When leveling, is the hit die only added to the maximum dice, not current dice?

Question

How are hit dice added when leveling takes place?

Hit Points are pretty straightforward in that the new hit points that are gained through a level advancement are only added to the character's "maximum hit points" at that time (PHB, p. 15). Their "current hit points" will not also go up until they take a long rest or use some other means to raise their "current" hit points (short rests, healer, potions, etc., etc.).

However, the PHB is a little more vague on hit dice. When a hit die is gained through a level advancement, how this should be added? Is it like hit points and only be added to their "maximum hit dice" until a long rest allows the character to regenerate some of their current hit dice? Or is the newly gained hit die added both to their "maximum hit dice" and the "current hit dice" they have available to use?

Answer 1

The relevant quote is this on page 15 of the PHB, or here in the basic rules:

When your character gains a level, his or her class often grants additional features, as detailed in the class description.

Nowhere does it mention that it takes any time to gain these features when you level. The only exception is that it explicitly states that leveling increases the hit point maximum. Since 5e defines current hit points and maximum hit points as specific terms, you are right to conclude that the you don't gain your new hit points until you rest. However 5e doesn't make that distinction in the terms of hit dice. Like your proficiency bonus, attribute gains, etc., it takes effect immediately upon leveling.

It is reinforced by the fact that Hit Dice advancement is in its own sentence:

Each time you gain a level, you gain 1 additional Hit Die.

Then the following sentence states:

Roll that Hit Die, add your Constitution modifier to the roll, and add the total to your hit point maximum.

Which make it a separate point.