The Undertale Demo is extremely brief, buggy, and less detailed compared to the final game. It begins when the protagonist first falls into the Ruins and ends with Flowey's judgment right after the encounter with Toriel.
Flowey's dialogue varies depending on the route taken.
- Pacifist Route: Flowey says the same things that he says on a Pacifist Route in the full game. However, Sans and Papyrus makes an appearance on the credits screen.
- Neutral Route: Flowey says a variety of different things depending on what actions were taken, which are the same in the main game. These are detailed on Flowey's page.
- Genocide Route: Flowey says the same things that he says on a Genocide Route in the full game. The credits screen displays red text, which simply reads "That was fun. Let's finish the job."
The Undertale Demo comes with a manual, which details the mechanics of the game and some of the characters. The manual changes depending on the actions taken throughout the demo:
- Killed Toriel: The final page of the manual displays a picture of Flowey grinning, and the message "Hahaha... You didn't even try to spare her. You murderer."
- Spared Toriel: Flowey is absent from the final page, and it simply says "Don't get too cocky."
- Genocide Route: All pages except the last and the first change to the Undertale logo with the words "That was fun. Let's finish the job." The first page remains normal, and the last page displays a faceless Flowey with no dialogue.
- The demo of the game is titled UnderTale rather than UNDERTALE.
- Toby Fox has mentioned that this capitalization of the game is still canonical.
- The manual was removed from the final version of the game; partly for being too complex to work with, and partly because the version of GameMaker Studio used in the final has more restrictions on editing the external files.
- All names can be used in this version, even names that are forbidden in the final version of the game.
- In the final game, if Toriel is killed, and then the protagonist goes to sleep, two messages are displayed. In the demo, only one of the two appears.
- If the protagonist returns to the starting point of the game, Toriel is standing by the golden flowers where the protagonist is first seen. She asks the protagonist not to worry about her, and say that someone must watch the flowers. This happens even if Toriel has died in her battle.
- Alt-Tabbing while moving has no effect on enemy encounters or walk animation in the Demo, unlike the final product.
- When decompiled, the demo contains a hidden message from Toby Fox. It asks for those who decompiled the demo not to spoil anything and not to modify the code until after it is released.
- If all the monsters for the Genocide ending are killed, but the Vegetoids in the room where "there is only one switch" are continually killed, the game returns to a Neutral Route.
- By killing these Vegetoids, it is possible to increase the LV stat over what is usually allowed.
- The first screen of the demo's introduction has a Loox instead of a Boss Monster. It also makes an appearance in the Neutral Route mock intro in the final version before the Photoshop Flowey battle.
- Pressing "C" while the "!" monster encounter icon pops up prevents the encounter from happening.
- On the Genocide route, when the kill counter is exhausted, Your Best Friend plays in place of the song found in the final game.
- Should one edit the SAVE file in the game files, it shows Room 271 (TESTROOM) instead of Room 326 (room_of_dog).
- Monsters which may be spared do not use yellow text.
- The game, if windowed, may not be closed by pressing the operating system's standard close button.
- The currency in the demo is GILD, rather than GOLD as in the main game.
- ↑ "The demo is out." - UnderTale. May 23, 2013. Twitter.
- ↑ "the demo is out now!" - Toby Fox. May 23, 2013. Twitter.
- ↑ "things:
1. encounter rate is higher in the demo + first-ish area
2. most rooms only have 1 encounter
3. can flee from them" - Toby Fox. September 20, 2015. Twitter.
- ↑ "Every capitalization is canon. Undertale. UnderTale. UNDERTALE" - Toby Fox. January 14, 2015. Twitter.