The Creation Kit requires the original Steam API DLL. You can use Root Builder to switch between the downgraded and original DLLs as needed. Root Builder allows you to set up the Creation Kit plus the original Steam DLL as a mod, making switching easy.

This error typically means your game isn't the latest Steam version. Verify your game files through Steam, ensure you're not using a pirated copy, and retry. The downgrader explicitly does not support pirated copies.

In addition to patching your game, here are several tips for optimizing your Fallout 4 experience:

Note: GOG’s latest default version may be higher; you might need to request the older patch from GOG support or a community archive.

: Major projects like Fallout: London officially rely on this specific version for a stable experience.

: This version was the final stable build before the major Next-Gen updates of 2024. Most essential mods, such as LooksMenu and various UI overhauls, were built specifically for this executable.

To ensure your portable version never accidentally updates, modify your appmanifest file.