Two quick Questions concerning Spell Casting Times:
#1: May I use a bonus action to cast e.g. Dragon's Breath on myself and use a magic action to "use it" the same turn?
#2: May I use a bonus action to cast Dragon's Breath on an ally and cast a cantrip (e.g. Ray of Frost) the same turn?
Example: Dragon's Breath
You touch one willing creature, and choose Acid, Cold, Fire, Lightning, or Poison. Until the spell ends, the target can take a Magic action to exhale a 15-foot Cone. Each creature in that area makes a Dexterity saving throw, taking 3d6 damage of the chosen type on a failed save or half as much damage on a successful one.
[...] You can take only one Bonus Action on your turn, so you must choose which Bonus Action to use if you have more than one available.
You choose when to take a Bonus Action during your turn unless the Bonus Action’s timing is specified. Anything that deprives you of your ability to take actions also prevents you from taking a Bonus Action.
Also, the second option is correct if we follow the updated 2024 rules about Casting Times:
On a turn, you can expend only one spell slot to cast a spell. This rule means you can’t, for example, cast a spell with a spell slot using the Magic action and another one using a Bonus Action on the same turn.
Edit: Following 2024 rules.
Two quick Questions concerning Spell Casting Times:
#1: May I use a bonus action to cast e.g. Dragon's Breath on myself and use a magic action to "use it" the same turn?
#2: May I use a bonus action to cast Dragon's Breath on an ally and cast a cantrip (e.g. Ray of Frost) the same turn?
Dragon's Breath: https://www.dndbeyond.com/spells/2619150-dragons-breath
Ray of Frost: https://www.dndbeyond.com/spells/2618930-ray-of-frost
Casting Time: https://www.dndbeyond.com/sources/dnd/free-rules/spells#CastingTime
Bonus Action: https://www.dndbeyond.com/sources/dnd/free-rules/spells#ReactionandBonusActionTriggers
Yes, both interactions (#1 and #2) are legit.
Just for completeness, you're choosing to use your Bonus Action before your Action (Magic in both cases), and that's correct.
Also, the second option is correct if we follow the updated 2024 rules about Casting Times:
Thank you very much for taking the time to answer in such detail.
I'm glad I could help!
Both interactions were also legit under the 2014 rules, by the way ;)