Metal Gear Solid V: The Phantom Pain Message Board  

Thanks a lot for the solution. I honestly had no idea Steam allowed you to roll a game back to an older build this way, and it worked perfectly.

For anyone else who wants to keep using the old trainer, here is a very short summary of what I did on my English Steam installation:

1. Backed up my MGSV save folders and the current game installation first.
2. Opened the Steam Console and downloaded the March 23, 2021 depot/build with:

`download_depot 287700 287701 2518831260221836800`

3. Waited for `Depot download complete`.
4. Copied the contents of the downloaded `depot_287701` folder over my existing `MGS_TPP` installation, replacing the files.
5. I left Steam's `appmanifest_287700.acf` untouched, so Steam still considers the current build installed.
6. Started the game normally through Steam. My existing save worked fine.
7. Launched the old CH trainer first, then the game, pressed F1 at the main menu, and everything works again - including Add GMP and Massive Materials, which were the options broken by the new game update.

One important note: `287701` is the English depot, so don't blindly copy the command above if your installation uses a different depot/language. Check SteamDB and the Steam Console guide first.

Also, make backups before doing anything. This is only a summary of what worked for me, not a complete rollback guide, so I would recommend anyone trying it to read a little more about Steam depot downgrading first, with or without the help of an AI assistant.

Thanks again for pointing me in the right direction, and more generally for the excellent support you continue to provide and the ongoing effort you put into keeping trainers working and supporting the community, even for games that are many years old. It is genuinely appreciated.