Spellstutter sprite + Xantid Swarm

[Waikiki]

My opponent attacks with Xantid swarm.

In his second main phase he cast Brainstorm.

Am I able to play spellstutter sprite or is this a spell when holding in hand(instead of a permanent)?

[Adan]

Actually every non-land card is considered a spell in every gamezone except in play and on the stack.

When your opponent attacks, the only thing you can do is activating activated abilities.

[RoddyVR]

Its a spell. You can't play it.

[Maveric78f]

But you could vial it and counter the brainstorm.

[yggdrasil]

Actually every non-land card is considered a spell in every gamezone except in play and on the stack.

Every non-land card is a spell ONLY WHEN ON THE STACK. Nowhere else.