The part of the True Strike spell I am concerned with reads:
Your magic grants you a brief insight into the target’s defenses.
I'd always considered this to be flavor text. But a friend of mine disagreed. He argued that this meant that the DM should give the player some insight into the target's defenses.
For example, the DM might say "You sense that the dragon is fortified against fire."
The thing is, I'm not sure he is wrong. Is this interpretation correct?
The "insight" into the target's defenses is only the insight you need to gain a momentary advantage. The spell does not say that you can know the monster's resistances, HP, or anything else, especially because such insights are decidedly not "brief". Instead, the kind of insight you get is as @MarcusYoder described in a comment:
You cast this on an orc with a sword, you might get the a gut feeling that, "He will step too far out of his stance on his next swing and be vulnerable." ... This insight wouldn't be applicable after next round.
The (settled) debate about fluff or non-fluff in 5e spells does not apply here: either way, it is still only a brief flash of insight and thus precludes any more detailed knowledge.
Whenever the system does give you some insight into a creature's mechanical stats, it is specific about what you learn and how you learn it. The Battlemaster's "Know Your Enemy" states:
Starting at 7th level, if you spend at least 1 minute observing or interacting with another creature outside combat, you can learn certain information about its capabilities compared to your own. The DM tells you if the creature is your equal, superior, or inferior in regard to two of the following characteristics of your choice...
If True Strike was to give you such information, it would say so explicitly.
Everything that's in a spells description is a part of the rules governing that spell. Being able to pick and choose parts of a spell as "flavor" and parts as "rules" gets to be subjective and is not clearly defined in the rules, as explained in this answer.
Therefore, following this logic, you do gain a "brief insight into the target's defenses". However, it's a brief insight, not a lasting one, so any lasting knowledge, such as a resistance or vulnerability, or the target's current AC or Hit Points is beyond the purview of the spell.
As a DM, I'd likely narrate the effect as if the "brief insight" were really only available as the attack is made.
The "brief insight into the target's defenses" is flavor text which explains why you gain the advantage.
No spell has "flavor" text.
I am using the word flavor to mean "text that has no rules effect", since a text can be flavorful but will still pertain to mechanics.
The other responses explain the spell in a manner that falls a little short: The DM can give you whatever brief insight they desire as part of the effect of the spell. The abstract effect of this insight is that you roll with advantage as the spell indicates, but the DM can hand out whatever insight he feels is appropriate.
There is no rule that specifies what "brief" means, so the plain english meaning should be used.
There are no "actual rules" and "ignorable stuff" in 5e rules. There is no flavor text, all rules are "actual" rules.
The True Strike description states
Your magic grants you a brief insight into the target's defenses.
The only mechanics the spell describes is the next turn advantage. Any additional effects are up to the DM.
the DM might say "You sense that the dragon is fortified against fire."
They might indeed. If you're the DM and you want to describe, how this "brief insight" looks like, it's up to you — this might make your game more interesting and flavorful.
