5.5 Truesight vs. Doppelganger

The title says it all. My party and I have a difference of opinion regarding a doppelganger's true form and another creature seeing it with truesight.

My thoughts are thus: 

Truesight

If you have Truesight, your vision is enhanced within a specified range. Within that range, your vision pierces through the following:

Darkness. You can see in normal and magical Darkness.

Invisibility. You see creatures and objects that have the Invisible condition.

Visual Illusions. Visual illusions appear transparent to you, and you automatically succeed on saving throws against them.

Transformations. You discern the true form of any creature or object you see that has been transformed by magic. <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

Ethereal Plane. You see into the Ethereal Plane.

It is the indicated line that we disagree on the ruling of. Nothing in the doppelganger statblock, nor the mimic or werewolf statblocks for that matter, states that the shapechange is magical in nature. With that, would true seeing actually see the creature(s) for what they are?

Any help would be appreciated.

I agree with you, but it's not actually clear. According to their description, Doppelgängers are "supernatural beings". Does that make their shapechange magic? Maybe!

Mimics are probably non-magical. Werewolves are probably magical, because the curse is magical in nature. Doppelgängers are <shrug emoji>. 

I made the decision I did because they're supposed to be a threat, and easy outs reduce that threat. (It's a clearer call in my current game, where I explicitly defined them as natural beings -- basically advanced octopuses; but that's house rules.)

In 2014, truesight could explicitly "perceive the original form of a shapechanger or a creature that is transformed by magic". In 2024, reference to shapechanger was removed. Note that some shapeshifting creatures (such as an ancient gold dragon) do explicitly change shape with magic, but many others do not.

I don't remember this being called out as a change in 2024, but it's definitely not the same wording so I'm inclined to consider it a real change.

The 2024 rules explicitly define what effects count as magical: 

“Magical Effect

An effect is magical if it is created by a spell, a magic item, or a phenomenon that a rule labels as magical.”

So, we can then look at the general rule for shape-shifting:

“Shape-Shifting

If an effect, such as Wild Shape or the Polymorphspell, lets you shape-shift, its description specifies what happens to you. Unless that description says otherwise, any ongoing effects on you—conditions, spells, curses, and the like—carry over from one form to the other. You revert to your true form if you die.”

Thus, shape-shifting isn’t per se magical.

Looking at the traits for the monsters in question:

“Shape-Shift. The doppelganger shape-shiftsinto a Medium or Small Humanoid, or it returns to its true form. Its game statistics, other than its size, are the same in each form. Any equipment it is wearing or carrying isn’t transformed.”

“Shape-Shift. The mimic shape-shifts to resemble a Medium or Small object while retaining its game statistics, or it returns to its true blob form. Any equipment it is wearing or carrying isn’t transformed.”

So, the Shape-Shift features for neither the Doppelgänger nor the Mimic are listed as magical, so True Sight wouldn’t really perceive their true form. 

A rememeber a similar question here: Would true sight work on a changeling

IMO, RAW, the difference between 5e and 5.5e Truesight means that the true forms of Doppelgangers, Changelings, Mimics, Imps, Night Hags or Oni are no longer revealed, since magic isn't involved.

I voted no because their true form has not been transformed by magic.

As others have pointed out, it depends on the version. That said, it's very clear in 2014 that it would reveal them, so I'm assuming (as is everyone here so far) that you are talking about 2024. I voted "No" on a pure RAW standpoint, but there is nothing preventing a DM from making those kinds of shapechanges "magical" and thus Truesight could reveal them.

Quote from sabin76 >>

As others have pointed out, it depends on the version. That said, it's very clear in 2014 that it would reveal them, so I'm assuming (as is everyone here so far)

 Well, he quoted the relevant rule, and it's the 2024 rule, so not really an assumption.

I think it's very silly to have a version of Truesight that doesn't reveal shapeshifters, but per RAW, the 5.5 version would not