is there a way to "Trick" steam into thinking i've installed a certain game?

[Dewa]

since a fast way to install something on steam is to have the existing game files, copy it to your common folder, and steam will automatically detect said files, shortening the download

the problem is how most games don't actually detect them properly

for example i still needed to download an extra 20GB when i was installing payday 2, even when steam detected the already copied files

[Perrywinkle]

usually copying the most recent version is enoug however some games need to be patched after the update like the mmos and some games with their own launcher, maybe because that game is multi-player need to patch live.

[Ogami]

If you copy over files, ALWAYS make sure you also copy over the correct appmanifest file from your steamapps folder. That tells Steam if a game is installed or not.
If you copy that with the game the game will always be recognized as installed without the need to download anything again.

[The Giving One]

デワ | Wolfie původně napsal:

for example i still needed to download an extra 20GB when i was installing payday 2, even when steam detected the already copied files

From what I know about that game, Payday 2 gets alot of updates, and sometimes they may be large. Maybe that was in the past and not so now, but my install of that game was getting many updates not too long ago.

Just do as said above and be sure to save the .acf file for that game. That is the file that tells Steam the game is installed. That file number is the same number at the end of the store page's URL for the game in question. And that file is located in the steamapps folder, so only having the game folder relocated to the common folder will not do. That .acf file must also be present.

EDIT..If there are updates available between your installations of said game, then of course Steam will download the files to update that game, unless you have the game set to only update on launch. And if it has been a long time between installations, then that update might be quite large.

You can also verify the game files if there is any doubt after taking the steps above.

https://support.steampowered.com/kb_article.php?ref=2037-QEUH-3335