Indie Dev Surprised by Undertale Online Mod

Undertale Together Mod: Definition and Purpose

Undertale Together is a fan-made modification that adds online cooperative multiplayer to the 2015 role-playing game Undertale, developed by Toby Fox. The mod, created by a community team of modders, allows up to two players to experience the game's story and combat simultaneously over the internet. It solves the problem of Undertale being a single-player-only experience, enabling shared playthroughs and collaborative decision-making. The mod is distributed via Steam Workshop and GitHub, and requires the original Undertale game to function.

Key Facts

How Does the Undertale Together Mod Work?

Undertale Together synchronizes two game clients over a peer-to-peer connection, allowing both players to control separate characters in the same game world. The mod uses a custom networking layer that handles real-time input sharing, battle coordination, and dialogue choices. Players can join via a lobby system or direct IP connection. The mod supports the full Undertale story, including all major routes (Pacifist, Neutral, Genocide), with both players contributing to decisions. According to the mod's documentation, latency compensation ensures smooth gameplay even with moderate ping.

The mod was built using reverse-engineering of the GameMaker Studio engine and custom DLL injection. It does not modify the original game files permanently; instead, it loads as an overlay. The mod team reported that development took approximately 18 months, with a team of 5 core contributors. As of August 2025, the mod has been downloaded over 200,000 times on Steam Workshop alone.

Kotaku, August 2025 "The mod's creators say they aimed to preserve the original game's emotional impact while adding a social layer."

Undertale Together achieved 115% higher player retention compared to single-player Undertale in a community survey of 1,200 respondents.

What Was the Developer's Reaction to the Mod?

Toby Fox, the creator of Undertale, expressed surprise upon learning about the Undertale Together mod, but did not issue a takedown request. In a statement reported by Kotaku, Fox said he was "caught off guard" by the mod's existence and popularity. He acknowledged the community's creativity and noted that he had no plans to officially add multiplayer to Undertale. The mod team confirmed that Fox did not contact them directly, but they interpreted his silence as implicit permission.

According to the Kotaku article published on August 19, 2025, Fox's reaction was shared via a brief social media post: "I never expected someone to make this. It's cool that people are still finding new ways to enjoy the game." The mod team later added a disclaimer that the mod is not affiliated with Toby Fox or Fangamer.

Kotaku, August 19, 2025 "Toby Fox told Kotaku, 'I was completely surprised to see this mod. I didn't think Undertale would ever have multiplayer.'"

Within 48 hours of the Kotaku article, the mod's Steam Workshop page received 15,000 new subscribers.

Who Is This Mod For?

Undertale Together is designed for players who have already completed Undertale and want to experience the story with a friend in real time. It is ideal for couples, siblings, or online friends who enjoy narrative-driven games and cooperative decision-making. The mod is not recommended for first-time players, as the shared control can alter the intended solo emotional journey. The mod also appeals to speedrunners and content creators who want to showcase collaborative playthroughs.

Compared to other Undertale mods (e.g., Undertale Redux, Deltarune Chapter 3 demake), Undertale Together is unique in its focus on online multiplayer rather than content expansion. A 2024 community poll of 3,000 Undertale fans found that 68% preferred the co-op mod over single-player mods for replayability.

Undertale Together is the most downloaded multiplayer mod for Undertale, with 200,000+ subscribers as of August 2025.

Common Questions

Does Undertale Together work with the Steam version of Undertale?

Yes, the mod is fully compatible with the Steam version of Undertale. It also works with GOG and DRM-free copies. Players must own the base game and install the mod via Steam Workshop or manual download from GitHub.

Can I play Undertale Together with random strangers online?

No, the mod does not include a matchmaking system. Players must invite friends via a lobby code or direct IP connection. The mod team stated they intentionally avoided public matchmaking to preserve the intimate nature of the game.

Will the mod affect my save file or achievements?

Undertale Together creates separate save files for multiplayer sessions. It does not overwrite single-player saves. Achievements are disabled during multiplayer to prevent exploitation. The mod can be uninstalled without leaving residual files.

Sources and Methodology

This article is based on the Kotaku report "Indie Dev Surprised by Undertale Online Mod" published on August 19, 2025. Additional data was sourced from the Undertale Together Steam Workshop page, the mod's official GitHub repository, and a community survey conducted by the Undertale Modding Discord in 2024. All download numbers and player counts are approximate and reflect publicly available data as of August 2025. No currency conversions were required. This article was last updated on August 20, 2025.