Undertale Yellow

Undertale Yellow

Prototype Info Prerelease Info Debugging Tools Unused Text... ... ... Bruh...

Unused NPCs#

There are 3 test NPCS in the game's code, meant to test interactions. One is of Testlet, the next is of Dalv, and the last one is of a now-deleted character.

obj_test_martlet#

An unused NPC that uses Martlet's sprites, presumably used to test cutscenes activated by interacting with an object. Upon interaction, it says the following dialogue (without portrait):

* Hello. This is MARTLET.

* Just kidding, I'm TESTLET.
* I'm here to show you
  how I can walk around.

* Watch and learn, kiddo!

* I bet you can't walk
  like that, scrub.

* Testlet, out!

obj_test_dalv#

An unused NPC that uses Dalv's sprites. It faces the left and walks to (300, 120) along the X axis, with walk speed set to 2 and animation speed set to 0.2, and avoids the player.

obj_test_follower#

An unused follower for testing, implemented very differently in comparison to the actually used followers.

It uses removed sprites named spr_rals_up, spr_rals_right, spr_rals_left, spr_rals_right; the name "rals" suggests that these sprites were of Ralsei from Deltarune. This is likely a leftover from development, when Ralsei was the first party member used to test the follower system. Ralsei sprites were also used in place of Clover's when testing swapping Clover's sprite out in different environments before the shader system was implemented.³

Unused Sousborg NPC#

In the Pacifist Route, after sparing Axis, a Sousborg NPC appears in the Chemistry Lab. This area is inaccessible in Pacifist, though.

Unused Attacks#

Bowll#

Bowll has an unused fourth attack, Bowll Test Song, which uses the Yellow Rhythm SOUL mode. As Bowll is immediately before El Bailador in the attack list, it was likely used to test El Bailador's attacks before the character was implemented.

This attack would've used the same patterns as El Bailador's attacks, but relevant script files, aside from a test script, no longer exist.

Flowey#

obj_flowey_1_attack_shoebox_creator and obj_flowey_1_attack_audience_creator are unused attack managers that are assumed to belong to Meta Flowey's 1st phase, due to following the same naming scheme as the standard attacks in it. They create and manage obj_flowey_1_attack_shoebox and obj_flowey_1_attack_audience respectfully.

• obj_flowey_1_attack_shoebox is an attack where a basket dumps rows of 2 boots (with holes to slide by between the boots) constantly over the SOUL.
• obj_flowey_1_attack_audience is an attack where an audience and a scoreboard appear, followed by the scoreboard lighting up the words "HATE", then the audience throws swarms of gravity-affected flowers at the battle box.
  • It would appear that "JOY" can also be displayed in place of "HATE". However, it seems only related sprites remained in the game, instead of actual related code. Given the sprites, it appears that plushes of the game's main cast (Dalv, Martlet, Starlo, Axis, and Ceroba) would be thrown at the SOUL and land on the ground. The Martlet one would break open on the ground and spill out stuffing.

Neither attack has battle box dimensions associated, causing a game crash. The videos shown have their box dimensions set to 140 x 140.

Unused Cutscenes#

Dalv Damaged Exit#

Watch on YouTubeDalv has an unused variant for the exiting Dark Ruins cutscene for if he was damaged in his fight. There are two versions of this cutscene, one for Pacifist and one for Neutral. This cutscene includes an unused animation of him using his lightning powers instead of breaking the boards with his hands. All of this actually worked correctly in the demo, and would work correctly in the final game, if not for the Dalv damaged flag never being set.

Test Cutscenes#

Objects obj_cutscene, obj_cutscene2 and obj_cutscene3 contain cutscenes that appear to be for testing, with the last object already present in rm_debug.

However, obj_cutscene and obj_cutscene2 misuse cutscene_npc_walk in a way that causes them not to proceed correctly. obj_cutscene also doesn't initialize cutscene_music, causing it to crash the game by trying to play Blossom, and obj_cutscene2 doesn't end itself. obj_cutscene3 works as-is. The videos here were recorded from a mod that has those bugs fixed and puts each of those objects in rm_debug.

