[RPG] What are the balance implications of using passive Arcana to identify spells for counterspell

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Xanathar's Guide to Everything (p. 85) gives an optional rule if a character wants to identify a spell:

Sometimes a character wants to identify a spell that someone else is casting or that was already cast. To do so, a character can use their reaction to identify a spell as it's being cast, or they can use an action on their turn to identify a spell by its effect after it is cast.

If the character perceived the casting, the spell's effect, or both, the character can make an Intelligence (Arcana) check with the reaction or action. The DC equals 15 + the spell's level. If the spell is cast as a class spell and the character is a member of that class, the check is made with advantage. For example, if the spellcaster casts a spell as a cleric, another cleric has advantage on the check to identify the spell. Some spells aren't associated with any class when they're cast, such as when a monster uses its Innate Spellcasting trait.

I'm thinking of completely replacing the check with comparing the passive Arcana score and the DC (15 + spell level). This would also not require a reaction to perform.

However, I'm not sure whether this will be balanced or not. The goal is to allow identifying spell before casting counterspell. The side goal is to reduce dice rolling.

One of my main concern is early level this would be useless, as even with proficiency in Arcana, you would only get at most +5 (+3 from INT and +2 from proficiency). Except if you have expertise, you will never be able to identify even 1st level spell. Without expertise, you will only be able to identify up to 5th level (10 +5 INT and +5 proficiency against DC 20 for 5th level spell).

Is this modification (completely replacing identification using passive score instead of actively rolling Arcana) underpowered? Can it be remedied by changing the base DC to 10 instead of 15? What are some other concern I might have missed?

This rule will also apply on other situations other than for counterspell.

Best Answer

I use this, with DC = 10 + the spell's level

And I have used it since before I read the rules in XGtE with absolutely no problems at all and my group really appreciated it.

This is also exactly what passive checks are for. If you notice something your brain immediately makes the link to your knowledge and you just know, you don't need to stop and concentrate to know something, you either know it, or you don't.

Notes

This makes Counterspell more powerful than it seems to be intended in XGtE, but I think that rule sucks and having a spell countered, or countering a spell is more fun than wasting a counterspell. But then as a GM I am on the players side and like them being informed about anything and everything.

If using DC10 as the base drop the advantage rule, because that boils down to +5 and means you start on a base of 15 passively so pretty much recognise everything.

I did try DC = 15 + the spells level, and this has the effect of making non-arcane spells more difficult to counter. I didn't like this personally, but it can be a flavourful touch. The DC15 is essentially taking into account that you have advantage on your own type of spells. I think here it is a group preference thing.

TL/DR

I find that more information in the hands of the PCs is better than less information, and any clever use of a passive skill such as this is very much within my style of DMing, but each to their own and this is steering away from RAW.