"meh, some things are better off forgotten"
– Sans, after vaguely remembering something . . . important. . .
Handplates is a backstory-based AU created by Zarla. It details the origins of Sans and Papyrus as the creations of Dr. W. D. Gaster.
In this AU, Gaster is Asgore's Royal Scientist, who decided to create two clones of himself as part of his research in breaking the barrier. While not knowing if they will be sentient, he takes steps to avoid emotional attachment to his creations, including accelerating their growth within tubes of magical fluid; this, he believes, will keep them from appearing too much like infants, thus limiting emotional attachment.
These clones, revealed to be Sans and Papyrus, are referred to by Gaster as 1-S and 2-P, respectively. The two skeleton clones were created to be used as tools to find a way of breaking the barrier and were not intended to be sentient. While nearly fully grown, they still act and look like children at the outset, which requires Gaster to choose whether he will show MERCY to his creations, or if he will use them to FIGHT against the barrier and, potentially, the humans.
Sans is very weak, manifesting the 1 ATK and 1 HP stats described in Undertale, but lacks his ability to bypass his opponent's invincibility frames. However, he is very clever, using snarky comments and derision against Gaster whenever he can. Papyrus, on the other hand, is considered by Gaster to be less intelligent than Sans, but Papyrus has much more HP and is more physically resilient. Both brothers are able to use healing magic (In this AU, all monsters can use healing magic, but not on themselves.), although Papyrus is much stronger at this magic than Sans is. Papyrus believes Gaster can change from his sadistic ways, though both Gaster and Sans do not believe that is possible. Gaster keeps the entire project as a secret from everyone, including King Asgore and Gaster's assistant, Dr. Alphys.