Will it break Counterspell if the trigger becomes "perceive" instead of "see"?

Question

The trigger for counterspell is originally:

which you take when you see a creature within 60 feet of you casting a spell

For a player character that is blind, I want to change it to this:

which you take when you perceive a creature within 60 feet of you casting a spell

which presumably would include blindsight. Of course, then it would cover 'hearing' the verbal component too. Presumably, a spell with verbal component can normally be counterspelled since you can see the a creature lips chanting the verbal components, so it wouldn't change much, but I'm looking for ways this change might be a bad idea.

Does this change break the game, somehow? If yes, how can it be mitigated?

Of course, I can just regard allowing blindsight count as 'seeing', but technically it is not, so an answer should not just use that as answer.

Answer 1

It makes it more powerful by including sound

Think of the following constellations:

• Evil Wizard A stands behind a pillar and casts a spell. You know he is there, but the pillar grants complete cover.
• Black Wizard B stands in a zone of Darkness and can not be seen. But he can be heard casting the spell.
• Blue Wizard C is protected from sight by Greater Invisibility. However, that does not muffle his sounds as he casts.

Under the old rule, those spells can't be counterspelled, because complete cover or darkness prevents you from seeing them, as does the Greater Invisibility's effect that is not ended by casting a spell.

Under the proposed new rule, you can counterspell all those spells of someone you can not see as soon as you can hear them: Hearing them is included in perceiving them.

Even Tremorsense and Detect Magic would suffice to counterspell

There are also other ways to perceive a caster without sight and hearing, such as tremorsense. With that, casting counterspell would work through a wall when you don't even hear the verbal component.

If the wall is less than the equivalent of 1 inch lead/1 foot stone/3 feet wood, Detect Magic would work too. A single foot of wood would swallow all sound by the way.

Answer 2

This won't break your game, but it is unneccesary

In principle this will allow you to counter a spell that you only can hear being cast. Spells however need a clear path to the target (p. 204, PHB). Counterspell says

You attempt to interrupt a creature in the process of casting a spell.

If you consider counterspell targeting the creature you interrupt, you still need line of effect to counter the spell, so it won't work to counter by ear if they are behind cover, and makes no difference unless there is no total cover and instead they are invisible.

Casting from behind cover

Spell descriptions do not need to use the word target, and have no formal targeting section in the header, so resolving what counts as a target falls to the DM in cases where it is not explicit.

I think most DMs would consider the other caster your target, but even if not, these spellcasters usually need line of effect to target you, so they also cannot target you from behind cover. In addition, there is a good number of spells that require outright line of sight (about a hundred, so more than a quarter in the core rules), and if they are not invisible, if they can see you, you can see them too. So in most cases, even if you do not need to target them, this again won't matter.

If countering someone in cover were allowed, it would help if they were trying to cast a spell on Self in hiding, which is again is about a quarter of them. They also might be able to place an area effect spell nearby that will affect you and it would help there, but then, they need to know or suspect that you have a wizard that can counterspell to even worry about this.

Invisibility

The other way for them to be able to cast on you while unseen is if they are invisible. Situations where the opponent casting a spell at you is also invisible are uncommon. They surely do happen, for example the Mage NPC or the Drow Mage NPC have greater invisibility and will make use of it if they can, but this will be far from a regular situation in most campaigns.

I think one aspect is that if you change counterspell in general, this is likely to hurt the PCs at least as much as the opposition, because now all their little tricks like casting from Darkness with Devil Sight, or casting while invisible due to greater invisibility, dark environments and Gloom Stalker multiclassing can now be countered. In addition, if that is how the spell works in general, enemy casters will know about this, and if they are aware you can use it, may try to avoid it by prioritzing casting their spells when out of range, or may be able to counter-counterspell you.

You don't need this change

Still, it will make counterspell even stronger, and bang-for-the-buck it is already one of the strongest spells in the game, if not the strongest, so that is probably not a good thing. And as @ThomasMarkov pointed out in a comment, blindsight counts as seeing, so it is not needed, if the reason you want it is because you have a blind PC with blindsight. They can just counter normally using their blindsight anyways (provided the blindsight has 60' range).