Cleanroom relauncher unofficial
This unofficial relauncher lets players start Cleanroom Forge for 1.12.2 without a separate installation, through MMC-based launchers or other launchers.
What it does
The mod handles the Cleanroom install and launch steps on the client. When an MMC-based launcher runs it, Cleanroom is installed automatically. Other launchers can start Cleanroom through the same mod. Java 21 must be installed for the mod to run.
A config file controls how the launch behaves. Options include using a local Cleanroom MMC pack, letting modpacks ship a default Cleanroom version and arguments, setting a proxy for MMC zip and library fetching, and choosing a custom library path so existing launchers can reuse libraries. A modpack can also ship its own default Cleanroom MMC pack, which does not get updated. If something goes wrong, deleting the config file and trying again resets the settings.
The mod also adds certificates to an old JVM when one is detected, and it adds the whole keystore for the latest Java. The author notes that updating the JVM is the preferred fix, and that Java 8u51 can cause a deadlock during multi-threaded fetching. The mod is open source under the MIT License and is published for Forge 1.12.2, with the latest file 0.8.4 for 1.12.2 dated 2026-10-10 on the beta channel.
Key features
Launch without installation
Cleanroom Forge starts without a separate installation step on the player's machine.
Automatic install through MMC launchers
MultiMC, PolyMC and PrismLauncher install Cleanroom automatically when the mod is used through them.
Launcher options in a config file
A config file sets a local Cleanroom MMC pack, default version and arguments, a proxy, and a custom library path.
Certificate and keystore handling
The mod adds certificates to an old JVM when one is detected, and adds the whole keystore for the latest Java.
Compatibility
Latest build for each game version and loader.
Only full game releases are shown. This mod publishes no snapshot builds.
Use cases
Reusing libraries
A custom library path lets the launch reuse libraries that existing launchers already fetched.
Shipping a modpack default
A modpack can ship a default Cleanroom version, arguments or MMC pack, so players start with the intended setup.
Who it’s for: Players who launch Cleanroom Forge for 1.12.2 through MultiMC, PolyMC, PrismLauncher or another launcher, and modpack authors who want to set a default Cleanroom version or pack.
Questions
Does this unofficial relauncher need Java 21?
Does the server need this mod?
What should be done if the launch stops working?
More from kappa-maintainer
What the author says
Official launcher ships with outdated Java 8u51 (released 2015), which doesn't contain certificates of later Let's Encrypt (used by Cleanroom maven) and Azure (used by libraries.minecraft.net). The mod will add those certs into JVM if old java detected. In fact, I added whole keystore from latest Java.
Official listing
- Modrinth page
- modrinth.com/mod/cleanroom-relauncher-unofficial
- Owner
- kappa-maintainer
- Published
- 2025-06-06
- Updated on Modrinth
- 2026-10-10
- Facts verified
- 2026-10-15