It has been established that a character cannot willingly choose to fail a saving throw. But can they willingly fail an ability check contest?
While most spells say that the target must succeed to fail a saving throw, the Telekinesis spell description states (in part):
Creature. You can try to move a Huge or smaller creature. Make an ability check with your spellcasting ability contested by the creature's Strength check. […]
I wanted to cast this on the party rogue so they could get up to the second-story window to sneak in; a rich man's Levitate spell. Alternately, cast it on a fighter and help him chase around the flying monsters.
The obvious workaround is to have them stand on an object that cannot contest the spell and lift that up instead, but then there is a chance of them losing their balance.
In the PHB/basic rules, it talks about ability check contests:
Sometimes one character's or monster's efforts are directly opposed to another's. This can occur when both of them are trying to do the same thing and only one can succeed, such as attempting to snatch up a magic ring that has fallen on the floor. This situation also applies when one of them is trying to prevent the other one from accomplishing a goal — for example, when a monster tries to force open a door that an adventurer is holding closed. In situations like these, the outcome is determined by a special form of ability check, called a contest.
This assumes that the two efforts are at odds. However, in this case a contest would not be required, as the characters are working together. But the spell description bypasses that and flat out says there is a contest – no mention of willing or unwilling.
So can my warlock give a boost easily, or is there a chance of failure?
Best Answer
They cannot choose to fail, but they can choose not to contest, which is effectively the same
The spell description of Telekinesis calls for the check regardless of the intents of the warlock and rogue. However, the rules for this situation encompass much more than the spell description.
The spell directs us to use the rules for ability checks
The spell description tells us to "Make an ability check with your spellcasting ability contested by the creature's Strength check." In doing so, it connects us to the rules for a contested ability check. The spell description does not supersede the contested ability check rules; this is not a case of specific over general. Rather, the spell description relies on our understanding of all the contested ability check rules. For example, the spell description does not state what to do in case of a tie; it assumes that we are using the contested ability check rules in their entirety.
The rules for ability checks say they are not needed if there is no contest
The contested ability check rules are divided into two sections. The first four sentences describe when to apply them, while the remainder describes how to apply them. The first section concludes with "In situations like these, the outcome is determined by a special form of ability check, called a contest." The concomitant is that in situations not like these, the contest rules are not applied, and that is what we have here - it is not the case that 'only one of them can succeed', nor that one of the PCs is 'trying to prevent the other from accomplishing a goal'. Because the warlock and rogue are explicitly working together to a common end, there is no contest.
No roll is needed
In the DMG section on Using Ability Scores we see that:
Since the answer here is yes, no roll is required.
Thus, even though the spell description calls for an ability check, this is based on the assumption that the target of the spell is resisting. The totality of rules surrounding contested ability checks specifically, and making rolls in general, tells us that no check is required.