In Chapter 4, the piano in the first sanctuary allows for a much more expansive range of notes than other in-game pianos. Accordingly, Susie and Ralsei have special reactions if the player inputs specific songs on it:

• Playing "Memory" results in Ralsei complementing it by comparing it to a lullaby.
• Playing "Your Best Friend" from Undertale results in Ralsei calling it "friendly and sweet," alluding to Flowey's introductory cutscene in that game.
• Playing "The Legend" flatters Ralsei, to which Susie responds by joking that Kris is flirting with him.
• Playing "Lost Girl" results in Susie wondering if she could learn it on guitar.
• Playing "Rouxls Kaard" results in Ralsei recognizing it as Rouxls' theme song; Susie then jokingly calls it the "theme of him getting his ass kicked."
• Playing "Heartache" (Toriel's boss theme in Undertale) causes Ralsei to describe it as "emotional" before Susie suggests playing it for Toriel.
• Playing "Undertale" results in Susie deriding the song as "crap."
• Playing "Dark Sanctuary" results in Susie pointing out that "Ralsei was humming it earlier."
• Playing the song on the Old Man's sheet music results in different responses depending on whether or not the player includes the octave switch: omitting it results in Susie & Ralsei commenting on its absence, while including it results in Susie commenting on how different it is.
• Playing the first four notes of "MEGALOVANIA" (Sans' boss theme from Undertale) results in the Annoying Dog abruptly running Kris over with the car from Cyber City's hidden alley in Chapter 2, which causes a smash to black before the game returns to normal. Attempting to replay the song results in Kris fumbling the fourth note.

Several other songs that produce reactions are transcribed unusually, and as such are more difficult to trigger if playing solely by ear:

• Playing A, D, E, F, D, D, C, D, B, A, D, E, D, C, D, D causes the pair to comment on the "annoying" nature of the melody.
• Playing "Don't Forget" (C, D, E, E, E, E, E, A, C, B, B, B, B, B, B, C, B, A, A, A, A, A, A, G, H, G, E, C) causes Susie to fall silent before calling the song "kinda cool."
• Playing "Spear of Justice" (E, D, B, C, A, B) results in Susie calling the song "majestic" before Ralsei suggests that the Old Man would enjoy it, referencing the song's nature as Spoiler:both Undyne and Gerson's leitmotif.
• Playing "Chopsticks" by Euphemia Allan, "Heart and Soul" by Frank Loesser & Hoagy Carmichael, or "You've Got a Friend in Me" by Randy Newman & Lyle Lovett (C, E, C, F, E, A, F, H, F, H, E) results in Susie scolding Kris for "messing around." Notably, all three of these are real-life, commercially released songs: the first two are common stock piano pieces, while the latter was recorded for the 1995 animated film Toy Story. Its inclusion likely references a series of comedic edits of the song posted to Tumblr by developer Toby Fox. One of these edits, "Friend inside Me", was previously the subject of tongue-in-cheek fan theories claiming that it would pertain to Chapter 3's hidden boss.