Where in the Comprehensive Rules is it defined that action spells cannot be played in response to something and must be played on an empty chain?

Ruling: Action spells can only be played during closed states, not in response to other effects. This means they must be played when the chain is empty.

Nuances:

• The relevant timing rule is found in the Reaction timing section of the rules, which specifies that reactions can be played during closed states (implying actions have different timing restrictions).