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Undertale/Debug Mode

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This is a sub-page of Undertale.

To do:

A debug mode can be activated by using a hex editor: change 00725B24 (in version 1.0), 00725D8C (in version 1.001), 00725DDC (in version 1.001 Linux), or 007748C4 (in 1.08) to 01 in data.win (Windows versions), game.unx (Linux), or game.ios (Mac OS X). A slow but friendlier way to enable/disable Debug Mode is to download UndertaleModTool, open the aforementioned data file with it, and run the builtin EnableDebug script; this works for almost every version of Undertale.

In the Windows 1.00 version, data.win can be extracted from UNDERTALE.exe with any decent archiving tool like 7-Zip or WinRAR. It's actually a self-extracting .cab containing all the game's resources along with the real executable, which can be run normally from wherever you extract the files to. game.ios can be found in the OS X version by browsing into the Undertale application using "Show Package Contents".

(Note: If you are playing the Steam version, you should extract every file except/delete steam_api.dll, or the debug mode won't be activated.)

(Source: mirrawrs)

Fun Events

When debug mode is enabled, all Gaster Follower encounters will be triggered, as well as Sans's fridge phone call and Alphys' pizza phone call.

Keyboard Shortcuts

Key Effect
G Opens dimensional box.
G (in shop) Sets gold to 5000.
O Opens a normally unused dialog to take items out of a dimensional box.
I Same as above, but to put away.
0-4 Changes the save file slot from 0 to 4. The game utilizes this feature only for temporary saves (to 9), or for the autosave before Photoshop Flowey (8).
S Opens the save dialog.
L Instantly loads your save file.
M Increase murder level by 1.
F Sets game speed to 60 (double normal speed), for the duration of a room. Does not affect sound.
P Toggle room speed between fast and normal.
R When pressed seven times in quick succession, restarts the game.
U Toggles Undyne phone calls. Turns the player blue if enabled, yellow if disabled.
V Shows room collision boxes.
W Slows game down to 10 FPS.
5 Increase battle group by 5. Does not roll over to 0 after 256.
Backspace Increases walk speed when held down.
Pg Up Increase "steps" by 100.
Pg Down Sets AT in the STAT menu to 989. As AT is recalculated every time you enter a battle, this is essentially cosmetic-only.
End Plays the next monster (Flowey if in a genocide run) dialogue from New Home.
Home Starts a battle with the current battle group. Default is 140 (So Sorry) in version 1.0 and 80 (Mettaton) in version 1.001. In later updates the battle group default is 57. Set 009F553C (in version 1.0), 009EB414 (in version 1.001), 009EB918 (in version 1.001 Linux) or 00BD8200 (in version 1.06) in data.win, game.unx or game.ios to change. List of battle groups here. If the battle group is invalid, the last dialogue script is shown in the battle box under the YOU WON! text. The battle ends if you spare, flee, try to use an item, or act. The game crashes if you try to fight. (NOTE: To put the number you want you will need to convert the number you want to hex.) This can also be done with the ChangeHomeBattlegroup community script in UndertaleModTool.
Insert Teleport to the next room in the room list. Places you in a specific position.
Del Teleport to the previous room.
F3 Places a file system_information_962 in your save folder. After game restart, puts you at the black screen after a genocide run.
F7 Adds 500 gold.
F9 Stops all sounds and music currently playing.
F10 Toggle collision with walls.
F11 Teleport to the room after Papyrus's boss fight (room_water1/idː82).
F12 Restarts room. Also sets the game to act as if you're at the True Pacifist ending.
R + 6 Teleport to Ruins tile puzzle room (room_ruins2/id:7).
T + 6 Teleport to outside of Ruins exit (room_tundra1/id:44).
W + 6 Same as F11.
F + 6 Teleport to Hotlands water cooler room (room_fire2/id:138).
E + 6 Teleport to New Home entrance (room_castle_front/id:219).
Y + 6 Teleport to True Lab entrance (room_truelab_elevator/id:244).
(0-4 functionality: tanwave)

Battle Shortcuts

Key Effect
1 Halves game speed. Cannot go below 1 FPS, or game crashes.
2 Doubles game speed. Cannot go past 2x normal game speed. Crashes game if pressed too many times.
8 Shortens current enemy turn.
9 Extends current enemy turn.
Space Shortens current enemy turn to 2 frames.
F Sets game speed to double normal speed.
W Slows game down to 10 FPS.
Delete Sets HP to 999 for current fight.
F6 Sets mercy to 0 and attack to 9999.
O + P Sets mercy to 9999, enabling the spare option instantly. Some enemies that cannot be normally spared turn into Aaron.

(Source: mirrawrs (pcy.ulyssis.be), Dobby233Liu)

Debug Flags

To do:
Which dialogue?
Flag Line in savefile Description
4 5 If set to 1, before Undyne's fight, her monologue will act as if you've killed no monsters, even if you've killed some.
26 57 Overrides murder level when set to a non-zero value.
28 59 If set to 1, holding C in dialogues will skip through them quickly.
69 100 If set to 1, changes Burgerpants' dialogue slightly.

