"Counter target ability from a [quality] source" -- changing [quality]?

Question

Some cards counter abilities, but only from some sources. For example, Ouphe Vandals can only counter activated abilities of artifacts.

I know that removing the source of an ability does not prevent the ability from resolving in the general case (the exception being relevant intervening if clauses). However, if I make a modification to the qualities of the source, does that affect what can target the ability, or are the qualities of the source "locked in" at the time of activation?

For a specific example: Mayael the Anima activates her ability. In response, I turn Mayael into an artifact using Liquimetal Coating. Can I then counter the ability (and destroy Mayael) with Ouphe Vandals, despite the fact that when her ability was activated, the ability did not come from an artifact source?

Answer 1

Rule 112.7 says

The source of an ability is the object that generated it. The source of an activated ability on the stack is the object whose ability was activated. The source of a triggered ability (other than a delayed triggered ability) on the stack, or one that has triggered and is waiting to be put on the stack, is the object whose ability triggered. To determine the source of a delayed triggered ability, see rules 603.7d–f.

So, if you want to answer the question "Is the source of this ability an artifact?" for example, you just have to answer "Is the object whose ability was activated an artifact?" That object exists independent of the ability and can become an artifact after its ability is activated. So yes, in your example you can target Mayael's ability with Ouphe Vandals.

In addition, as 3C273's answer quotes, rule 608.2b says, in part

If the source of an ability has left the zone it was in, its last known information is used during this process.

This means that in your example, if Mayael left the battlefield after Liquimetal Coating's ability resolved, it's last known information would include being an artifact, and Ouphe Vandals' ability would be able to target its ability. It would be inconsistent if this was different from the situation where it doesn't leave the battlefield.

Answer 2

I didn't find a precise rule either, this answer is then partially guesswork. However, considering that, according to the rule below :

• that the source of a spell seems to be a characteristic pointing at a card in a certain zone;

• that the rules mention ability sources in the same paragraph as target re-checking:

I would expect that Vandal Ouphe would see the ability as coming from Mayael who is now an artifact. Somehow, I feel this is the most natural way of understanding the rule. If they wanted the source to be the card in it's original state, they would have said it clear and the bolded line concerning the last known state of the card would not have been needed.

About your example, I am not 100% sure you can cast a spell after Liquimetal Coating has resolved but before Mayael's ability. This, however, does not change the answer of the question you're asking. (Edit : apparently you can.)

608.2b If the spell or ability specifies targets, it checks whether the targets are still legal. A target that's no longer in the zone it was in when it was targeted is illegal. Other changes to the game state may cause a target to no longer be legal; for example, its characteristics may have changed or an effect may have changed the text of the spell. If the source of an ability has left the zone it was in, its last known information is used during this process. The spell or ability is countered if all its targets, for every instance of the word "target," are now illegal. If the spell or ability is not countered, it will resolve normally. However, if any of its targets are illegal, the part of the spell or ability's effect for which it is an illegal target can't perform any actions on that target, make another object or player perform any actions on that target, or make that target perform any actions. If the spell or ability creates a continuous effect that affects game rules (see rule 613.10), that effect doesn't apply to illegal targets. The effect may still determine information about illegal targets, though, and other parts of the effect for which those targets are not illegal may still affect them.