Assuming you have obtained a legitimate copy of Lunar Client JAR 1201, here is what the file structure looks like (decompiled):

  • Anti-Cheat (Acorp): Even in JAR form, version 1201 contains legacy Acorp native calls. This often causes issues if your OS is newer than Windows 10 version 1809.

    • If you are chasing the performance of old Lunar Client but cannot get version 1201 to work, consider these alternatives:

    If you need version 1201 specifically (perhaps because a mod or server requires that exact build), follow these safe steps:

    Once you have the JAR and attempt to run it via command line (e.g., java -jar lunarclient-1201.jar), you will encounter issues. Here are the top three errors and fixes.