Cheats & Secrets - Undertale

Cheats & Secrets Guide

Overview

Undertale has no traditional cheat codes (e.g., invincibility, level skips) that you can input via a keyboard or menu. However, the game is packed with developer-intended secrets, hidden events, and file-based modifications that act as "cheats" for experienced players. This guide covers all known safe secrets, Easter eggs, and the powerful FUN value mechanic.

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1. The FUN Value

The FUN value is a hidden variable (0–100) that controls rare events. It is randomized per save file but can be manually set via file editing (PC only). Each FUN value triggers unique encounters or dialogue. Below are known values and their events:

FUN Value	Event

| 1–100 | Random chance for various encounters; higher values increase rarity.
| 56 | The Man behind the tree appears in Snowdin Forest (grey door event).
| 61 | The Wrong Number Song phone call from "Dad" in the Napstablook fight.
| 66 | The Gaster Follower in the grey door room (Western room).
| 90–100 | Increased chance for Gaster Follower encounters overall.
| 100 | Triggers "Gaster's Theme" audio during the grey door sequence.

#### How to Check Your FUN Value (PC Only)
1. Navigate to `%LOCALAPPDATA%\UNDERTALE` (Windows) or `~/Library/Application Support/UNDERTALE` (Mac).
2. Open `file0` or `undertale.ini` with a text editor (e.g., Notepad).
3. Find the line `FUN=`. If missing, add exactly `FUN=56` for the Man behind the tree (most famous secret).
4. Save the file and relaunch the game.

> Warning: Editing save files can corrupt your progress. Always back up `file0` first.

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2. File Editing (PC Only)

Beyond the FUN value, you can modify other flags for specific outcomes:

Set `LOVE=0`, `HP=20` to simulate a fresh start (not recommended for story integrity).

Set `route=1` to force Genocide Route flags (used for speedrunning).

Set `true_pacifist=1` to unlock the True Pacifist ending without requirements (triggers final fight skip).

3. Hidden Rooms & Locations

Requirements: FUN = 66 or 56 (or high FUN), after completing Snowdin Forest.

Location: In Snowdin Forest, after the bridge with the sentry station, go west until you see a grey door (normally closed). It opens if FUN > 50 and you have not killed Snowdrake.

Contents: A room with three shadow creatures (Gaster Followers). Interact with them for cryptic lore about W.D. Gaster.

Requirements: No kills; be friends with Napstablook.

Location: In MTT Resort after the elevator, enter the house with the crying ghost. Go upstairs and walk through the wall to the right of the bed.

Contents: A TV playing a video of Sans telling a knock-knock joke. Also a hidden key item: "Mystery Key" (unlocks the dog shrine).

Location: In the snowed area of Snowdin (near the inn), there is a dog statue. Use the Mystery Key from MTT Resort to unlock it.

Contents: A random dog (Annoying Dog) spawns and gives you the Annoying Dog key item (used in the True Lab for a secret scene).

4. Easter Eggs & Developer Messages

Trigger: FUN value between 90–100, or specific file editing.

Effect: During the Napstablook fight, when you SAVE him, instead of the normal phone call, you hear a voice saying "Hello?". This is a reference to an early version of Toby Fox's song.

Trigger: FUN = 56, after Snowdin Forest (before the trash zone).

Effect: In a room with a large tree, examine the tree to see a shadowy figure (W.D. Gaster). He says one of several cryptic phrases before vanishing.

Trigger: Call Papyrus on the phone multiple times during the Sans fight in Genocide Route.

Effect: Sans will sometimes joke about the calls. This is very rare and requires over 100 calls.

Location: Waterfall area, near the gap that the bird normally carries you across.

Secret: If you do not call the bird (by not interacting with the phone), the bird will eventually appear anyway after 30 seconds of waiting.

Trigger: During Sans fight, if you have the FUN value set to 100, Sans' Gaster Blasters will occasionally say "Gaster" in a distorted voice (file name: `gasterblast.wav`).

5. Secret Bosses

Undertale has no hidden optional bosses besides the mandatory ones (Toriel, Papyrus, Sans, etc.) — but two fights have major secrets:

Secret: Sans' fight is almost impossible to beat without exploiting his pattern. There is no skip or cheat to win instantly, but you can use the "Mercy" exploit (see Section 6).

Secret: During the final fight, if you lose all hope (die repeatedly), a secret dialogue triggers where Flowey comments on your determination.

Exploit: You can skip the fight entirely by editing `true_pacifist=1` in `file0` (see above).

6. Exploits & Glitches (Safe & Intended)

How: In any battle, select Mercy > Action > Check then Mercy > Spare repeatedly. This can restore HP in some fights if the enemy is scripted to spare you after a check (e.g., Mettaton EX).

Note: This is borderline a glitch. It works only in specific scripted encounters.

Description: During the Sans fight, if you select Mercy > Spare at the very start before Sans attacks, the fight will end immediately (Sans laughs and skips the battle). This is a deliberate joke by Toby Fox.

Trigger: After buying the Annoying Dog item from the Dog Shrine, return to any room with snow. The dog will appear and steal your item. Not a cheat, but an Easter egg.

Description: If you repeatedly die to a boss (e.g., Undyne the Undying), the game will eventually offer to reduce the difficulty (enemies deal half damage). This is a built-in assist, not a cheat.

7. Other Notable Secrets

The Memoryhead Room (Waterfall): After the piano puzzle, enter the dark room and examine the coffin. If you have the Annoying Dog key item, a head appears with a memory.

Temmie Village: Sell your refrigerator and other items to Temmie for coins; then buy the Temmie Armor for 999G. This armor reduces enemy damage to 1, making it the best defensive item in the game (but it costs a lot).

Sans' Workshop: After True Pacifist, check the basement of Sans and Papyrus's house. It contains blueprints for the Sans' Room seen during Genocide.

The Grey Door Wrong: If you enter the grey door room with FUN exactly 100, the door will not open; instead, a text reads "(The door is locked.)"

8. Ethical Considerations

No Console Commands: Undertale has no built-in developer console. All "cheats" require external file editing.

Genocide Route: Exploiting file editing to skip Genocide fights defeats the purpose of the game's moral choices. Use only for testing.

Safe Backups: Always backup your `file0` before editing.

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For a deeper dive, refer to the Undertale Wiki and The Cutting Room Floor for unused content and beta secrets.