In Some Of The Neutral Endings, Sans Has Been Shown To Have Told Papyrus That His Friends Simply "went

In some of the neutral endings, Sans has been shown to have told Papyrus that his friends simply "went on a vacation" when they've been killed by you/frisk because he doesn't have the heart to tell him what really happened, like how he omits the truth of what happened to papyrus to Toriel in order to preserve the "perfect little angel" image she has of Frisk

Papyrus "accepts" that in his presence but the second he's not in the room, he drops his cheery and naive facade to tell you how he really feels

So if anything, they're both protecting eachother from less than savory truths, but yeah, he has some streaks of lying

you're RIGHT thank you anon. i forgot about those endings entirely. those are a good bit of characterization to keep in mind in any conversation about sans and lying. thank you! let's think about this...

personally, in my characterizations of sans, i've always interpreted sans's "vacation" excuse as part of his nihilism--if you know you're stuck in a time loop, and you know you've reached the "end" of the time loop, then everyone who died is actually about to come back. Combined with some pretty heavy depression, you might think of everyone who died as just getting to skip out on the sad epilogue. especially since sans doesn't die in literally any epilogue*, I've always thought of that as less protective and more...conflict avoidant? cracks in the facade?

like, at the end of the loop, his actions no longer matter. the human is not in the underground, and they're the one with all the power. outside of that phone call, he has no further effect on them and therefore the world in general, outside of his relatively small circle. so i think the things said about sans in those phone calls reveal a different side of him. after all, no one will remember this soon, so he doesn't have to keep up the "i'm a normal guy who only knows about regular linear time and therefore believes in a future" thing. so...yeah. permanence isn't permanent. everyone is on vacation. they all get to take a little dirt nap and he's just waiting for them to come back. why break toriel's heart when this is all going to end halfway through that conversation? I think it's less about lying to deceive (after all, Toriel and Papyrus are both perfectly capable of finding the truth), and more about the interesting shapes sans twists his understanding of reality into in order to understand that people are dead without taking the time to process that grief. He's mostly telling himself that people are on vacation. They're fine, maybe even lucky, and they'll come back soon so he doesn't have to worry about it. that's what allows him to function and even help rule** in bad endings, and how he's able to avoid totally breaking down after papyrus's death in those runs. no one is dead. they're just resting.

That said, he does say that to Papyrus knowing that it's literally, factually not true. He super does lie either to impart his horrible coping skills or to avoid having a conversation he doesn't like. It's also a really bad sign for his relationship with reality that he's just saying this with no regard for how insane he sounds. And, given how he does step up to rule in the King Papyrus ending...maybe he doesn't assume a reset is coming and I'm wrong about all of that. But my interpretation is that "everyone is on vacation actually" and "haha nah my brother is fine he's just...not around...ever..." are both so transparent that they can't be genuine attempts to deceive. Frankly, Sans is just a better liar than that.

So...tl;dr: Sans does lie! I was wrong about that. but I think he lies to protect himself more than anyone else. I don't really see it as something so...altruistic? controlling? condescending?? as lying for Papyrus and Toriel's sake. he just can't handle confronting reality, and god forbid he have a conversation about it.

*Sans may or may not die in the no mercy but in any case the world ends like 3 minutes after that so...not much of an "epilogue" to live in. he survives basically as long as the timeline does.

**In the King Papyrus ending, Sans does "paperwork" and dictates policy decisions such as how humans will be judged (which, his policy there is interesting in and of itself). Papyrus says Sans is "working hard." My takeaway from this is that Sans is doing most of the logistical ruling, while Papyrus works on the absolute disaster of morale. Unclear whether Sans has a role in Alphys's ending. He seems to drift towards whoever is closest to him of the survivors, and help them with whatever they need.