Imagine the following situation: Bobby and Tommy, 2 characters with the Counterspell spell prepared, face against Freddy, an evil priest (who does not have Counterspell). Freddy casts a high level spell. Bobby attempts to counterspell it (and must roll to do so) and Tommy does not react, believing that Bobby will be successful in his roll. But Bobby rolls too low and his Counterspell fails. At that moment, Tommy wants to attempt a Counterspell himself, after seeing his friend fail.
Considering that the reaction must be taken precisely when the spell is cast, is it possible for Tommy to actually wait for another Counterspell to fail before attempting his own Counterspell ?
Best Answer
Maybe - it's basically up to DM fiat
You've hit upon a genuinely contentious question. Let's work this one through...
Why you shouldn't allow a second counterspell
Counterspell uses a reaction, and a reaction is:
If counterspell has to be cast as an instant response to a trigger then once someone decides not to initially cast Counterspell, they cannot choose to do so later, the instance has passed.
There is no rule suggesting that a failed Counterspell has lengthened the time that it took the opposed caster (Freddy) to cast that spell (due to mental interference or whatever). So the instant trigger, that Tommy needs to respond to, has not been lengthened by the failed attempt to Counterspell (made by Bobby).
Let's look at this another way. The trigger for Counterspell is:
So, when Freddy begins casting his spell, provided they can see him, both Bobby and Tommy meet the conditions to cast Counterspell. However, if Bobby chooses to cast it initially and Tommy doesn't, once Bobby's attempted Counterspell has failed, does Tommy still meet the trigger condition in order to now attempt his own Counterspell? Is Freddy still casting a spell or has he, in the elapsed time, already successfully cast it? The rules are unclear. The casting time of most combat spells (1 action, or 1 bonus action) is pretty abstract, but neither of these periods of time is long (arguably significantly less than six seconds, as one character could cast up to three spells in one round of combat, if they used action (cantrip), bonus action and reaction to do so).
It could be argued that the only non-meta sign that Tommy would see, that Bobby's attempted Counterspell had failed, would be Freddy successfully casting his spell, at which point it is obviously too late for him to attempt to counterspell.
Why you should allow a second counterspell
Counterspell reads:
Combat is an abstraction, so while in-game both counterspells would be rationalised as being resolved simultaneously, they in reality are not resolved that way at the table.
The first attempted Counterspell (Bobby's) will be resolved, at the table, before the casting of the antagonist Freddy's spell has been resolved. As soon as the first Counterpsell has failed Tommy (the second caster) could attempt to cast Counterspell, using his own reaction, in response.
As Rubiksmoose said in his answer:
Taking this to it's logical conclusion, if Tommy's Counterspell also failed, a third caster could then also attempt to counterspell, and then a fourth, and so on.
This might seem extreme but allowing it is arguably balanced. Each Counterspell attempt consumes a reaction and a spell slot, so the more characters that try to Counterspell the same spell the better it is for their enemy, who has always used only one slot to cast their own spell.
So, how should you rule?
Well, that's entirely up to you. Both approaches seem to have benefits, I'd consider which you think your players will find more fun.
Would you players enjoy the drama when the second caster comes through with an important save, at a crucial moment, after the first attempt at Counterspell had failed? Mechanically this might to be the higher tension and more exciting option.
Or, would your players prefer the jeapoardy of having to decide what their PCs would do in that situation simultaneously, before either counterspell is resolved? They'd run the risk of a wasted spell slot if both succeed. This seems like it could be the more realistic roleplaying decision, and narratively satisfying, and also gives both players a meaningful decision to make.