To do: Add some more elaboration/quality to the actions in the transcripts

obj_cutscene

Watch on YouTube

obj_cutscene2

Watch on YouTube

obj_cutscene3

Watch on YouTubeobj_cutscene3 has Starlo walk into the player and initiate the fake battle cutscene with Rephil. The fake battle appears exactly as it does in-game:

Neutral Hotland Rooftop Cutscene Duplicate#

obj_hotland_roof_cutscene_neutral_2 appears to be an old revision of obj_hotland_roof_cutscene_neutral. It only contains the LV 1-2 dialogue variant and is missing the quick-save before the fight with Meta Flowey.

Unused Items#

Various unused items exist in the game data.

* "Super Ammo" - Weapon AT 15
* (Regular toy gun ammo sold
  at your favorite convenience store.)

* (You load the Super Ammo.)

* (Your gun now feels lighter
  than your conscience.)

"Golden Scarf" - Armor DF 25
* (It's emitting a violently
  golden shine.)

* (You put the Golden Scarf on.
* Your tremendous taste for fashion
  makes monsters go easy on you.)

* Info message

* (You scarf down the Homemade Cookie.)

The Homemade Cookie is a little odd. It does not have overworld use text, so using it outside of battle softlocks the game. Its check text just uses the default check message, since its actual check is set in an unused script. The check is meant to read:

Heals 20 HP
* (A little burnt on one side, but
  looks yummy anyway.)

Unused Rooms#

rm_live_buffer#

An empty room. Based on its name, it appears to be related to GMLive in some way, similar to room_gms_debug_failsafe in Deltarune Chapter 1&2.

rm_dunes_30_old#

An older version of Oasis Valley, without the café and the fancy building leading to the Wild East.

rm_dunes_35#

Related to the previous entry; this room was supposed to lead to the café before it became a part of Oasis Valley.

rm_steamworks_07#

An old version of Steamwork's first steam vents section, containing a much harder version of it.

rm_steamworks_17b_old#

An old version of a room with a console that opens a gate in the Steamworks, lacking a section with steam vents.

rm_build_ending#

Another empty room. Based on its name, it may have been used to inform playtesters of a build's ending at some point.

rm_battle_flasback_parent#

This needs some investigation. Discuss ideas and findings on the talk page. Specifically: Since some GM update rooms can have children rooms (RPG Maker reference?), so maybe this has something to do with that.

An empty room (notwithstanding three effect layers) that appears to serve as the base of the overworld portions of Meta Flowey's 1st phase.

rm_debug#

This room only contains obj_cutscene3.

Unused Summary Screens#

rm_score_room#

rm_snowdin_14b_yellow	rm_score_room

This slightly more crowded copy of the cabin of Honeydew Resort (rm_snowdin_14b_yellow) appears to have served as a roundup for a playtest build. As you can't move around, you can't see everything happening in it. You get to see some neat data, though.

• COMPLETION TIME: In the format mm:ss
• TOTAL DEATHS
• DALV KILLED
• MARTLET CHASED OFF: Interestingly, the corresponding variable in this screen is named martlet_killed.
• ENEMIES KILLED: As global.kill_number[0] is set to 9999, this number is normally ridiculous.
• MO ITEMS BOUGHT
• SLURPY SAVED
• MICRO FROGGIT FOUND: NO/YES/KILLED

rm_summary#

Another unused summary screen resides in a room named rm_summary. It shows the following data:

• ROUTE: Pacifist/Neutral/Geno
• FULL NAME: Either Clover or Gun-Hat, depending on the name given to Flowey in Snowdin
• PLAYTIME: In the format hh:mm:ss
• LEVEL, TOTAL EXP, DEATH COUNT
• FUN VALUE: Displayed as 420 instead if it's 100, though Fun Values only range from 1-80.
• SLRUPY SAVED: No.../Yes
• GOLD SPENT, TOTAL GOLD: Amount of gold spent/picked up; only tracked if scr_debugmode is enabled. So, for most players, 0.
• BOSSES KILLED: Can contain Da (Dalv), De (Decibat), St (Starlo) and Ax (Axis), or simply be NONE
• ENEMIES KILLED
• TIMES HIT: Counts number of times Clover was hit when scr_debugmode was enabled. So, for most players, 0.
• TIMES MO BROKE THE LAW: Random integer between 100-9000, with a 1/100 chance for another number between 1-100 to be added to it each frame
• Additionally, "#breaktheshufflers" is drawn at y=340 (and invisible as a result).

