That’s not circular though. Each step follows logically from the last. It’s also not accurate to the way those of us arguing the outcome isn’t uncertain. ...
PHB 174 establishes that - The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure. When the outcome is uncertain, the dice determine the results.
This is silent on who decides that an outcome is uncertain.
DMG 237 establishes that a DM should - Only call for a roll if there is a meaningful consequence for failure. When deciding whether to use a roll, ask yourself two questions: Is a task so easy and so free of conflict and stress that there should be no chance of failure? Is a task so inappropriate or impossible-such as hitting the moon with an arrow-that it can't work? If the answer to both of these questions is no, some kind of roll is appropriate.
A DM consults themselves, in making their decision.
PHB 6 establishes that - the DM listens to every player and decides how to resolve those actions. Sometimes, resolving a task is easy. If an adventurer wants to walk across a room and open a door, the DM might just say that the door opens and describe what lies beyond. But the door might be locked, the floor might hide a deadly trap, or some other circumstance might make it challenging for an adventurer to complete a task. In those cases, the DM decides what happens, often relying on the roll of a die to determine the results of an action.
DM can decide that a challenge makes something that would otherwise be certain, uncertain.
PHB 185 establishes that - Roleplaying is, literally, the act of playing out a role. In this case, it’s you as a player determining how your character thinks, acts, and talks. Roleplaying is a part of every aspect of the game, and it comes to the fore during social interactions.
And for completeness we have the
Sage Advice - An NPC ability check can't force a PC to think/feel/do/sense something.
PHB 7 - Specific Beats General - can among other things create an exception that makes something that would be certain, uncertain, or that forces a PC to think, feel, do, or sense something.
This is silent on who decides when that applies.
DMG 5 - Master of Rules.
DM decides when and how to apply the rules.
A possible exit to our dilemma is this. Determining how your character thinks, acts, or talks is not the same as determining means of resolution. Nor is it a general fiat over outcomes. It is a fiat over how those outcomes impact on how your character thinks, acts, or talks. This is consistent with all the text, and requires no circular arguments.
An example, chosen to get at an egregious case and show it to be resolved.
DM The tea-lady glowers, and threatening to revoke your biscuit privileges, wants you to sit back down.
Describes circumstances.
PC Is this pre-school? I'm going to stand up to her.
Stand up, get it?!
Says what they want to attempt.
DM How about those biscuit privileges? Are you just sacrificing those?
DM puts something on the line.
PC Yeah, right, and she has that amazing shortbread. Okay, I'm going to sweet talk her some.
Chooses their approach.
DM Okay, the challenge here is that this lady has dealt with generations of all-too-clever students like yourself, her fierceness is legendary. It's a contest, your CHA against hers. You can include Persuasion, she includes Intimidation. Roll.
Says how it will be resolved.
Rolls - NPC wins.
DM Remember that sweet, sweet shortbread. That's now a thing of the past, and frankly this woman is pretty scary. You might feel somewhat intimidated. Want to sit back down?
Narrates outcome.
PC in world 1 - Yeah, okay, I'm sheepish and slink back to my seat.
Roleplays taking into account the outcome.
PC in world 2 - Nah, I stay right where I am.
Roleplays, applying their PHB 185 fiat.
So adjusting my answer to
@Swarmkeeper - on balance, the whole text gives it to DM to decide there are factors that make something a player character attempts uncertain, and if so at DM's discretion call for an ability check. Players don't get to decide how outcomes are reached (how they are resolved) only how their character thinks, acts, or talks. Unless there is a set limit or s>g exception, a game outcome can't override how their character thinks, acts and talks. At the same time, DM, not player, is master of rules.
For me it helps to make sure the basic loop, described right up-front in PHB 6, is followed thoughtfully. Frame interactions from the point of view of what the player characters are attempting: they are the protagonists. Explain what the challenges are, what's at stake, etc, and then as DM decide how that will be resolved. That yields good consistency with the text as a whole.
@Maxperson @Bill Zebub @Lyxen for vis.