I'm coming back to this because there is precedent elsewhere in the PHB for luck. Specifically the Luck feat, where multiple uses of Luck cancel each other out. I would apply that in this case too.
Hunh? I don't see the parallel. There's a general rule for *spells* not stacking with other castings of the *same* spell, and one could apply that precedent to non-spell abilities. Where is there a precedent for two *different* abilities not stacking with each other?
With the Lucky feat, that specific rule applies when when A attacks B, both A and B have the Lucky feat, and both of them try to apply Lucky to that attack roll. That's a very specific rule, about a corner case. It's parallel to advantage and disadvantage negating each other.
Stacking for damage is very different to stacking for success due to bounded accuracy. ACs and DCs are bounded but HPs are unbounded.
Cogent point. Attack rolls, saving throws and stat checks are all based on a single d20 (though d4-d12 are explicitly possible modifiers), and that is substantially different from damage dice, hit dice, etc.
If four clerics are casting Guidance, and they roll 1, 1, 2 and 4, then only the 4 applies; that's RAW. If you ruled that Song of Rest and Cutting Words work the same way (when more than one bard sings during a party's short rest, or more than one Lore Bard applies Cutting Words to a particular roll), I'd consider that RAI. If you ruled that when people apply Guidance, Bardic Inspiration, Dark One's Own Luck and Bend Luck all to a single d20 stat check, only the largest modifier of ANY of those rolls applies, then I'd call that a house rule.
I'm curious whether you select the largest modifier by absolute value, since Bend Luck can be a penalty and Bardic Inspiration can be used as Cutting Words.
Here's three examples for comparison.
Example A: Bob the Bard applies Cutting Words to Vrax's stat check (for -3), while Vrax applies Dark One's Own Luck (for +2). Vrax rolls 15. What's the final result?
Example B: Bob the Bard applies Cutting Words to Vrax's stat check (for -3), while Otto the Other Bard applies Bardic Inspiration (for +2). Vrax rolls 15. What's the final result?
Example C: Susan the Sorcerer applies Bend Luck to Vrax's stat check (for -3), while Olive the Other Sorcerer applies Bend Luck (for +2). Vrax rolls 15. What's the final result?
