"BROTHER, IF YOU HAVE ANY PROBLEMS, JUST... LET ME KNOW, OK?"

Oh boy.

Sans had a problem.

If Sans didn't say anything, Papyrus would guess that he was being secretive, and would not leave him alone until Papyrus found out EVERYTHING. But, if he did spill the beans... In any case, Papyrus would end up knowing, so the question is really a matter of sooner or later.

Sans asked frisk, and they said later. But Sans was unsure, both of the choice in general and of frisk, who could potentially be lying, or possessed by chara, whose opinions were untrustworthy. Or both. In the end, it was that mindset more than either logical or emotional reasoning that led him to telling Papyrus.

"paps, i need to tell ya something."

Sans told Papyrus everything, from the resets, to every route and every option within every route. And at the end of it all, Papyrus stood (he had refused to sit, claiming that he needed excersize more than he needed cushioning) with a blank face, and after a second, he asked:

"WAIT, WAS THAT ALL?"

Sans was baffled. His brother, the adorable cinnamon roll, somehow had found out in the past about the whole situation.

"what do you mean, p-paps?"

"SANS, I KNEW ABOUT THE RESETS AS LONG AS YOU HAVE! EVEN LONGER, EVEN! I HAD KNOWN ABOUT YOUR BACK ROOM, SANS, AND ONE DAY, WHILE YOU WERE AT GRILLBYS, I HAD SNUCK IN, AND TAKEN A LOOK. IT WAS DIFFICULT, BUT THE GREAT PAPYRUS FIGURED OUT THE MEANING TO THOSE! AND AS FOR THE ROUTES... FRISK TOLD ME, AFTER I TOLD THEM THAT I KNEW ABOUT THE RESETS."

So, that's why frisk said to let Papyrus find out later rather than sooner. Oh well, Sans didn't care that this was a Pacifist Timeline, frisk's head WAS going to end up on a stake. Or a steak about to be fed to the dogs, Sans didn't care which.