Does not So Fast work against Adaptatron?

To determine if Not So Fast can counter Adaptatron's ability, we must analyze whether the ability "chooses" or "targets" a friendly unit or gear.

Ruling

Yes, you can use Not So Fast to counter Adaptatron's ability.

According to the card text, Adaptatron's ability reads: "When I conquer, you may kill a gear. If you do, buff me."

• Not So Fast requires the spell or ability it counters to "choose a friendly unit or gear."
• Adaptatron's ability requires the selection of a gear to be killed. Under Rule 352.10.c, an ability that requires the controller to select a specific object to be affected (such as "kill a gear") constitutes targeting. Because the ability requires a target to be chosen upon resolution, it meets the requirement for Not So Fast.

Reasoning

1. Targeting vs. Selection: Rule 352.10.c clarifies that abilities requiring the selection of a specific object, such as "kill a gear," function as targeting effects. Because Adaptatron requires the selection of a gear to be destroyed, it is considered to be targeting that gear.
2. Not So Fast Requirement: As established in FAQ #5930, abilities that require a valid target to be selected are subject to counter-effects that interact with targeting. Since Adaptatron targets a gear, it is a legal target for Not So Fast.

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