"Yeah," Sans says, and it's not great, but Sans already realized Papyrus wasn't quite following the conversation when he asked where they were going, so additional confirmation doesn't shake him too much. "Which is more proof you need a break. Don't worry, I won't say I told you so."

Of course, in totally-saying-I-told-you-so-by-saying-I-won't, Sans is trying to give Papyrus something more normal to focus on: exasperation with Sans.