Uncanny Forethought was a feat from the 3.5 supplement Exemplars of Evil (page 26). This supplement was released in September 2007, very late in the 3.5 design cycle. Just two months later, 4th Edition was announced, and I assumed that anything that came out that late in the lifetime of 3.5 was... let's just say not designed with the level of care given to earlier products. In other words, things like Uncanny Forethought were broken and best left on the same garbage heap as Divine Metamagic + Persistent Spell + nightstick, or Assume Supernatural Ability.
For the longest time, I thought this about Uncanny Forethought, until I saw Brilliant Spell Preparation. I think it might be instructive to analyze the differences between the two.
Here they are side-by-side:
Brilliant Spell Preparation
Prerequisites: Int 13, ability to prepare 3rd-level spells.
Benefit: Select one class for which you prepare spells of 3rd level or higher. Once you select a class, it can't be changed. When you prepare spells for that class, you can leave one spell slot open as a special slot. The slot must be at least 2 levels lower than the highest-level spell you can cast. You can then prepare a spell in this special open slot as a standard action instead of it taking 15 minutes.
Special: You can take this feat multiple times. Each time you do, you can leave an additional special slot open.
Uncanny Forethought
Prerequisites: Intelligence 17, Spell Mastery
Benefit: When preparing your daily allotment of spells, you can reserve a number of spell slots equal to your Intelligence modifier. As a standard action*, you can use one of these slots to cast a spell that you selected for the Spell Mastery feat. The level of the slot used must be equal to or greater than the level of the spell you intend to cast.
Alternatively, as a full-round action, you can use a reserved slot to cast any spell that you know. The spell is resolved as normal, but for the purpose of the spell, your caster level is reduced by two.** The level of the slot used must be equal to or greater than the level of the spell you intend to cast.
*I'll assume for sanity's sake that the intent here was that if a spell had a casting time longer than a standard action, you would use that instead. Likewise, the increase in casting time was identical to that of adding metamagic to a spontaneous spell. WotC never got around to errata for the later 3e material.
**Let's assume that a spell with an effective caster level of -1 or 0 automatically fails; in 3e getting two feats of your choice at 1st level and an ability score of 17 was unheard-of for casters unless you were using the flaw system, which was broken as hell.
For the moment, let's ignore the second paragraph of Uncanny Forethought; we can come back to it later.
At first, Brilliant Spell Preparation seems inferior in every way. It requires you to take it multiple times for multiple slots rather than scaling with your key ability score, it does not work with your highest-level spells, and casting the spell from the reserved slot requires an additional standard action.
But Brilliant Spell Preparation doesn't require you to select a set of spells ahead of time. In addition, the level of the additional slot isn't set at the time you take the feat; it scales over your career. A wizard could take it at 5th level to gain a floating 1st-level spell slot, and at 17th level it will be a 7th-level slot, without taking additional feats.
Meanwhile, Uncanny Forethought relies on your taking Spell Mastery feats, whose benefits are set in stone at the level you acquire them. When you gain access to a new level of spells, you must take Spell Mastery again to use them with Uncanny Forethought, which in many cases creates the same level lag as Brilliant Spell Preparation.
Now, the second paragraph of Uncanny Forethought is somewhat more troubling. It can work with any spell, just like Brilliant Spell Preparation, but without the 2 level lag of the latter.
With Brilliant Spell Preparation, the standard action required to fill the reserved slot does not have to be taken during combat, such that a few seconds' warning before hostilities obviates the action cost, while the versatile use of Uncanny Forethought always costs additional time during combat.
Out of combat, despite its ability to work with your highest levels of spells, Uncanny Forethought is at somewhat of a disadvantage due to the hit to caster level that affects even lower-level spells.
In instances where there is no warning (which are the times when "I planned for this" is the most welcome thing one could hear), it would appear that Uncanny Forethought has the edge, because the standard action to prep the spell is a turn of spellcasting that is lost, while the unrestricted version of Uncanny Forethought merely increases the casting time to take up your move action for one round. But note that a full-round casting time has a significant disadvantage: every other combatant gets to take their turn in between you declaring you will use the spell and the time the spell actually goes off, and unlike with Brilliant Spell Preparation, you have to be casting for that entire time. If you successfully cast defensively, your opponents can still take a regular attack and force one or more insanely difficult concentration checks on you. They can also see you chanting and waving for six seconds and decide to duck behind a wall, cast an appropriate protective spell, or take other countermeasures.
The one-slot-per-feat nature of Brilliant Spell Preparation compares unfavorably with Uncanny Forethought's power to automatically keep pace with your key ability score without additional feats, but Uncanny Forethought has a feat tax in the form of Spell Mastery. The additional reserved slots from a higher ability score only work unimpeded with spells you have Spell Mastery for; thus, while there is more versatility in one direction, there is less in another. With a little bit of planning, the action disadvantages of Brilliant Spell Preparation can be overcome, while those of Uncanny Forethought are baked in, with an additional penalty of -2 to caster level.
So, which is better, design-wise? Play-wise?
Would you permit Uncanny Forethought in your game?
If not, would you permit it if using the feat's second option imposed burn (like the kineticist class feature) or had a daily limit? If not then, either, would you permit it with the secondary feature removed?