In the near future I'm going to play in a one-shot (DND 5e) and I might want to play a level 4 divination wizard. When I read Portent I saw the following line: When you finish a long rest, roll two d20s and record the numbers rolled.
And now I'm wondering if the start of the one-shot counts as having had a long rest. Because there is a good chance I won't get to set up Portent otherwise (since it is a one-shot).
It would be...unusual...if your character has never taken a long rest. Unless for some reason your character has spent their long rest resources between the last long rest and the beginning of the story, which I would find quite odd, you definitely start the game with all of your resources, because they recharged whenever you took a long rest last.
But ask the DM just in case - I’ve played in a game where the party started out in prison with no hit dice and the casters had limited spell slots. There are scenarios where it makes sense for a divination Wizard to start the game without portent available, but I’d certainly expect to start most games assuming I’ve had a long rest.
Creating Adventures (DMG 71)
Creating adventures is one of the greatest rewards of being a Dungeon Master. [...] When you design an adventure, you call the shots. You do things exactly the way you want to.
("You" in the above refers to the DM.)
A Clear Focus on the Present (DMG 71)
Instead of dealing with what happened in the past, an adventure should focus on describing the present situation, what the bad guys are up to, and how the adventurers became involved in the story.
(Emphasis mine in the above.)
How to Play (PHB 7)
- The DM describes the environment.
All the above rules are indirectly giving the answer to your question. It is up to the DM to decide how the adventure starts, including whether you've had a long rest and have gained any benefits from your class abilities.
Just ask them. Getting a technically correct answer from internet people will not sway them either way on whether you can have your portent dice at the start, or not, as they may decide that the opening scene begins with you exhausted and your resources drained.
Also, I have never seen any rule that explicitly states the exact conditions of how an adventure starts, in regards to resources, so you are unlikely to find any RAW source to tell you an answer one way or the other.
Ask your GM before or after setting up the adventure hook, what the state of your characters and equipment is, if you have charges of this and that ability or if you are presumed to have used your special abilities, such as Portent. A good three examples:
If your GM describes you all in your underwear, sitting on a tree hanging from a cliff and tired to the bone from having to stay awake for the last 3 days lest falling down, then that is your state: without equipment, fatigued, and most definitely not rested and most likely with reduced capabilities.
If your GM describes you just arriving at the inn after a long day of overland travel, you might have had your Long Rest in the morning, but also would most likely be only worse for the travel-wear, possibly with some status effects from the travel exertion.
If your GM describes you as having just woken up in the inn, then you are most likely well-rested and had your long rest - unless they describe you as having a massive hangover.