A background that changes depending on the route the player was on would have faded in; however, the background sprites have been removed, and as the result only a pitch-white room will be shown, rendering the data illegible. 3.5 seconds after the fade in, the OST Kanako (or Greenhouse at 0.3x, if the player was on the Genocide route) will play, but will not loop.

The background image for the Neutral route would later be shown as part of the Undertale Yellow digital artbook. It is an artwork not featured in the game but shown several times in promotional material.⁴

Based on the mention of a "completion cards channel" and the associated text in the artbook, this screen seems to have been used in playtest builds; playtesters were meant to send a screenshot of this screen to the aforementioned channel in a Discord server or the like.

Unused Graphics#

Editor Sprites#

The random encounter generator object (obj_rndenc), despite not being visible, has this sprite regardless. It is a winking face with a red right eye and a pink blush. Perhaps it's the evil cousin of Super Mario Bros. 2‍ '​s smiley faces.

Sans#

Sans' left-facing and up-facing walk cycles exist in the game data alongside three of his talk portraits. His font and sound font are also in the game.

Sans' sprites are part of the WildEast texture group. Developer Spasco originally implemented him as a FUN value event, in which he would be on a seat in the Saloon and would walk off-screen after Clover talked to him.⁵ According to the art book, this was cut due to the implications that his appearance would have had on the game's story.

Ceroba#

Ceroba has two talk portraits that go unused. It's unknown where they would have been used, especially the first one.

Ceroba's Animated Hair#

Ceroba's hair-tails that appear in her Pacifist boss fight have 14 frames of animation showing them waving up and down, but the object's animation speed multiplier gets set to 0 at multiple different points, causing only the first frame to ever render. A shader is instead used to create a more subtle waving effect.

Specifically, the functions gml_Object_obj_ceroba_hair_Create_0, gml_Object_obj_ceroba_hair_Step_0, and gml_Object_obj_ceroba_hair_Other_7 all set image_speed to 0, and the latter two also set image_index (current frame number) to 0. However, attempting to simply remove/change these calls so the animation plays will result in broken graphics in-game, as the shader used for the hair relies on U/V coordinates being passed in, which are normally only set based on the first frame. To fix this, open gml_Object_obj_ceroba_hair_Draw_0 and add the line uvs = sprite_get_uvs(sprite_index, image_index) right before the last shader_set_uniform_f call. Alternatively, the shader can be disabled entirely by removing the first 6 lines of this function.

While the above bug could potentially be reason the animated hair was cut, it's also possible that it was removed simply for looking too silly, especially for the mood at this point in the plot.

Sprites	In-Game

Bartender#

A version of Dina, the bartender in the Wild East, with a pink color pallete. It has been mentioned by developers that this was her original color pallete, but it was changed because it was too clashing with the saloon, and was too similar to the colors of other shopkeepers.

El Bailador Light Cones#

In-game, the light cones on either side of El Bailador are magenta, light green, and dark blue to match the colors of the notes in the battle box. However, light cones colored red, yellow, light blue, and pink can be found in the files. Interestingly, all of these colors combined with the used ones (besides magenta, and missing orange) happen to match up with the colors of the SOULs down to the exact hues. Could SOUL mechanics have been a part of this fight in a previous lifetime?

Coffins#

Just like Undertale, a room containing the coffins of the previous humans who perished in the Underground exists, in the same spot as the original game. Normally, the player can only see five of the coffins, with two more being off-camera. However, inside the file exists a total of eight coffin sprites (stored as frames in spr_coffins), with three of them being absent from normal gameplay: frame 2 containing a purple SOUL coffin, which is not present in the room at all, frame 6 being a duplicate of the blue SOUL coffin (frame 4), and frame 7 being an empty coffin without a colored SOUL and lacking a placard.

