Can you store a casting of Counterspell in a Shield Guardian?

Question

Would you be able to store a casting of counterspell inside of a Shield Guardian? Their Spell Storing trait states:

A spellcaster who wears the shield guardian's amulet can cause the guardian to store one spell of 4th level or lower. To do so, the wearer must cast the spell on the guardian. The spell has no effect but is stored within the guardian. When commanded to do so by the wearer or when a situation arises that was predefined by the spellcaster, the guardian casts the stored spell with any parameters set by the original caster, requiring no components. When the spell is cast or a new spell is stored, any previously stored spell is lost.

My concern is that Counterspell doesn't specifically target a creature and can only be cast as a reaction to another spell being cast.

Answer 1

RAW, not usually.

A spell with a casting time of 1 Reaction can only be cast in response to its trigger:

Some spells can be cast as reactions. These spells take a fraction of a second to bring about and are cast in response to some event. If a spell can be cast as a reaction, the spell description tells you exactly when you can do so.

RAW, you can only cast counterpsell in response to its trigger, which is:

you see a creature within 60 feet of you casting a spell.

Notably, a spell cast by the shield guardian is cast requiring no components. Since the shield guardian's spellcasting requires no components, you cannot see it casting a spell, so you cannot cast counterspell on it.

Maybe if the Shield Guardian casts a spell with a magic item.

This Q&A outlines the details of counterspelling spells cast through magic items: When casting a spell using a magic item, is the casting itself visible? The rules for casting spells through magic items say:

Some magic items allow the user to cast a spell from the item, often by expending charges from it. The spell is cast at the lowest possible spell and caster level, doesn’t expend any of the user’s spell slots, and requires no components unless the item’s description says otherwise.

So if the particular magic item's description says the spell cast includes perceptible components (material or somatic), then you could counterspell the shield guardian and store a counterspell.

And to be totally clear, counterspell does target the opposing spellcaster. From the Sage Advice Compendium:

Does counterspell target the caster or the spell you’re trying to counter?

Counterspell targets the other spellcaster.

Answer 2

Maybe

Whether or not this is possible depends on whether you interpret the rules such that counterspell targets a creature or not.

Counterspell says

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.

I think the interesting thing to point out here is that the counterspelling caster is the one making the check, not the entity whose spell is getting countered. If the interrupted creature were the target, I would expect it to have to make the check.

As it is, Counterspell doesn't really declare a target, though it speaks about the interrupted spell as though the interrupted spell is the target.

Normally, spells are not among the things that one can target. There is some gray area on this matter though due to fact that sometimes wording is vague or specifically overrides the general rules

Based on the popularity of the competing answers to this question, there is some disagreement on whether counterspell targets the caster.

I believe it does not, and if it doesn't target or affect a creature, it therefore can't target the shield guardian to be cast on it and stored by the guardian.

Some spells create an area of effect and could be said to fall on a creature without directly targeting it. For example, a group of creatures engulfed by a fireball might be said to have the spell cast on them even though fireball doesn't directly target creatures. But counterspell is not an area affect and its wording focuses on the spell being interrupted and doesn't really deal with its caster at all.

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If you interpret the wording of counterspell such that it does target the caster, then there's this to bear in mind: counterspell has a casting time of one reaction that features the caveat:

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

This means that you can only cast counterspell in response to someone/thing else casting a spell. You might consider having the guardian cast a stored spell which you then counterspell (meaning it would take two spellcasts to store a counterspell).

It's also worth mentioning that, because counterspell is a reaction to another spellcast, it requires the caster to use one turn to command the guardian to cast the spell on its turn (and provide the necessary parameters for that spell to be cast) and then wait to cast counterspell during the guardian's turn.

Also of note: the counterspell interrupts the guardian's casting of the first spell. Since the first spell is not successfully cast, it is instead replaced by counterspell.

Answer 3

Yes

It’s a spell of 4th level or lower so it meets the only criteria. Indeed the optimum choice is to cast it at 4th level.

A spell can be “cast ... on the guardian” even if the normal targeting and triggering conditions were not met. For example, you can store a Fireball even though it targets a point in space or a Shield even though it targets you and requires you to be hit by an attack or magic missile.

Unlike Glyph of Warding, the effect does not limit the spells that can be stored except by level.

In the alternative ...

You can’t. Nor can you store any area of effect spell or a spell that targets “you” or a spell with multiple targets.