Can a defending player respond to an attack chain (started with a reaction spell) with an action spell?

Ruling: No, only reaction spells can be added to a chain. Action spells can only start a chain when you have focus, while reaction spells can both start and be added to a chain.

Sequence:

• Action spells can only be played when you have focus and only to start a chain
• Reaction spells can be played to either start a chain or be added to an existing chain
• When responding to an attack chain, only reaction spells are valid responses