Train#

A full train sprite can be found in the game and is placed outside of the player's view in the Wild East. Developer Deadkat made this sprite after misunderstanding he was meant to create a fake train for the trolley problem test, and since it was so detailed, the developers left it out-of-bounds so it could still be included.⁶

Flowey Fight Sprites#

Unused sprites meant for the "intermission" attack (the long one with the hands and the creepy face in the background) of Flowey's first phase. It seems like a different creepy face would've shot "HA!"s at the player at some point.

Two images of the Polygonal portion of Flowey's second phase glitching out. According to developer AeroArtwork, these were intended to show up during the phase randomly and sporadically, but were scrapped because they were found too distracting.

GameMaker Electricity Particle#

A lightning particle texture from GameMaker's particle emitter presets.

Pre-Assembled Monster Battle Sprites#

Assembled sprites of Ceroba, Dalv and Frostermit with lines over them, possibly used to assist in aligning the separate parts of their unassembled sprites.

spr_ceroba_body_battle	sprite678	sprite781

Unused Zenith Martlet Frames#

During the No Mercy route's final boss, there is a section where Martlet takes off and very shortly returns to the battle box as an actual obstacle that the player needs to dodge. The animation of her returning has some unused frames in which her sprite color shifts from blue to white, telegraphing that she herself has become an attack. In-game, however, the first couple frames are skipped entirely.

Uncropped Honeydew Resort Credits Layout#

spr_credits_resort	credits_resort

For some inexplicable reason, an uncropped version of the background used during the scene in the credits where Dalv plays his organ to an audience at Honeydew Resort can be found with the exact same filename as the used background, only without the "spr_" prefix that all other sprites have. Of note is that every object not seen in the small window used in the credits is missing, except for the carpet in front of the bar, oddly enough. (The door is still present because that's a part of the background itself, not a separate sprite.)

Silver Compound#

Code, dialogue, and a sprite for a silver compound in the Chemistry Lab's compound mixing room is left in the game.

Miscellaneous#

A light cone and blob/stone. The light cone is similar in color to the magenta one used in-game, but it's slightly off.

Unused Battles#

MasterSwordRemix#

Objects named obj_masterswordremix_body_a, obj_masterswordremix_sword_a, and obj_masterswordremix_shield_a exist, appearing to be meant for a self-insert of developer MasterSwordRemix. obj_masterswordremix_body_a specifically seems to be based on an early version of obj_crispy_scroll_head_a. This in fact appears to be a removed enemy - there's no trace of associated sprites, dialogue, or other code.

MasterSwordRemix's self insert, Rodney, actually does appear as the keyboard player of the band in Honeydew Resort and UG Apartments' hotel.

Penilla/Crispy Scroll#

A functioning battle against Penilla and Crispy Scroll is in the game, but is only accessible by cheating. This battle was present in the demo, but was cut. It is in the list of obj_debugger battles.

Insomnitot/Know Cone#

Another functioning battle against Insomnitot and Know Cone is in the debug menu but not the unmodified game.

Unused Audio#

To do: Add info from dev streams

Unused Music#

There are a few unused tracks in the mus directory.

dalv_diary.ogg
This track would have been played while reading through Dalv's diary entries; however, the game only tries to stop and not play it.

micro_temperature.ogg
This track was likely intended for the Golden Coffee room in Snowdin. The final game uses "Fun-Sized" instead.

1st Anniversary Livestream Developer Comment: "A track for the Micro Froggit room in Snowdin. Opted to reuse Fun-Sized instead to tighten the OST (aka remove unnecessary filler)."

final_stand.ogg
This track uses Martlet's leitmotifs and is generally more cheerful compared to both of Zenith Martlet's themes.

