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

Yes, Counterspell absolutely works against spells cast as a reaction!

When you cast Counterspell, the target creature must make a Constitution saving throw. If they fail, the spell dissipates and whatever they used to cast it—whether it was an Action, a Bonus Action, or a Reaction—is wasted D&D 5e SRD 5.2 — Spe....

However, there is one important condition: to cast Counterspell, you must be able to see the creature casting the spell, and the spell they are casting must include Verbal, Somatic, or Material components D&D 5e SRD 5.2 — Spe.... Since both Shield and Counterspell have components (for instance, Counterspell has a Somatic component), you can counter them as long as the caster is within 60 feet and visible to you D&D 5e SRD 5.2 — Spe....

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