Deltarune

This page details bugs of Deltarune.

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To do: There is a lot to unpack here. Deltarune is quite a strangely coded game, not maybe quite as much as its parallel story Undertale, but enough to have a lot of dubious coding practices that lead to a lot of bugs if you know where to look. I ask that maybe we hold off on documenting bugs for Chapter 5 until the Deltarune team has stopped rolling out patches for that chapter. I wanted to write about the "Ralsei in the Flower Kingdom Early" bug but it got patched while I was sleeping... On that note, preferably only document bugs that affect current builds of the game for now.

Chapter 1#

Jevil ShadowCrystal Chapter Select Bug#

To do: I know for a fact that this bug does not affect Chapter 3 or 4 as for the former, the chapter literally ends with the secret boss so someone would've caught it by now, and as for the latter, I recently tested it. Chapter 5 is quite recent so I haven't tested it yet. Does anyone have any insight as to whether this occurs in Chapter 2?

If the player defeats Jevil in Chapter 1 by either means of mercy or violence, and then completes the chapter by defeating King, sealing the Dark Fountain and returning home without ever saving their game, the savefile will not be correctly awarded a ShadowCrystal for the file in the Chapter Select screen. The game is supposed to track which ShadowCrystals have been obtained in which chapter save slots, with it correctly handling awarding ShadowCrystals for the other chapters, even if the player doesn't save their game at any point after collecting the ShadowCrystal and completing the chapter.
This occurs because the function responsible for handling whether the player should be awarded a ShadowCrystal in the Chapter Select screen, scr_store_ura_result, is not ran when completion data is created and is only ran when the player saves or when a savefile is created. This bug only occurs on Chapter 1 on various patches of the game, with it being resolved in Chapter 2, 3, 4 and 5.

Chapter 2#

Super Mario World Screen Transition Crash#

To do: Does this occur on the console versions?

Doing something that causes the Deltarune application to temporarily lose focus, such as pressing Alt + Tab ↹ or changing the Fullscreen settings with F4 on Windows for example, during Queen's laughing animation at the end of the Cyber City section of Chapter 2 as the screen transition occurs, will cause the game to crash with the following error:

##############
ERROR in
action number 1
of Step Event2
for object obj_circlezoom:
Trying to set a surface target that does not exist.
at gml_Object_obj_circlezoom_Step_2
##############
gml_Object_obj_circlezoom_Step_2 (line -1)

This occurs because of how the object responsible for the transition, obj_circlezoom, handles the transition. It uses a Surface, a type of texture in GameMaker that can be drawn to like a canvas. Notably, surfaces are volatile, and are freed from memory when the program loses focus. The crash occurs because obj_circlezoom does not have a failsafe for if the surface it is trying to access doesn't exist.
This crash currently impacts every Windows version of the full game, as well as the demo.

Spamton NEO Big Shot Spamming#

Watch on YouTube

To do: Was the most straightforward and simple video I could find demonstrating the bug, but there should be a better one

Pressing and releasing Z (or whatever key "Confirm" is bound to) and holding ↵ Return (vice versa also works) at the same time on a keyboard during Spamton NEO's bossfight will cause Big Shots to be fired in quick succession, greatly reducing the difficulty of Spamton NEO's bullet patterns. This also works with a controller, via A (or whatever key "Confirm" is bound to) and Up.

As of v1.06 of Chapter 2, this bug was addressed, not with a patch, but rather turned into an intended feature. In Spamton NEO's enemy object, several variables labelled as funnycheat track how many times the player has spammed Big Shots using this glitch, When the variable funnycheat becomes greater than 5, Spamton will become enraged (visually indicated by his face turning red from anger followed by a car horn sound effect), causing the fight to slightly increase in difficulty.

It is also worth noting that this glitch can not be accessed on console versions of the game, only the PC versions.

Chapter 3#

Invincibility Inconsistency#

In Chapter 3, the invincibility frames between hits are inexplicably 10 frames less compared to the other chapters. As Deltarune runs at 30 frames per second, this means that in Chapter 3 you have 0.3 seconds less leniency when getting hit.

