I just realized that somebody with the War Caster feat might be able to cast a helpful spell such as cure wounds or death ward as a reaction when an ally leaves their reach. However, the text of War Caster, and I believe the rules for opportunity attacks which it relies on as well, say that a creature must be hostile to provoke this reaction. It just seems silly to me that you could technically cast a healing spell as a reaction if and only if the person you are casting it on is really angry. My questions are twofold, and as follows; would you let a player use this (probably) unintended application of the feat, and what could be some of the most helpful reaction spells if it were allowed?

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

You can't use War Caster to cast a beneficial spell on an ally as a reaction.  All it can do is cast spells on enemies.  There's no unintended application.

Quote from 6thLyranGuard >>

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

You can't use War Caster to cast a beneficial spell on an ally as a reaction.  All it can do is cast spells on enemies.  There's no unintended application.

 It feels weird that you can do more to help your enemies than your friends just because of the fact that they're your enemies.

Reactions are a weird thing: you can hit a door with an axe once every six seconds but if it starts provoking opportunity attacks you can hit it twice as often.  How does that make sense?

War Caster is intended to let a spellcaster like a wizard use their reaction to do something more dangerous than poke at an enemy with a dagger since their melee attacks are often going to be unimpressive.  If you want to cast a buff spell on your opponent instead of some flavor of elemental death you technically can but it's hard to think of a reason to do so.

Hostile creatures are inclined to get in your way, which is the opposite of what an ally PC is.

Quote from Quar1on >>

 It feels weird that you can do more to help your enemies than your friends just because of the fact that they're your enemies.

 It also feels weird to want to help your enemies in this circumstance.

I still like the idea of casting Haste on the enemy and then dropping concentration, of course you'd have to convince the enemy to be willing at the time it was provoking an opportunity attack. Maybe "Here, I'll make you go fast since you are running away" would let you trick a creature to be a willing target.

If the GM even entertained that idea it would probably require a DC 30+ Deception check.  And it's a rather weak use of a 3rd level spell slot.

This works under the 2024 rules.

Quote from WhitestElric >>

This works under the 2024 rules.

 Casting a spell as a reaction for an Op Attack works, but not healing your buddies.

Reactive Spell. When a creature provokes an Opportunity Attack from you by leaving your reach, you can take a Reaction to cast a spell at the creature rather than making an Opportunity Attack. The spell must have a casting time of one action and must target only that creature.

Quote from Tigero >>

Quote from WhitestElric >>

This works under the 2024 rules.

 Casting a spell as a reaction for an Op Attack works, but not healing your buddies.

Reactive Spell. When a creature provokes an Opportunity Attack from you by leaving your reach, you can take a Reaction to cast a spell at the creature rather than making an Opportunity Attack. The spell must have a casting time of one action and must target only that creature.

The argument is that since it doesn't say "a hostile creature" anymore, you can use a reaction to cast a healing spell at an ally that moves out of your reach.

I don't think this is really intended and I probably wouldn't allow it at my table, but you can make an argument that it's allowed under RAW in the 2024 version.

One can also make the argument that only hostile creatures provoke opportunity attacks.

Quote from 6thLyranGuard >>

One can also make the argument that only hostile creatures provoke opportunity attacks.

Well, again, that's how I generally rule it, but the specific language around this did actually change in the 2024 rules and a strict reading of it says that this is no longer required.

In the 2014 Player's Handbook, it said you can make an opportunity attack "when a hostile creature that you can see moves out of your reach".

In the 2024 Player's Handbook, it says you can make an opportunity attack "when a creature that you can see leaves your reach". The word "hostile" was removed.

Quote from Tigero >>

Quote from WhitestElric >>

This works under the 2024 rules.

 Casting a spell as a reaction for an Op Attack works, but not healing your buddies.

Reactive Spell. When a creature provokes an Opportunity Attack from you by leaving your reach, you can take a Reaction to cast a spell at the creature rather than making an Opportunity Attack. The spell must have a casting time of one action and must target only that creature.

 Personally, Warcaster is broken. I learned this after my friend cast Power Word Kill on me after I left his reach

A spellcaster of 17th level or higher is rather broken already.

Quote from 6thLyranGuard >>

A spellcaster of 17th level or higher is rather broken already.

 But it gets more broken for the reason i said above

Eh, not really.  It's not like the feat lets you cast more 9th level spells than you already could.

Quote from 6thLyranGuard >>

Eh, not really.  It's not like the feat lets you cast more 9th level spells than you already could.

 there are ways of doing that

Citation needed.

Quote from Scienceboy1 >>

 Personally, Warcaster is broken. I learned this after my friend cast Power Word Kill on me after I left his reach

You seem to have an unusual definition of friend. Also, in a campaign where that sort of thing happens, you should really be able to tank a power word kill.