His fight is widely considered the most difficult in the game among players.

Sans uses bones and straight beam projectiles that emerge from skeletal faces, known internally as Gaster Blasters, in his attacks.

He is the only enemy in the game that has the first turn rather than the protagonist, which can catch the unwary off-guard.

This results in the famous Game Over screen that is only seen on this occasion.

If this happens, death is inevitable, although if the SAVE file is hacked to get unlimited health, Sans continues to shrug, but the bones disappear and the game cannot progress without save reloading or other hacks.

He also starts to use Gaster Blasters more frequently and aggressively, and starts using vertical bone slides rather than just the horizontal variation he uses during the first half.

After a while, he does his biggest attack, which features elements from previous attacks, as well as new ones, and finishes with a spinning ring of Gaster Blasters.

After getting past his mercy offer, the fight then includes Sans constantly flickering the screen as he transitions through his attacks, uses his Gaster Blasters more often, and even attacks the protagonist as they navigate the menu (though menu attacks only deplete their HP to 1 and cannot kill them).

After enough turns, Sans then performs an attack lasting roughly 40 seconds, launching some attacks shown previously at the protagonist, including a large array of bones, a bevy of Gaster Blasters, and many slams against the Bullet Boards walls (the protagonists HP is depleted by 1 for every slam but stops once their HP reaches 1, preventing the attack from killing them).

Sans abstains from attacking and stalls his turn out indefinitely, preventing the protagonist from accessing the menu in hopes of the protagonist becoming bored as time passes.

After several minutes and dialogue, Sans eventually falls asleep, allowing the protagonist to push the Bullet Board towards the FIGHT button.

After pressing it, Sans wakes up and dodges the first attack, only to be ambushed and struck down with another automatically-performed attack, instantly finishing Sans off for good.

Unlike most enemies, however, he starts to leak a red fluid of some sort (which is hypothesized to be injected determination, ketchup, or even blood), then limps off-screen before dying, raising the protagonists LOVE to 20 and HP to 99.

Some attacks involve platforms the protagonists SOUL can rest upon safely.

From his false plea for the protagonist to stop fighting onwards, Sans also attacks the protagonist in the action menu.

Spare dialogue if the protagonist has previously tried sparing Sans.

One cannot do anything other than restarting the game to progress.

Ultra Sans Fight