Chapter 3	Chapter 5, for example

TV World Particle Bug#

In battles that take place in backstage rooms of TV World, particularly a specific fight against two Ribbicks, the objects responsible for creating particles around the overhead lights in the overworld are not properly deactivated or hidden during battle, which allows the particles to continuously spawn and be visible during battle.

Screenshot	Particle circled

"BONUS ZONE" Text Layering Bug#

If the player enters the secret Bonus Zone room in TV World, and then attempts to use the Cell Phone key item after entering, the "BONUS ZONE" text in the room will erroneously display over the textbox, which is likely the result of a layering bug.

BreathAXl ACT#

If the player has the TripTicket in their inventory, started any encounters with Ribbick in them, and has gotten the Rabbick easter egg after getting the enemies to 100% Mercy using the CroakOn/CroakOnX ACTs, the red X that's intended to appear for Ribbick's CroakOnX ACT will also be visible in Rabbick's ACT menu as well, overlapping with the "BreathAll" ACT text.

Chapter 4#

To do: Multiple bugs need to be documented here, including but not limited to: Balthizard's max recruit count is intended to be 4 according to scr_recruit_info, but is shown to be 5 in battle. (This is due to the enemy's Other_20 script being overwritten with old draw code, causing it to not fetch its parent object's own Other_20 script.) A now-patched debug leftover in Chapter 4 involving the money fountain in the Dark and Second Sanctuaries, where pressing P caused the waterfall to split open. Document this here and figure out when this was patched. Hammer of Justice's laughing sound effect sometimes doesn't play, likely due to a timer variable in his enemy object's code not being reset properly.

BlueRibbon Treated as Secret Boss Reward#

Chapter 4 has a rather nasty bug related to the BlueRibbon armor item that can be won from Chapter 3's Ball Machine as a gold prize. This bug is best explained by first explaining how Castle Town's cliff hole works.
To the west of Castle Town, at where the player first falls down into Castle Town in Chapter 1 after escaping Lancer in the Cliffs, there is a small hole. Interacting with it, depending on the save file slot the game is being played on, can give the player the reward items for the Secret Bosses if the game detects completion data suggesting that the player has beaten the bosses before. The key problem that causes this bug to occur is an oversight with how the rewards for the first two secret bosses are handled, extending to every other secret boss. Jevil and Spamton NEO, the game's first two secret bosses, drop either a Weapon or an Armor depending on if they were beaten with violence or pacifism respectively. To make coding simpler, their Weapon and Armor drop pairings share an ID (for example, DevilsKnife has a Weapon ID of 7 and JevilsTail has an Armor ID of 7). Due to this, the cliff hole was programmed to only need to remember one ID. However, this causes an issue with the Knight, Chapter 3's secret boss. The Knight can only ever be beaten with violence and drop a BlackShard, which has a Weapon ID of 26. Yet, the game will still check for the presence of an Armor with ID 26 in the player's inventory, as to prevent duplication, which just so happens to be BlueRibbon. What this means is that if the player has BlueRibbon in their inventory, they cannot get the BlackShard from the cliff hole.
This bug also has the effect of BlueRibbon having the properties of a Secret Boss reward, such as not being able to be thrown away.
This bug was patched in Chapter 5 v0.0.253 on July 31st 2026. The bug was not present in any versions of Chapter 5, but in this patch, the fixes were backported to Chapter 4.

Balthizard Mercy Number Bug#

When using the ACT "ShakeX" against a Balthizard, if there is a healing number belonging to a party member on screen as Balthizard attempts to create the number to show Mercy gained from the ShakeX ACT, it will instead increment the healing number instead of creating its own. This is purely visual and does not actually affect the amount of healing the party member receives. This happens because in the code for obj_balthizard_enemy, there is not enough proper error handling for using a proper instance of obj_writer, the instance which is also used for displaying healing numbers.

Susie and Susie#

When using the Dual Heal ACT in the Titan Spawn and Titan battles, the text that appears when the spell has been cast will erroneously display as "* Susie and Susie cast DUAL HEAL!".
The reason for why this happens is due to how the string for this line is set up in scr_spelltext. In the script, the line is set up as "* ~1 and Susie cast DUAL HEAL!/%", with "~1" being replaced by global.charname[2], which fetches Susie's name. global.charname[2] should instead be replaced with global.charname[3], which would fetch Ralsei's name.