Just that. At loading MorePlanets complains it needs version 10.7 and that I only have 10.6, but I do have 10.7 freshly downloaded. It just doesn't make no sense.

I only have these mod and it run properly, maybe you should try download mixin booter on modrinth.

I tried the modrinth download, same exact result.

For the sake of completion, I tested some more. Java 8 + Forge does boot. Java 25 + Cleanroom does not (reported behaviour). Java 21 + Cleanroom also does not work.

I was going to blame Cleanroom's implementation, but I decided to test MorePlanets GC4.0.6 with SteveKunGLib 1.3.1 and that does boot with no issues.

I have no idea about java programming, but I suspect some check is failing only on cleanroom minecraft for some reason. Should I bring this up to their team instead? Is this something that could be on your end of the code?

Yeah it might because of that. But you can leave this issue open until they fix with newer java version. Or my guess some mods included older version of MixinBooter inside?

After testing some more, apparently is Cleanroom by itself. It packs mixinbooter inside, so even if you don't add mixinbooter it will act as such. Which is very weird because I'm using release 3.0.20 which released 2 days ago, but mixinbooter 10.7 relesed back in september. I might bring it to thewir attention, because this behaviour indicates that they might not have merged mixinbooter and cleanroom builds for some reason or something. Maybe this info helps you too. Thanks for replying, I know this might be an edge case, I'm just really looking forward to Cleanroom being the de facto way to play 1.12.2, it seems like a fantastic idea.

i dived in code of cleanroom and found way to fix it just modifying forge_early.cfg config.
set CUSTOM_BUILD_IN_MOD_VERSION to true and MIXIN_BOOTER_VERSION to 10.7 and my game booted. it may cause some troubles, but commits in cleanroom say that they doing it time to time without modifying any of mixinbooter code.