Would it be sensible to give someone disadvantage on a saving throw they didn't see coming?

Such as an invisible dragon breathing on them from behind?

Even the surprised condition doesn't give disadvantage on saves. The barbarian's Danger Sense gives them advantage on Dex Saves if they can see the effect -- implying that if they don't see it, it's just a regular save. And Dex is about the only saving throw type (maybe also Strength) where I think being able to see it might really make a difference. Considering how many spells have saving throws and can be cast at range or only need hearing, disadvantage feels like a lot.

I respect that. Follow up retort. Dexterity is dodging or making it so you aren't hit by all of it. If you don't know to dodge it, wouldn't it be sensible to have dis?

Quote from 17ArtificerSpecialist >>

I respect that. Follow up retort. Dexterity is dodging or making it so you aren't hit by all of it. If you don't know to dodge it, wouldn't it be sensible to have dis?

 No?

Just look at the barbarian ability - it gives advantage on dexterity saving throws when they see the effect coming. This implies that they get a normal roll when they can't see it coming. If the designers decided that dex saves do not receive disadvantage for everyone if they can't see it coming then perhaps they had a reason in mind?

Maybe dexterity in this context isn't entirely about "dodging". For example, how do you "dodge" a 20' radius fireball when you are 10' from any of the edges? Certainly you aren't moving out of the way or out of the area of effect ... so what does dexterity mean in this context? The fireball engulfs the character, they can't see the part behind them, they can't see the part above their head. In addition, a fireball is "instantaneous", is there even enough time to react to it even if you do see it coming?

I'd suggest that your desire to impose disadvantage on dexterity saving throws when a character can't see the effect coming is based on an assumption about the reason for the dexterity saving throw having something to do with dodging. On the other hand, perhaps it has to do with reactions, a character with a high dexterity is able to cover parts of their body quickly to reduce their exposure to the effect. This might be easier if they see it coming (as in the the barbarian ability) but the baseline design assumption appears to be that the characters could be unaware of the effect since awareness has no effect on dexterity saving throws as written.

Could you house rule it this way? Sure. However, keep in mind that your NPCs, when surprised by an invisible PC casting a fireball will also have disadvantage on dexterity saving throws since they didn't see it coming. PCs will use this to set up lots of situations forcing the other side to have disadvantage.

One last point, the level 9 arcane trickster rogue has an ability that forces disadvantage on saving throws for their spells if the rogue is hidden from the target when the spell is cast. Again this is one special class ability that could force disadvantage on saves and it requires the rogue to be hidden. So, disadvantage on saves could be a special ability but it would not make sense in general even for dexterity saves.

Just look at condition descriptions. Invisibility and blinded do not give disadvantage to saving throws. Other conditions do, like being restrained. Generally, for something to negatively affect a dexterity saving throw, it has to keep them from moving around.

The other thing is dodging it doesn't mean your fully aware its happening, you might stub you toe and stop your foot from continuing its momentum cause of the initial pain. So in a ideal world If all of a sudden I'm engulfed in fire I'm going to try and get out asap (aka dex saving throw). Because I was in the fire and made my save I did take damage but not as much as if I stood there and just bathed in (aka half damage). My saving throw is the reaction to me starting to be in pain and realizing what is happening and trying to move. Barb danger sense/rogue evasion is like the step better where they have the spider tingle. They know something bad is going to happen and have a moment to prep or maybe see the way out or something to hide behind...etc.

Did I move in time...
Yes cause I made the saving throw and got out of the major damage area.
No, then my reflexes where not on point and I just took the fire to the face.

Also just to point out (and I can be wrong and might very well be) Invisibility does not make the dragon silent. Something that big moving makes a lot of noise. Also does not stop things from moving if the dragon nocks it around. So as a Dm I would look at exactly how much this dragon can "hide" 

I wouldn't think it's reasonable to give disadvantage on saving throw against an invisible dragon breathing since while the dragon might be invisible, it's breath isn't and so a character should be able to try to save against such effect normally.

Unless you are using the optional rule Facing from the DMG, this shouldn't be an issue since PCs are assumed to be watching all around them for danger.

A creature can normally target only creatures in its front or side arcs. It can’t see into its rear arc. This means an attacker in the creature’s rear arc makes attack rolls against it with advantage.

Quote from 17ArtificerSpecialist >>

Would it be sensible to give someone disadvantage on a saving throw they didn't see coming?

Such as an invisible dragon breathing on them from behind?

 Generally speaking unless the condition says so, it doesn't happen. Meaning Invisibility doesn't give disadvantage to saves.

Sometimes saves are knowingly getting out of the way. Sometimes saves are being able to roll with a blow you can't avoid. Sometimes saves are just dumb luck.

There are real life cases of people being in the middle of a barrage and not getting hurt while everybody else was blown to pieces. 

You might be thinking of "Hiding" rules.

If a character is successfully hidden from an adversary then it can gain advantage on its attack roll.

This is the way an attack targeting an unsuspecting player shows how its much easier to hit.  Any attack could be surprising in the heat of battle and the dexterity save represents flexibility and reaction to a threat.  It might be the whistling of an arrow through the air or the whooshing of an approaching ball of flame, it could even be the beginning of an explosive blast nearby or another beginning effect of a spell that causes a character to leap like a startled cat, turn from the blast, shield themselves with arms or actual shield or just generally react in a way to reduce the effects.  It's also a part of why many things will still do half damage or have a partial effect to represent the character was caught in the effect but was merely better at putting some degree of defense up.

Another point to consider is that Dexterity is part of calculating initiative.  This shows that it's not merely flexibility, balance and fine or precise movement but reflexes and reaction time.

I would say no because of the wording on the Arcane Trickster's "Magical Ambush" ability. Targets you cast a spell on that require a save have disadvantage on that save if you're hidden from them.

Given that this is a subclass feature, it implies that this isn't the normal way saving throws work.

I would say make them roll for surprise first.  If they save they do not get hit. If they fail then the save for the breath. 

Also feel the need to point out that invisible and undetected are two very different things. If you are not discerning that, you are vastly overpowering invisibility.

By RAW, you know exactly where an invisible dragon is unless it has used the Hide action. You hear it, you smell it, you see distortions in the air when it moves, however you want to describe it. But all it does is grant disadvantage when attacking it and prevent things that require you to "see" the target.