1st Anniversary Livestream Developer Comment: "The precursor to "Retribution". A fun track, but a bit too peppy for the brutal gauntlet of Martlet's second phase."

prebattle2_yellow.ogg "Enemy Approaching" from Undertale. There is a chance this was intended to replace prebattle1_yellow (which ended up becoming the normal Dark Ruins encounter theme) after the intro Froggit fight.

This is named mus_battle1 in the Demo and is referenced by two enemy objects, however there obj_radio handles battle music instead of enemy objects.

shop.ogg "Shop" from Undertale. While gml_Object_obj_shop_generator_Create_0 defines it as the music of all shops except for the Wild West's gun shop, those shops and the gun shop's Genocide variant always inherit music from the overworld as hardcoded in scr_summon_shop.

danza_attack([01-10]/_finale_[01-02])_yellow.ogg

Attack segments of "DANZA", El Bailador's battle theme, with the orchestra hit notes. Aside from scr_audio_fade_out_battle and scr_audio_stop_sound referencing all of these probably by mistake, danza_attack_01_yellow is only referenced by test song data. Notably, some of the note patterns are slightly different from how it appears in-game.

Early/Alternate Tracks#

cafe_arcade.ogg

A chiptune version of "Café Dune". When the object obj_dunes_35b_dynamic_music is present in rm_dunes_35b, this plays when you approach the arcade machines in the room. There's also a chance this was originally intended for Mew Mew Love Blaster or another arcade game in the Oasis Café.

Duplicate Tracks#

detour.ogg Duplicate of detainment.ogg (Detainment 014).

justice.ogg

A version of gameover_yellow.ogg (Justice) with slight mixing differences. Interestingly, while the Demo's game over theme has the filename mus_justice, it (or at least the metadata) matches the final version.

Unused Sounds#

snd/mail_jingle_alt.ogg This appears to be an alternate version of the jingle heard when interacting with a UGPS sign. For some reason, it takes Doki Doki Literature Club!‍ '​s theme leitmotif; perhaps this was a inside joke of the devs.

snd_talk_sans Sans' talking sound from Undertale.

sndfnt_sans A weird, presumably dummied-out sound, supposed to be for Sans according to the filename. This is a duplicate of "House Guest" in v1.1 of the Demo.

Undertale Customs Leftovers#

These appear to be artifacts of Undertale Customs, another Undertale fangame developed by Yellow programmer mysteryracer before joining the development team.

They are usually guarded under the condition global.game_mode == "customs".

Undertale Items Text#

The check text for every item in Undertale can found in a unused object named obj_text_info_item_selection_pause_screen_overworld.

Mart-2-Go#

Text related to a shop named Mart-2-Go from Customs was copied over to the Yellow game mode. There's no code to set up its items and song in gml_Object_obj_shop_generator_Create_0, and sprites for its background and shopkeeper no longer exist. The shopkeeper, the self-insert of mysteryracer in Customs, notes that you are not supposed to enter this shop upon entrance.

Before buying something

• Uhh... so how did you get here, anyway? [Upon entrance]

Exit

• [Upon exiting]
  • Alright, catch you on the flip side!
  • Peace out!

Oddities#

Sprite Naming#

A number of duplicate sprites exist in the file that don't follow usual sprite naming (spr_). Those include Toriel overworld sprites, her dialog faces, Frostermit sprite in his hidden and regular form, and 2 frames of Kanako's dialog sprite talking with her eyes closed.

Internal Music Names#

While the internal names of music mostly reflect their names in the OST, some have notable differences.

Flowey Background#

To do: A whole section for stuff like this. IDK what to name that so...

The background of the 1st phase of Flowey's boss battle has colors; however, it shows up distorted and in grayscale in-game.

Flowey Dance#

Flowey can be seen dancing during the 1st phase of his boss battle. The sprites are disconnected from his main sprite, and due to opacity effects applied in-game, the sprites are difficult to make out. Here is what they would look like without any filters applied.

Audio#

• All music in the game has a sample rate of 32kHz to presumably save space, except "Main Menu 5 (Steamworks)".
• The following music are embedded in data.win for some reason:
  • Main Menu 5 (Steamworks) (mus_menu_steamworks)
  • AFTERLIFE (mus_f_phase2_gray)
  • Retribution (mus_retribution)
• "AFTERLIFE" is also in mono for some reason.

