells of x level they check that box. So as much as it can be said, I think they should qualify for these. Because you learn so few mysteries you will have to carefully select what you are using for your reserve, and you can only use a small number of them. But they certainly fulfill the spirit, and at will fundamentals show the designers aren't against shadowcasters having something to fall back on. Very good find!

[Zaq]

The tricky part is that by RAW, just like you said, only their highest-tier mysteries are considered to be arcane spells. SLAs and Su abilities don't power reserve feats, so when you reach a new tier of mysteries, unless your new highest-tier mysteries have the right keywords, you stop being able to power your reserve feats, which is horribly awkward no matter how you slice it.

(There's also the unfortunate fact that since only your highest-tier mysteries work here, once you cast them, you can't fall back on your reserve feats anymore, thereby running into nearly the same old "lack of staying power" problem that the reserve feats are trying to help us avoid. Wizards can have a similar problem, but compared to Shadowcasters, they nearly have to be trying for the issue to apply in quite as harsh a fashion.)

I'm in favor of a non-RAW ruling that mysteries of any level can be treated as spells for the purposes of reserve feats. I still wouldn't play a Shadowcaster without a little bit more houseruling to make them less constrained and more robust, but it's a baby step in the right direction.

[DrMotives]

Not that any of my players have shown any interest in being a Shadowcaster, but I could see Zaq's suggested houserule as allowing uncast mysteries to power reserve feats even if they count as SLAs or supernatural abilities. Also, using the bonus feats either per the book (half the paths the SC has access to) or the suggested fix of 1 bonus feat per completed path, I can see adding reserve feats to the list of allowable choices to enhance the SC versatility. This would certainly made "Dark Air or Water" very attractive, if only for the ability to power more different reserve feats than any other mystery.