Bethesda mod archives

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The Gamebyro engine can load files from specialised archives into the game on startup. This allows DLCs and mods to package all their files together without worrying about them being accidentally overwritten and further simplifies the modding process by minimising the number of files. These archives can be either BSA (Oblivion, Fallout 3, Fallout New Vegas and Skyrim) or BA2 (Fallout 4 and Fallout 76) format files. Extracting or modifying archives should be considered an advanced modding technique and in most cases is unnecessary to allow the game to function correctly.  



Mod archive compatibility

The BSA and BA2 archives are not automatically compatible with all games. Even among the variations of the same file type, they can cause unwanted side effects such as preventing the game from starting up. The table below lists the types of BSA and BA2 and how they behave with each Gamebyro title. The version of the archive can be determined by inspecting the file header. 

Game BSA v103 BSA v104 BSA v105 BA2 v1
Oblivion Compatible Ignored Ignored Ignored
Fallout 3 Untested Compatible Untested Ignored
Fallout New Vegas Untested Compatible Untested Ignored
Skyrim (2011) Limited Support Compatible Incompatible Ignored
Skyrim Special Edition Incompatible Incompatible Compatible Ignored
Fallout 4 Ignored Ignored Ignored Compatible
Skyrim VR Untested Incompatible Compatible Ignored
Fallout 4 VR Ignored Ignored Ignored Compatible
Fallout 76 Ignored Ignored Ignored Compatible

Below is a more detailed explanation of each status. 

  • Untested - This combination has not been tested, feel free to update this article if you are able to test. 
  • Compatible - This is the intended archive format for this game and is fully compatible. 
  • Limited Support - This archive type is not intended to be used with the game but does appear to work. The exact impact on loading times, stability or general performance is unknown. It is not recommended to continue to use this combination.
  • Incompatible - This combination will cause a critical error, usually crashing the game. 

Mod manager archive checking

Some mod managers include a feature that will automatically check archive versions and identify any mismatches. At the time of writing, only Vortex and Wryebash include this feature. 

Extracting and packaging archives