While working on a character concept around using an opponent's strength against them, I thought of an ability that would make this cool for magic users. Initially, I thought it would be a bard subclass feature, but I don't think I want to make a whole subclass for it. Instead, I made it a feat:

Spell Mimic

Prerequisite: At least one spell slot

You are practiced at imitating the movements and words of other. Refining your skills you gain the following benefits:

• When a creature that you can see casts a spell of 1st level or higher, you can use your reaction to carefully study their movements and wording. For the next minute, you know the spell and can cast it using your spell slots.


• On your turn, you can cast the spell as normal. Casting the spells in this way still requires all components of the spell unless you have a way to avoid them.


• Your spellcasting ability for this spell is the same as the original caster's. You use your own corresponding ability score to calculate your spellcasting ability modifier.

Once you use this ability, you cannot use it again until you complete a short or long rest.

This feat is basically a watered-down version of the arcane tricksters 17th level Spell Thief ability. It is slightly more available and more interesting though.

Is this feat balanced against other available feats?

Potential problems I've identified:

• Warlocks and clerics teaming up to use the warlock's higher-level slots for healing magic immediately before a short rest.


• Warlocks upcasting other classes' spells. Basically warlocks in general are a concern.


• It makes half-casters even weaker as they have fewer slots to spend on mimic spells.

Spell Mimic is too flexible

I would not say that:

This feat is basically a watered down version of the arcane tricksters 17th level Spell Thief ability.

Because in many ways the Spell Mimic feat is better than the Spell Thief feature:

• The Spell Mimic's trigger is less specific than the Spell Thief's.


• The Spell Mimic cannot fail, whereas the Spell Thief can (and does more often than not).


• The Spell Thief's ability to reproduce a spell is implicitly limited to 4th level spells slots, whereas the Spell Mimic can even work with 9th level spells.
  


• The Spell Mimic recovers on short rests while the Spell Thief does not.

I'll grant you that in some respects Spell Mimic in inferior to Spell Thief:

• Spell Thief preserves the memory of the new spell for much much longer than Spell Mimic.


• Spell Mimic is implicitly useless with spells that have a casting time greater than 1 minute, whereas Spell Thief is not.


• the Spell Mimic may not use their prefered spellcasting ability, whereas the Spell Thief always uses intelligence.

But Spell Mimic also lacks effects that make Spell Thief better against foes and inefficient with allies:

• Spell Thief negates the triggering spell, whereas Spell Mimic does not.


• The target of Spell Thief forgets the triggering spell whereas the target of Spell Mimic does not.

This means Spell Mimic is best suited to reliably double the efforts of your allies instead of "using an opponent's strength against them".

In practice, Spell Mimic gives a caster temporary access to the spells prepared by the rest of the party when they are most needed and effective. This flexibility combined with some teamwork makes Spell Mimic unbalanced.

Whenever the party really needs something (whether it be heals, buffs, control, or raw damage) as long as at least one other caster can provide it, the character with Spell Mimic can double their efforts by mimicking their most suitable spell. Whereas it is much rarer for an opponent to use a spell worth mimicking in any given circumstance.

Then there are also class-specific shenanigans, like:

• A Warlock can use buffs (especially ones with long duration) and heals they usually don't have access to before a short rest.


• A Sorcerer can make up for their notoriously small amount of spells known.


• A Wizard can create contingencies of mimicked spells.


• A multiclass of two casters can mimick spells of a higher level than they can learn.


• A Tempest Cleric can spam a couple max-damage chain lightning.


• Etc.

My bare minimum recommendation to balance Spell Mimic is to somehow discourage targeting allies and limit it to spells of 5th level or lower.

However, it may be easier to achieve a feat that uses an opponent's strength against them via modifying something less convoluted than Spell Thief. I would explore an effect similar to the Ring of Spell Turning with adjustments like removing the advantage on saves and limiting which spell levels can be turned to match the growth of a half-caster.