I was looking into creating a wizard that remains hidden for the most part from combat (potentially even to go as far as to hide in a fellow party members' backpack, they are a halfling) and thought about this question when it came to using the senses of my familiar as created by Find Familiar.

Find Familiar
While your familiar is within 100 feet of you, you can communicate with it telepathically. Additionally, as an action, you can see through your familiar’s eyes and hear what it hears until the start of your next turn, gaining the benefits of any sp⁠ecial sense⁠s that the familiar has. During this time, you are deaf and blind with regard to your own sen⁠ses.

So looking through the senses of my familiar, in this instance let's say it was a Raven and I can see the battlefield through his senses as he flies about 20ft above us providing an aerial view.

Casting Time: 1 reaction, which you take when you see a creature within 60 feet of you casting a spell
You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell’s level. On a success, the creature’s spell fails and has no effect.

Following the earlier example let's go even further and say that I can see the battlefield from inside the fighters' backpack (hidden completely) and someone, let's say a sorcerer within a 60ft range of the two of us decides to try and cast a spell, since I am in the backpack hidden away I usually would not be able to see out of it, except in this scenario I am using my familiars's senses to see and I can see the enemy sorcerer and the fighter on the battlefield.

Would I then be able to cast counterspell to stop the sorcerer?

  • \$\begingroup\$ Welcome to the site! Please take the tour when you get the chance. \$\endgroup\$ – Oblivious Sage Oct 3 '19 at 14:25
  • \$\begingroup\$ @NautArch I can see through the senses of my Raven who is above us looking down on the fight taking place \$\endgroup\$ – JMC Oct 3 '19 at 14:36
  • \$\begingroup\$ Ah Gotcha! I'm not sure about whether you can hide in someone's backpack, but that's another question :) \$\endgroup\$ – NautArch Oct 3 '19 at 14:39
  • \$\begingroup\$ Ah okay thanks for the pointer, I will retract the tick for now and wait and see \$\endgroup\$ – JMC Oct 3 '19 at 14:44
  • \$\begingroup\$ I went ahead and marked this as a duplicate. Please let us know if there is a fundamental difference and what it is if that question doesn't address your concern. \$\endgroup\$ – NautArch Oct 3 '19 at 15:37

According to these similar previously-asked questions, it appears that the answer is no.

Your character needs direct line of sight from his location.

To target something, you must have a clear path to it, so it can’t be behind total cover. If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.

Incidentally, I recall older editions distinguishing between line of sight and line of effect - but I don't recall having encountered that in 5e.

  • \$\begingroup\$ Ah damn... would have been fun to be a secret caster hiding away but that ruling does seem fairly explicit, nevermind eh? \$\endgroup\$ – JMC Oct 3 '19 at 14:41
  • \$\begingroup\$ Working on that as we speak. \$\endgroup\$ – Joshjurg Oct 3 '19 at 14:41
  • 1
    \$\begingroup\$ Thanks! Hopefully that's a little better. \$\endgroup\$ – Joshjurg Oct 3 '19 at 14:47
  • \$\begingroup\$ Selected this as accepted because it answered the particular scenario I was presenting with the obstacle only being line of sight, the other answer was very helpful to me as well but I can only select one "correct" one \$\endgroup\$ – JMC Oct 4 '19 at 9:20

Unfortunately, this won't work

There's a couple of reasons why as well.

Familiar is only able to deliver touch spells cast by you

The first is that the standard Wizard casting find familiar(PHB, 240) is limited on what spells can be cast:

Finally, when you cast a spell with a range of touch, your familiar can deliver the spell as if it had cast the spell. Your familiar must be within 100 feet of you, and it must use its reaction to deliver the spell when you cast it. If the spell requires an attack roll, you use your attack modifier for the roll.

Counterspell(PHB, 228) is not a touch spell, and thus you can not cast it 'through' your familiar.

The familiar doesn't have a spellcasting ability

Let's look at the other side of this. Can the familiar cast it with you seeing through it?

The answer here is no as well. The familiar doesn't have a spellcasting ability so can't actually cast anything. Additionally, the ability to see/hear through it is only that. You are still you, you just can see and hear through the familiar. But counterspell requires the distance to be from you as the caster.

Whose reaction is it, anyway?

The final bit here is that you've got the problem of counterspell being cast as a reaction. Is it the familiar's reaction? If so, then we're at no because the familiar can't cast a spell on their own. Is it the Wizard's? Then we're back to the requirement that the only spells you can cast through your familiar are touch spells.

But can you use the Wizard's reaction by seeing through the familiar? That' still a bit tricky and probably a "no", because the spell is cast from the wizard and still requires the wizard to be in range, at which point, it's just the wizard doing all of this.

  • \$\begingroup\$ I'm not entirely sure the OP wants to cast the spell through the familiar. It is still the wizard casting the spell at a target within 60', but its just that the wizard is peering out using the familiar's senses. So is the caster still 'seeing' the target for the purposes of spell targeting? \$\endgroup\$ – PJRZ Oct 3 '19 at 15:32
  • \$\begingroup\$ @PJRZ That's why I tried to cover all the bases (including that), but I also cover the wizard casting. \$\endgroup\$ – NautArch Oct 3 '19 at 15:33
  • \$\begingroup\$ Ah, last paragraph. Sorry. I feel that this should be the focus of the answer though, as the OP isn't asking about casting through the familiar, which is what most of your answer is about \$\endgroup\$ – PJRZ Oct 3 '19 at 15:34
  • \$\begingroup\$ @PJRZ I'm debating about rearranging, but on further review this also does look like a duplicate of the first linked question in the comments. \$\endgroup\$ – NautArch Oct 3 '19 at 15:36
  • \$\begingroup\$ Yeah the idea was only to use the eyesight of the familiar in question since everyone was still in range, however I didn’t even think about your other points so I appreciate you pointing them out :) \$\endgroup\$ – JMC Oct 3 '19 at 16:05

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