Music Metadata#

Various music in the game has some metadata leftover such as encoding and date information.

Revisional Differences#

To do: Expand on what was changed. The original release notes aimed to reduce spoilers. Might need to double check the release dates of the patches here.

v1.0.0#

Launch version of the game.

v1.1.0#

The v1.1 patch was released on December 14th for ModDB and December 16th, 2023 for Game Jolt, with the following changes.

Major Changes#

• Added two new accessibility options:
  • Easy mode: When enabled, the Hats now provide DEF values for an easier experience (can be changed mid-playthrough)
  • Auto-Rhythm: When enabled, automatically hits the notes in the El Bailador fight without having to press a button, if you are in the correct lane
• Reworked the final pacifist boss fight attacks to be less chaotic and unpredictable
• During the Axis chase sequence, Axis was supposed to slow down after every time he catches you. It now works as intended
• Added an Act heal to the final No Mercy route boss (Also fixed a crash caused by this Act option being partially implemented but not working)

Crashes and Softlocks#

• Fixed a crash caused by using the “shoo” command twice during a dunebud duo fight
• Fixed a crash caused by interacting with an object while completing the minecart puzzle
• Fixed being able to backtrack on the second steam puzzle, which could lead to a crash and your save being ruined
• Fixed a crash caused by pressing Z on frame 1 of the attack targetting cursor
• Fixed a crash caused by a demo savegame being present on your system
• Fixed a softlock caused by blocking Mo’s path as he leaves
• Fixed a softlock caused by interacting with the crystal tree in Snowdin from the left
• Fixed a softlock in the final pacifist boss fight where the selection soul would disappear from the menu
• Fixed a softlock caused by sending one of the mine carts off-track
• Fixed an issue that caused one of Cactony’s attacks to never end
• Fixed a softlock caused by mashing through the text fast in the raft room in Snowdin
• Fixed a softlock caused by turning around immediately upon entering one of the rooms in the late-game
• Fixed a very specific bug that caused you to get stuck on a bridge upon retrying a fight under it
• Fixed a crash during the Guardener fight that could happen on her last line of dialogue
• Fixed an unrelated softlock in the Guardener fight
• Fixed a crash on the final No Mercy boss caused by getting hit on the same frame as you defeat the boss

Other#

• Fixed the tutorial buttons being swapped in the arcade minigame, also added additional information to the tutorial screen
• Changed the Delta Rune Patch gold requirement from 150 to 100
• Increased the item stock in the final shop of the No Mercy route (Applies on save reset)
• Fixed mistakes in the credits
• Fixed several line breaks, typos, and mistakes in the text
• Altered one of Sir Slither’s act dialogues to make the sparing condition more apparent
• Added an autosave after the final pacifist boss as a temporary help with reported performance issues in this cutscene
• Added the Auto-Fire toggles to the death screen of relevant fights so you don’t need to restart the game and fights to change it
• Fixed a mood-ruining issue in the final act, in which Chujin’s television would glitch out, blue screen, display an image of Chujin, and have blaring sounds during a cutscene; caused by a fun value of 60-62’s event occurring at an unintended time
• Other minor fixes

v1.2.0#

Released on July 15th, 2025.

• The credits were updated and changed, like removing Rylarolls due to her being removed in 2019 on sexual misconduct cases.
• Several typos were fixed.
• There was a fix that attempted to fix the final Pacifist cutscene slowdown.
• A No Mercy checkpoint that was missing was added to the Steamworks area. (The checkpoint would be bugged.)
• Further fixes to the game were made.

v1.2.1#

Released on July 18th, 2025.

• The Fixed No Mercy checkpoint in SteamWorks which was bugged in 1.2 was fixed.

v1.2.2#

Released on November 21st, 2025.

• Shayy's name was removed from the credits after accusations of sexual misconduct and abuse of power by them were released.

References#

Undertale Yellow Wiki