These rely on a file called CPKREDIR.asi or d3d9.dll . They are old, unstable, and cause the game to crash on launch if you are using Windows 11.
However, there is a universal cry of frustration echoing across Reddit, Steam forums, and Discord servers: sonic generations mods not working link
Here is the harsh truth: Sonic Generations does not natively support mods. If you are manually dropping .cpk or .ar.00 files into the game folder, you are doing it the , and it will fail 90% of the time. The #1 Fix: Stop using the vanilla exe. You need HedgeModManager (HMM) . These rely on a file called CPKREDIR