If I play Falling Star and my opponent reacts with Not So Fast, then can I react with Abandon to counter my own Falling Star to get it back to my hand?

Ruling

Yes, you can use Abandon to counter your own Falling Star in response to an opponent's Not So Fast.

Because Riftbound uses a chain system based on LIFO (Last-In, First-Out), players receive a window of priority to add new reactions to the chain before the current top item resolves. If your opponent plays Not So Fast in response to your Falling Star, you have the opportunity to respond by playing Abandon before the Not So Fast resolves.

Breakdown of Chain and Timing

1. You play Falling Star: It is added to the chain.
2. Opponent plays Not So Fast: It is added to the chain, targeting your Falling Star.
3. You play Abandon: You may add this to the chain in response to the Not So Fast. Because Abandon is added after Not So Fast, it resolves first.

Resolution

• Abandon resolves: It counters your Falling Star and returns it to your hand.
• Not So Fast resolves: Because its target (your Falling Star) has already been removed from the chain by Abandon, the Not So Fast has no valid target and fails to resolve.

Addressing Your Concerns

• Not So Fast Timing: Per FAQ 8733, Not So Fast can be used to counter the initial cast of Falling Star. However, as a reaction, it does not resolve immediately. You maintain priority to play further reactions, such as Abandon, before the chain resolves.
• Abandon Interaction: By playing Abandon in response to the Not So Fast, you successfully target your Falling Star while it is still on the chain. This removes the spell from the chain and returns it to your hand before the opponent's Not So Fast can resolve.

Note: I could not fully verify this ruling against the retrieved rules.