Can Action-speed spells like Grand Strategem be used in response to "When I Attack" triggers like Anivia's?

Ruling: No, Action-speed spells cannot be used in response to "When I Attack" triggers during the initial chain of a showdown because the initial chain is closed and Action-speed spells can only be used in open chains.

Sequence:

• When I Attack triggers automatically start the initial chain in a showdown
• The initial chain must resolve first
• After the initial chain resolves, the chain becomes open and Action-speed spells can be used
• As the attacker, the opponent gets the first opportunity to use a spell after the initial chain

Nuances:

• Reaction-speed spells can be used to react to "When I Attack" triggers when you gain priority
• Adding abilities and permanents being played give no priority