Game Pass save format (internals)

This page documents how a Game Pass / Microsoft Store save is packaged on disk, for anyone working on the editor or curious about the format. You do not need any of this to edit a Game Pass save: the desktop app and CLI unpack, edit, and repack the container for you. For the user-facing workflow (opening, editing, converting) see the Game Pass saves guide.

Why it differs from Steam

A Steam save is a folder of loose .sav files - one per world region, plus a player file per character. A Game Pass save packs that entire world (every WorldSave_* region and every player) into one compressed blob inside an Xbox Connected Storage ("wgs") container. The save content is identical to Steam (the same GVAS data); only the packaging differs, which is why a world converts losslessly between the two.

Four nested layers

From outside in:

1. wgs container - containers.index maps logical container names to GUID-named folders; each folder holds a container.N manifest pointing at a GUID blob file.
2. ABF_SAVE_VERSION bundle - the world blob is a small archive: a table of contents (member path + save class + size) followed by one Oodle-compressed stream of every member.
3. Headerless members - each packed save is the GVAS property body only; the save class lives in the table of contents. The editor splices a class-matched header back on to read it, and strips it again on write.
4. GVAS - the same save data the Steam version stores. See the player and world save schemas.

Oodle (de)compression uses the same library the editor already uses for the game's pak files; it is downloaded on demand if it isn't already present.

Account ids

Game Pass players are owned by a 16-19 digit Xbox account id (XUID) instead of a SteamID64. Each <XUID>_<id> folder (the one containing containers.index) is one Xbox account's save store. Converting to Steam can optionally re-home the player to a SteamID64; see Converting between Steam and Game Pass.