A showdown has started, the attacker passes focus. The defender plays an action (Smoke and Mirrors) and passes focus. Can the attacker now play an action card or must it be a reaction only?

According to Rule 346 of the Riftbound Core Rules, when the last item on the chain resolves and the game returns to an Open State during a showdown, focus passes to the next player.

Regarding your specific scenario:

1. Attacker passes focus.
2. Defender plays an Action (Smoke and Mirrors). This opens a chain and the state becomes Closed.
3. Defender passes focus. (Note: In a closed state, the active player [defender] passing doesn't end the showdown; it passes priority to the attacker to respond).
4. The Chain resolves. Once the Smoke and Mirrors spell finishes resolving, the chain is empty and the turn returns to an Open State.

According to Rule 346, because the chain has finished resolving and the state is now Open, focus passes to the player who did not have it, or moves to the next player in the sequence. In this case, focus returns to the attacker.

Ruling: Yes, the attacker can now play an Action card. Because the chain is empty and the state is Open, the attacker (who now has focus) is no longer restricted to only using [Reaction] cards. They may play any legally timed card, including an [Action] card.

The showdown will only end once both players pass focus consecutively without playing any cards (Rule 347.3.a).