Does Counterspell work on a spell cast as a reaction, like Shield or Counterspell itself?

Yes, absolutely! You can use Counterspell to interrupt a spell being cast as a reaction, including Shield or even another Counterspell.

The trigger for casting Counterspell is simply seeing a creature within 60 feet casting a spell that includes Verbal, Somatic, or Material components D&D 5e SRD 5.2 — Spe....

Since Shield requires Verbal and Somatic components D&D 5e SRD 5.2 — Spe..., and Counterspell requires a Somatic component D&D 5e SRD 5.2 — Spe..., both produce perceptible casting requirements and are perfectly valid targets to be counterspelled.

However, there is a very important catch in the 2024 rules! D&D 5e 2024 introduces the "One Spell with a Spell Slot per Turn" rule, which states that "on a turn, you can expend only one spell slot to cast a spell" D&D 5e SRD 5.2 — Spe....

This creates a major tactical difference compared to earlier editions:

• Cantrip + Counterspell: If you cast a cantrip like Fire Bolt on your turn, you haven't expended a spell slot yet. If an enemy tries to Counterspell your cantrip, you can absolutely use your reaction and a spell slot to Counterspell their Counterspell!
• Spell Slot + Counterspell: If you cast a leveled spell like Fireball on your turn, you have now expended your one spell slot for that turn. If an enemy uses their reaction to Counterspell your Fireball, you cannot cast Counterspell on that same turn, because it would require spending a second spell slot D&D 5e SRD 5.2 — Spe.... (A teammate who hasn't taken their turn yet could still jump in and Counterspell for you, though!)

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