(Source: mirrawrs (pcy.ulyssis.be))

Area Specific

First Sans Encounter

Press J to zoom in on Sans, make a rimshot sound, and crash the game.

Alternate Reflection

Toby Fox messing with us.

The reflections of yourself in the puddles in Waterfall are replaced with a sprite of an unknown character when Space is pressed. According to the game's files, the character seems to be Chara, but they also look like Kris from Deltarune. The related variable in the code that controls showing this alternate reflection is called death, which may indicate that this was once used after you sold Chara your soul.

Monster Kid Climb

Maybe next time, we'll try a relaxing shade of green.

Two numbers will appear over the player's head, starting at -1 and 0. The first number indicates the value an alarm is at (which alarm is determined by the second number), while the second number changes the first one and indicates Monster Kid's state.

Fight Specific

Lesser Dog

Key Effect
J Increases head height.

Greater Dog

Key Effect
Space Changes the "ignore" value to 3, making the enemy act as if it has been ignored 3 times.
(Source: Original TCRF research)


Key Effect
Space Sets Papyrus's next attack and dialogue to "THE KING WILL TRIM A HEDGE IN THE SHAPE OF MY SMILE!!!"
O Decreases the turn counter by 1.
P Increases the turn counter by 1.
(Source: Dobby233Liu, OceanBagel)


Key Effect
3 Increases the emotion variable, used during the fight to affect her status.

(Source: /u/mirrawrs)

Mad Dummy


A red number starting from 300 appears attached to the right of the Mad Dummy, indicating its health. Pressing A will set the number to 180, make the dummy float around, and make its next attack the "FINAL ATTACK!"


Note: Undyne cannot be spared with the key combination O+P.

To do:
Pressing Space + direction keys while the distortion effect is on does some stuff.
Key Effect
7 Sets the player's HP to 999.
A Decreases turn counter by 1.
U Sets Undyne's HP to 2.
(Source: Original TCRF research)


I think it's a metaphor for his impatience.

A turn counter counts up from 0 in the top left corner of the screen. The higher the counter is, the harder Asgore's attacks are and the less defense Asgore has. Press A to decrease, press S to increase. If above 20, it is set back to 20 each time Asgore does his orange-blue attack. If set manually to above 22 or below 1, crashes the game when Asgore attacks.


I think he's enjoying it.

The text "debug" in yellow comes down on the right side of the screen alongside Flowey. The intro and game over screens are skipped after the first death.

Key Effect
Home Attack Flowey instantly. Does 6000 damage if all soul indicators are darkened.
Pg Up Darkens the soul indicators in order, increasing damage and affecting the next soul battle. Disables darkening when all are darkened, and the next time a soul battle occurs, the game will crash.
Right Shift, Left Shift, X Darkens the soul indicators in order, increasing damage and affecting the next soul battle.
Q Starts the battle instantly during the intro. Does not stop the intro, so you cannot move and the attacks miss you until Flowey is closer.
W Slows game down to 10 FPS. If used in Flowey's death state, starts the attack after Flowey's health is restored.
E One burst of three stars from each eye.
R One burst of seven stars from each eye.
I Makes Flowey laugh and restart his attacks, including in a soul battle.
O Purple Soul fight (books)
P Green Soul fight (frying pan)
A Toggle flamethrower
S Saves battle state. Shows "FILE 2 SAVED" at bottom left corner.
F Doubles game speed. Lock on pellet attack.
G Orange Soul fight (glove)
H One hand-gun attack
J Yellow Soul fight (gun)
K Aqua Soul fight (knives)
L Loads battle state. Shows "FILE 2 LOADED" at bottom left corner. If there is no save, crashes game.
C Toggles Flowey's mouth flap waving around.
V Single vine attack.
N Single F-Bomb.
M Ball of teeth/dentata attack.
Shift Deactivates souls one at a time.

(Source: /u/Nox13last)


Key Effect
C Toggles Asriel's floating around and staying still above the middle of the screen.
Space Same as above, but closer to the top. Asriel must be still for any of the below keys to work. Attacks stack and start instantly.
G Chaos Buster/Chaos Blaster
B Shocker Breaker/Shocker Breaker II
W Chaos Saber/Chaos Slicer, slows the game down to 10 FPS.
S Star Blazing/Galacta Blazing

Undyne the Undying

Note: Undyne cannot be spared with the key combination O+P.

Key Effect
7 Sets HP to 999.
A Decreases turn counter by 1.


Key Effect
E Sets Sans' next attack to his special attack.
Q Sets Sans' next attack to the part where he spares you.

(Source: /u/sheik999)


Undertale Logo Before Entering Snowdin

For some reason, setting global.debug to 999 allows you to rotate the Undertale logo with the Left and Right keys if you didn't kill any monster, like in the Demo.

(Source: Original TCRF research)