In "Take [action]. When you do, [effect]", is the action a cost or an instruction?

Question

I've been trying to understand Wick's Patrol's triggered effect.

When Wick's Patrol enters, mill three cards. When you do, target creature an opponent controls gets -X/-X until end of turn, where X is the greatest mana value among cards in your graveyard.

If my library has 3 or more cards, it's very easy to resolve. Mill 3 of them, then choose a target for the reflexive triggered ability. But what if I have only 2 cards? Or none?

The rules for mill¹ say that if you're instructed to mill more cards than you can, you mill as many as you can. So my first answer was, you were told to mill 3, so you mill as many as you can, and whatever number you milled, you've followed the "mill 3" instruction and get to shrink a creature. In this interpretation, the "when you do" can only fail to go off if there's some effect making it illegal for you to mill cards, and doesn't make the -X/-X conditional on the number of cards milled.

But another knowledgable player pointed me to rule 118.12:

118.12 Some spells, activated abilities, and triggered abilities read, “[Do something]. If [a player] [does, doesn’t, or can’t], [effect].” Or “[A player] may [do something]. If [that player] [does, doesn’t, or can’t], [effect].” The action [do something] is a cost, paid when the spell or ability resolves. The “If [a player] [does, doesn’t, or can’t]” clause checks whether the player chose to pay an optional cost or started to pay a mandatory cost, regardless of what events actually occurred.

That player's claim is that the "Mill 3" of this ability is a cost, despite being worded as if it were an unconditional instruction. And since you can't choose to pay that cost (701.13b again), you can't benefit from the related effect.

If the ability were worded "Mill 3 cards. If you do, ..." I would agree, because that is the specific wording that 118.12 uses. But the ability actually uses "When you do" wording from 603.12², which says nothing about costs, just that you follow the given instructions, and the delayed trigger checks whether you followed them.

If milling 3 is a cost, as in 118.12, then you can't pay it and can't do anything. But I don't see what rule allows me to understand the milling as a cost: it looks like an instruction. Of course, I would welcome an answer explaining why this is the case.

But if milling is not a cost, then it seems to me you should mill as much as you can up to the limit of 3, whether that number is 3 or fewer. Then do you get the reflexive trigger afterwards if you only milled 2? If you milled none at all? On the one hand you did follow the "Mill 3 cards" instruction, but you followed it by milling a smaller number of cards. And as 701.13b says, you can't mill more cards than are in your library, so it seems a bit silly to argue that you did so.

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¹ 701.13b A player can’t mill a number of cards greater than the number of cards in their library. If given the choice to do so, they can’t choose to take that action. If instructed to do so, they mill as many as possible. Similarly, the player can’t pay a cost that includes milling a number of cards greater than the number of cards in their library.

² 603.12 A resolving spell or ability may allow or instruct a player to take an action and create a triggered ability that triggers “when [a player] [does or doesn’t]” take that action or “when [something happens] this way.” These reflexive triggered abilities follow the rules for delayed triggered abilities (see rule 603.7), except that they’re checked immediately after being created and trigger based on whether the trigger event or events occurred earlier during the resolution of the spell or ability that created them.

Answer 1

The instruction preceding a reflexive triggered ability is not a cost, but the ability does not trigger unless the instruction is completed in full.

Rule 118.12 lists a specific set of text patterns in which the resolving spell or ability asks a player to pay a cost and then creates an effect if the cost is or is not paid. In addition, no other rule specifies that the beginning of the reflexive ability text counts as a cost. Every other kind of cost is specifically stated somewhere in the rules, so we can conclude that the instructions at the beginning of a reflexive ability are not a cost.

However, if you cannot mill 3 cards, you still follow the instruction as much as possible and mill whatever cards are in your library, but the reflexive triggered ability does not trigger because its trigger condition has not been met. The card The Royal Scions has an ability with a reflexive trigger that can similarly fail to meet its conditions:

Draw four cards. When you do, The Royal Scions deals damage to any target equal to the number of cards in your hand.

That card has a ruling that says

If you draw fewer than four cards while resolving the last ability of The Royal Scions, its reflexive triggered ability doesn't trigger.

Answer 2

It's both a cost and an instruction, but it doesn't really matter.

If "mill 3" is a cost, you can't pay it by 701.13b and can't benefit from the effect.

If it's an instruction, you didn't fulfill the trigger events by 603.12, and can't benefit from the effect. Either way, if you only have 2 cards in your deck, you mill 2 and don't get the effect - the distinction seems almost academic.

The reality is that the question presents a false dichotomy, suggesting the text must be either a cost or an instruction, but costs themselves can be instructions - "sacrifice a creature" could appear as a cost or an effect, but is an "instruction" either way. The rules for costs suggest that costs are indeed expressed through instructions (emphasis mine):

118.1 A cost is an action or payment necessary to take another action or to stop another action from taking place. To pay a cost, a player carries out the instructions specified by the spell, ability, or effect that contains that cost.

A card may tell you to do something, which is an instruction (MtG has no specific definition of "instruction" as far as I know, so we must use a colloquial definition). If that something is required to take or prevent another action, it is also a cost (as specified by the game's definition of a cost).

As an aside, the only real difference between the "when you do" phrasing found on the card and the "if you do" phrasing explicitly mentioned in 118.12 is the timing of the effect and whether you can respond to it - "when" creates a new trigger which can be responded to, while "if" continues the resolution of the previous trigger and cannot be responded to. In either case, though, the prior action is necessary to take the following action specified in the conditional instruction, which by definition makes it a cost according to 118.1.

Answer 3

The "mill 3" of Wick's Patrol is a cost. If you have less than 3 cards remaining in the library, the instruction does not count as completed and you don't get Wick's Patrol's trigger, but you still have to mill as many as you can.

I agree there seems to be no rule that directly says reflexive triggered abilities are to be treated as per 118.12, that the [action] is always a cost. However, Mill specifically and 609.3 in general agree that if you have less than 3 cards remaining, you cannot "mill 3" and thus did not take the action required for the reflexive triggered ability, though you still have to mill as many as you can:

609.3. If an effect attempts to do something impossible, it does only as much as possible.

701.13b A player can’t mill a number of cards greater than the number of cards in their library. If given the choice to do so, they can’t choose to take that action. If instructed to do so, they mill as many as possible. Similarly, the player can’t pay a cost that includes milling a number of cards greater than the number of cards in their library.

The same argument should hold for any other partially completable instructions, such as the hypothetical "Sacrifice two creatures. If you do, [instruction]". If you have only one creature, you would have to sacrifice it, but you wouldn't get to do the instruction.

In the end, there is no practical difference between "[do something]. If you do" and "[do something]. When you do" as far as [do something] is concerned. They are both a cost in that if you didn't do it, you don't get the following effect. I would argue on those grounds that in both cases "do something" is a cost, but partial completions, where possible, do not count as having done the thing.