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NeoForge 1.21.9 incompatability #5670
Description
Operating System
Windows
Description of bug
On MultiMC V 0.7.0-4235 win32, I'm attempting to set up a NeoForge instance, but I get the following error:
Using onesix launcher.
Failed to start Minecraft:
java.lang.NoClassDefFoundError: cpw/mods/modlauncher/Launcher
Steps to reproduce
Create an instance named "1.21.9 w NeoForge"
Edit Instance on the Version panel
Note: "Minecraft version 1.21.9"
Install NeoForge v. 21.9.9-beta
Note: Version 21.9.0-beta through 21.9.9-beta do not appear to work
Launch
Suspected cause
I'm guessing NeoForge altered a class that causes cpw.mods.modlauncher.Launcher to freak out.
I'm happy to supply detailed log info if this can not be reproduced or the cause appears unknown.
This issue is unique
- I have searched the issue tracker and did not find an issue describing my bug.
EDIT
I should note the above actions cause no issues with MC+NeoForge for 1.21.8, and the NeoForge change log https://neoforged.net/news/21.9release does suggest major changes that may be causing problems for the launcher.
Activity
phit commentedon Oct 3, 2025
PrismLauncher/ForgeWrapper#9
Lordingard commentedon Oct 4, 2025
Resolved now, need to be implemented in MultiMC ;-)
GameLord2011 commentedon Oct 10, 2025
Yeah... that'll (probably) never happen. Prism is under a licensce that the maintainer (not pinging him) doesnt like, and it's a sharealike liscence, which means that if he wants any code from there, he will need to reliscence this project.
phit commentedon Oct 10, 2025
ForgeWrapper is using MIT license, so that doesn't apply here.
GameLord2011 commentedon Oct 10, 2025
Yea, I misread the repo that it was refrencing sry.
Romaq commentedon Oct 11, 2025
I'm not clear on the upshot of all this, but does it mean a problem updating MultiMC for deploying Neoforge Instances for 1.21.10?
Romaq commentedon Oct 14, 2025
As this was closed as completed, would this mean we are only waiting for the next update of MultiMC to be able to use NeoForge with Minecraft 1.21.9+?
phit commentedon Oct 14, 2025
no new release needed, should be fixed now
Romaq commentedon Oct 18, 2025
Thank you. I did not recognize the assets would update but not MultiMC. I report all is good now.