[He relaxes a little, just enough to drop the battle-ready stiffness, as Sans's demeanor picks up, but continues listening solemnly. With these last months, he can recognize a little better the signs of Sans being earnest.

That said, his takeaway isn't one of thinking Sans sounds crazy. The bits and pieces of things Flowey's alluded to, the strangely knowing demeanor Frisk stepped forward with, the deja vu... It would be so easy for Papyrus to assume that Sans is talking literally, here. But it would sound outrageous, if he assumed so and said so and his brother doesn't mean anything like that. So he prevaricates, and reassures instead.]

Y-yeah, I can tell. All the work you've been doing, trying to save the world... Even if it's still hard, [even if Sans has messed up a few times that he doesn't want to describe,] you're doing a lot more than you used to. That's progress!