So I'm a new DM but a long time player, recently one of my party members asked if he could change one of the casting times of a spell to a reaction. I don't see anything overtly broken about this switch but I don't really want to mess with the core mechanics of the spells. Would anyone have any insight on whether it's broken or maybe I can be a little lenient on this rule

Which spell is the player asking about, and what is their end game? If it is an offensive spell, making it a reaction would allow them to effectively double their damage output every round without a cost.

The feat War Caster allows for a spell with a 1 action casting time to be cast as a reaction, but only under the condition of an Opportunity Attack.

If you decide to go along with the player's request, it should probably have a constraint similar to Hellish Rebuke, which can only be cast in response to taking damage. Or, increase the spell level.

It depends on the spell, but most of the time this is going to be very powerful.  Since a character gets both an Action and a Reaction in a turn, this would potentially allow them to cast this spell and another one in the same round.  What spell are they asking about, and what world be the conditions for using the Reaction?  (See the spell 'Shield' for an example)

He wants Rimes Binding Ice and I think it's too powerful of a spell too make a reaction combined with sculpt spells i think it becomes too devastating to change that much, thank you for the advice:)

Hah, yeah. That would be a gnarly one to have as a reaction.

You could nerf its potential: There's a rule that if you cast a spell as a Bonus Action, you can't cast any spell other than a cantrip that turn. Perhaps you could modify this and include Reactions as well as including the round rather than just turn; it would still be really powerful (a levelled spell and potentially two cantrips in a round?), but at least there's a cost to it.

If they want to cast it as a Reaction, then they can use the Ready action on their own turn - so effectively they lose their normal action in order to gain the benefits of casting it as a Reaction on somebody else's turn.

I think the player is specifically wanting it as an actual Reaction (not just readying an action). Probably either to preserve action economy, or to prevent wasting spell slots. I don't think the Ready action is what they are asking for.

Quote from Kotath >>

Quote from Linklite >>

I think the player is specifically wanting it as an actual Reaction (not just readying an action). Probably either to preserve action economy, or to prevent wasting spell slots. I don't think the Ready action is what they are asking for.

 

Reaction spells still use spell slots, so that wouldn't be it. Readying an action can preserve spell slots, though, since you have that extra conditional control.

Preserving action economy, though, that is DPS.

 

 Using the Ready action consumes a spell slot if you use it or not. If it's actually a Reaction spell, then it's only if you actually cast it. Only Readying a spell actually wastes a slot (not getting into things like Counterspell, successes on saved etc).

What was the proposed trigger for this reaction version of Rime's Binding Ice?  All reactions have a specific trigger condition that must be met to use them; that's why they're called reactions.  You are reacting to an occurence in the world.

There already is a way to turn a spell into a reaction. Or an attack, or other kind of action. You do so by using the Ready action on your turn to say that, over the next round, if certain criteria are met, you use your Reaction to do X.

For example, a held action could look like "I Ready an action to cast Fireball if the bandit takes another step closer." In that case your Action is Ready, and Fireball works off your Reaction.

Quote from Yurei1453 >>

What was the proposed trigger for this reaction version of Rime's Binding Ice?  All reactions have a specific trigger condition that must be met to use them; that's why they're called reactions.  You are reacting to an occurence in the world.

Right, this is what I was wondering.  Making the spell ONLY castable as a Reaction could actually make it worse, not more powerful.  If the trigger is sufficiently rare.  Why does the player want to change the spell this way?  For an in game advantage?  Or an RP concept that they are willing to nerf themselves to get?

This is an instance where it's totally okay to say no. No roleplaying concept requires changes to mechanics - it's about description and emphasis. Readying is an existing mechanic that provides risk/cost in exchange for the added flexibility.