For those who want to actually use Quack Prep in their own game, here is the technical breakdown.
Undertale’s engine (GameMaker Studio) uses a global random number generator that advances with every single action:
Quack Prep exploits the fact that certain RNG seeds produce "safe" patterns. The "quack" sequence—typically Up, Down, Left, Right, Up, Up, Menu, Cancel, Left, Right, Menu, Cancel, Down repeated in a specific rhythm—advances the RNG by a fixed number of steps. By performing this sequence in a safe room (like the hallway before the Sans fight, or the bridge before Undyne), you align the RNG to a known favorable seed.
In Undertale fan circles, "quack prep" is a joking/misheard phrase from Mettaton's quiz show (the "Button" segment). The actual line is something like: "Correct! Prepare for..." but fans often mishear it as "Quack prep!" — especially since Mettaton's EX form has a duck-themed transformation (legs → duck feet).
| Step | Action | Purpose | |------|---------------------|-------------------------------------------| | 1 | Press Up (x2) | Advance RNG by 2 frames | | 2 | Press Left + Right | Create a "null move" that doesn't move you| | 3 | Open Menu (ESC) | Force a state change | | 4 | Navigate to ITEM | Seed manipulation | | 5 | Cancel out (ESC x2) | Reset input buffer | | 6 | Rapidly tap Z (x4) | The "quack" itself (mimics duck call) | quack prep undertale
Do this exactly three times. Then, walk into the boss arena. The first attack will often be a predictable pattern, giving you precious extra frames to react.
Note: Quack Prep does not work on console versions of Undertale (Switch, PS4, Xbox) due to differences in the RNG implementation. It is exclusive to the PC version.
Deep within the lore of Undertale, if one delves into the game's text dump, is the character Gaster. W.D. Gaster is the game’s most enigmatic figure—a skeleton scattered across time and space, accessible only through hidden "FUN" values and debug modes. His dialogue is usually garbled, his entries in the True Lab written in a strange font.
However, the infamous Vietnamese guidebook had a different interpretation. When describing Gaster—or perhaps misinterpreting a line of code or a sprite—the guidebook referred to him not as a brilliant scientist, but as "Quack Prep." For those who want to actually use Quack
The name is absurd. It strips the spooky, glitched aura from Gaster and replaces it with the image of a duck in a chef’s hat. Perhaps he is a duck being prepared for dinner? Perhaps he is a chef who specializes in duck? The ambiguity only fueled the fire.
To understand Quack Prep, you need to go back to early Undertale speedrunning (circa 2016-2017). Runners noticed that Undertale’s RNG—which governs enemy attacks, item drops, and dodge patterns—was not truly random. It was deterministic based on the number of frames since the game booted up and the sequence of player inputs.
One particular runner (known by the pseudonym "Duckspeaker" in old forums) discovered that by inputting a frantic, specific string of directional taps and menu opens before a fight, they could force Sans to use his easiest attack patterns first. When asked to explain their “preparation method” in a Twitch chat, they jokingly typed: "I just quack at the game until it behaves."
The name stuck. Quack Prep evolved from a joke into a formalized tech. Today, you will find it referenced in: Quack Prep exploits the fact that certain RNG
The keyword "quack prep undertale" can be deconstructed into two parts:
Thus, Quack Prep is the act of performing a precise sequence of seemingly random inputs (often on the keyboard or controller) during a safe moment in Undertale to “prepare” the game’s internal logic for a favorable outcome in an upcoming battle. It is most famously used in Genocide Route runs against bosses like Undyne the Undying and Sans.
Although Mettaton NEO is a one-hit kill for the player (he dies instantly), advanced Quack Prep is used in "Lowest Possible LV" runs to manipulate his pre-battle dialogue, skipping a 10-second unskippable cutscene.