Does not at fast work against Harnessed Dragon ability?

To determine whether Not So Fast can counter Harnessed Dragon's "When you play me" triggered ability, we must address the apparent contradiction in the provided context.

Ruling

Yes, Not So Fast can counter Harnessed Dragon's "When you play me" triggered ability.

Addressing the Contradiction

The confusion stems from the general nuance in FAQ #3684, which states, "No current cards can counter unit abilities." However, FAQ #9377 and FAQ #11128 provide specific, governing rulings for the interaction between Not So Fast and triggered abilities originating from units (such as Elder Dragon).

In the context of Riftbound rulings:

1. Specificity overrides generality: While FAQ #3684 serves as a general warning that there is no broad "Counter Unit Ability" mechanic, Not So Fast is a specific, conditional card.
2. Interaction mechanics: Not So Fast specifically targets "an enemy spell or ability." Harnessed Dragon's "When you play me" effect is a triggered ability that enters the chain (see FAQ #4606). Because this triggered ability chooses a friendly unit (from the perspective of the Not So Fast player), it satisfies the specific targeting requirements of Not So Fast.
3. Official Precedence: The RiftJudge FAQs (#9377, #11128) regarding Elder Dragon (a unit) confirm that Not So Fast successfully counters such triggered abilities. These take precedence as they explicitly address the mechanic of countering triggered abilities generated by units.

Sequence of Interaction

If you choose to use Not So Fast against Harnessed Dragon:

1. Play Phase: The opponent plays Harnessed Dragon. The unit enters the battlefield.
2. Triggered Ability: Harnessed Dragon's "When you play me" ability is placed on the chain, choosing your unit as a target.
3. State Change: The game state becomes CLOSED (a chain exists).
4. Reaction: You play Not So Fast as a [REACTION], targeting the triggered ability.
5. Resolution (LIFO):
   • Not So Fast resolves first. It counters the Harnessed Dragon ability (Rule 425.1).
   • The countered ability is cleared from the chain and does not execute (Rule 425.1.a). Your unit is not killed.

Note: This is my interpretation based on the specific interaction rulings for Not So Fast provided in the context, which supersede the general nuance found in FAQ #3684.