The 2024 Blade Ward cantrip allows the caster to force disadvantage to a melee attack roll as a reaction. Every round, if desired.
Casting Time: Reaction, which you take in response to a visible creature targeting you with a melee attack.
Effect: You trace a sigil of warding, imposing Disadvantage on the creature’s attack roll against you.
By your rules text the decision to use or not use this needs to be taken BEFORE the monster has succeeded or even rolled.
That's a fairly big difference, analysis-paralysis wise. If you simply use it every round (against the most fearsome attack you think you'll suffer) there is no time lost to the decision point at all.
And it only helps against melee attack rolls; arguably the least disruptive kind of success.
Using Silvery Barbs to force a NPC to reroll a really great social roll is FAR more impactful on the story overall. And if three PCs team up to force a big monster to reroll no less than three verified successes (not to mention criticals) per round that's just an auto-win button right there.
So thanks Mirror. My reply to Ungehuer then becomes:
No that's not what I was thinking of, not foremost in my mind in any case. By that I mean I wasn't aware of this playtest update, but if I knew what I know now, it would not be.
(Do casters generally use their reactions? Because if they don't, this is just free disadvantage. Why ever not take this cantrip? I guess it makes counterspelling a fair bit worse since you can't semi-automatically spend your reaction on this if you want to be a counterspeller. And you probably want Shield still, for when you're swarmed by many smaller foes)
Still, it doesn't trigger my annoyance to nearly the extent Silvery Barbs does. A game where anything more powerful react-to-success-wise than Blade Ward 24 being limited to once per short rest or something (and NOT once per spell slot per day!) would be a better game than what D&D has turned into in 2023.