For advanced users who need the absolute latest build:
There is only one safe way to obtain this extension. You must install the official Steamworks Extension Pack via the Clickteam Fusion 2.5 installer. steamworks.mfx download
Cause: The extension works, but Steam is not running, or the steam_appid.txt file is missing.
Fix: In your game's build folder, create a text file named steam_appid.txt. Inside, put only your Steam App ID (e.g., 480 for the example Spacewar game). Never distribute this file in your final Steam build. For advanced users who need the absolute latest
There is no single "official" standalone download, because the extension is part of a larger SDK package. Here are the legitimate ways to obtain the file: but Steam is not running