I don't know what particular weight you are giving to "begin hiding" - I read your post but to be honest am not 100% sure that I followed every move your were making. To me, "beginning hiding" just means being still and quiet and hoping not to be noticed. (Which means it will auto-fail if you can be seen.)
Let me see if I can be more clear then. Ultimately, I'm trying to discuss whether Crawford's statement on Halflings (and Wood Elves) is ambiguous as you and Hriston claim, or if holes that have arguably been identified in your interpretation render it implausible to the point that the ambiguity disappears.
The original argument Hriston put forth (that you joined) in regards to Wood Elves was that Crawford was saying that Wood Elves who had previously taken the hide action while unobserved could merely
remain hidden while in natural light obscurement. Others argued that Crawford was saying that Wood Elves in natural light obscurement could take the hide action to
become hidden (i.e. “begin hiding”) even while observed.
The conversation has since moved on to Halflings. In the exchange ending in post
#832, Hriston agreed that Crawford’s use of the word “vanish” implied a “change of state”, and thus could not be referring merely to
remaining hidden because remaining hidden doesn’t involve a change of state.
I believe that concession is fatal to Hriston’s argument that Crawford was ambiguous with respect to whether Halflings can take the hide action while observed. This is because (as you have agreed) the context of Crawford’s sentence about the Halfling ability is the same as the context of his preceding sentence, namely, hiding “in full view”, because of Crawford’s use of the adverb “though”. (For a more thorough exposition, see the above link.)
Hriston has since countered (indirectly, he wasn’t replying to me) that “in full view” does not mean “observed”. His newest argument is therefore that Crawford was saying that a halfling can take the hide action in full view so long as no one is present. I argued that this was ridiculous in post
#887, which sparked the current exchange between you and me.
Since both you and Hriston apparently would indeed permit a human to take the hide action while in an empty room, I’m left wondering what circumstances exist under Hriston’s latest interpretation of Crawford where a halfling could take the hide action but a human cannot. It’s evidently not an empty room, because that doesn’t qualify as “in full view” from the people (if any) outside it. So I asked you for a different example where a halfling was in full view and no one was present where you would permit the halfling to take the hide action but would not permit a human to do that same.
Does that make sense?
I believe I can infer your answer from your example below.
That said, here's an example that makes sense to me (adapted from my post just upthread of yours):
A is a human spy. B is a halfling spy. They are a team.
A to B: "Be careful - there are observers nearby, and we don't want to be spotted by them!"
Those nearby observers haven't seen the spies yet.
B tags quietly along behind Spy A. When A is spotted, B has a chance to remain unnoticed (contested DEX (Stealth) vs WIS (Perception) check).
I will state what I think is going on using the Sage Advice language:
Normally, you can’t hide from someone if you’re in full view. B, though, who is a lightfoot halfling, can try to vanish behind a creature that is at least one size larger, such as A. B's hiding attempt is assured of success; a Dexterity (Stealth) check is required as normal, and an observant foe might later spot B: “I see you behind your team-mate, you tricksy halfling!
If the observant foes WIS check fails, however, then B has indeed vanished behind A.
(The above is all predicated on it being the general case that people can't hide behind other people, because when you're behind someone else you're still in full view.)
It’s unclear when exactly the Halfling is taking the Hide action. It sounds like you might not require a declaration (and a roll) at your table until later, but if it were my table the halfling has effectively taken the Hide action if he agrees to comply with A’s directive to “be careful-there are observers nearby”. Regardless of when you roll, I think you would agree that from this point forward, the halfling has a chance to be hidden behind the human? If so, from a rules standpoint, that’s when the hide action was taken.
At the time the Halfling starts "being careful", from your reply to my empty-room hypothetical, may I correctly infer that you would consider both the halfling and the human to
not be “in full view” of the nearby observers? If so, to this point you and I agree, even if we’d make the roll itself at different points in time.
Where we disagree is when you say “If the observant foes WIS check fails, however, then B has indeed vanished behind A.” As I discussed in the first of the above links, every definition of “vanish” that I looked up either required or emphasized the sudden nature of the disappearance. In your hypothetical, the halfling hasn’t disappeared from anyone at all, let alone suddenly. The observers were, and (assuming they didn’t beat the Halfling’s Hide check) still are unaware of the Halfling’s existence.
Ergo, I do not agree that the halfling has “vanished” behind A at the point in time where A becomes observed. It’s slightly more reasonable (but not much) to say that the halfling vanished behind A at the point in time where the halfling started to “be careful” (i.e., at my table, took the Hide action) because at least something at that point changed state. But at that point the Halfling wasn’t in full view from the nearby observers,
and I believe you’d let a human start to “be careful” at the same time the halfling did*? Accordingly, I don’t see any way where your example fits Crawford’s statement that a halfling (and only a halfling) can “vanish” behind another creature (while in full view).
*(Even though a second human would automatically be seen at the same time A is, the human starting to “be careful” at the same time the halfling would have matters if the duo might be heard by the nearby observers prior to being spotted.)
Based on your use of the word “vanish” above, I’m concluding that you disagree with Hriston (and most everyone else) and believe that “vanish” can plausibly mean “remain hidden” and does not require a “change of state”? If so, your argument is valid, even though I find your use of “vanish” utterly at odds with the normal usage of the term. I therefore cannot agree with your claim that Crawford’s statement regarding Halflings is ambiguous--your interpretation is simply too much of a stretch for me to consider plausible.
[MENTION=6787503]Hriston[/MENTION], in pemerton’s example above, do you consider the human and the halfling to be “in full view” of the “nearby observers” at the time the halfling starts to “be careful” (i.e. takes the hide action)? If so, can you please explain why the duo are in full view in pemerton’s example but not in my empty room example? If not, can you please provide a different example where a halfling, but not a human, can take the hide action while simultaneously “in full view” and “unobserved”?