Uncanny Dodge and Protective Field (Or other damage reduction abilities) Order of Operations

Question

I had a scenario come up in game this past weekend that I wasn't sure how to adjudicate. The rogue was hit by a big hit (>30 damage) and decided to use uncanny dodge to reduce the damage, but at the same time the Psi Warrior decided to also reduce that damage using his protective field.

Which order should the abilities be applied?

1. Uncanny dodge first, then protective field. Basic PEMDAS, we do division before subtraction.
2. Protective field first. While uncanny dodge does not give resistance to the damage, it functions like resistance by halving the damage and as such should come after.
3. Whichever order the rogue wants it to be in. Based on the rules for effect timings at the start of a creature's turn, the creature being affected gets to choose the order. (Though in this case that is certainly going to be option 1 for the maximum damage reduction).

Answer 1

Uncanny Dodge activates first

The Rogue's Uncanny Dodge states:

[...] when an attacker that you can see hits you with an attack, you can use your reaction to halve the attack’s damage against you.

Meanwhile, the Psi Warrior Fighter's Protective Field states:

When you or another creature you can see within 30 feet of you takes damage, you can use your reaction to expend one Psionic Energy die [...]

So one occurs when an attack hits you and the other is when you actually take damage. Thus, Uncanny Dodge must be used before damage is rolled and also before Protective Field. The fact that Uncanny Dodge must be declared before damage is rolled is also supported in the following:

• Does the Rogue have to declare that they're using Uncanny Dodge before or after damage is rolled?

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Unless hits and damage are simultaneous

However, you could also subscribe to the idea that getting hit and taking damage are simultaneous events, and thus these features should trigger simultaneously, at which point, you could default to the optional rule from Xanathar's about resolving simultaneous events where whoever controls the creature whose turn it is decides the order. Or you could make any number of houserules, but I have personal used the rule in Xanathar's for almost every case of simultaneous events even though I do not myself consider getting hit and taking damage to be simultaneous. For more reading see:

• Are getting hit and taking damage simultaneous events?

Answer 2

Borrowing from this thread: What happens if multiple effects could be resolved upon being targeted with an attack?

An optional rule from Xanathar's states:

If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character’s turn, the player decides which of the two effects happens first.

Whoever is controlling the creature that did the damage (presumbly you, the DM) gets to decide how things pan out according to that rule.

Another options include letting things play out in the order in which players declared. ie. Whoever declared they'd use an ability first has their ability resolved first. You could also resolve things according to initiave order.