First off, I'll give some background on why I'm asking this: I enjoy Steam achievement hunting, so I have no intention of unlocking all achievements.
However, there are several games (E.g. Spiral Knights) where I have every achievement but one. This wouldn't bother me, except these achievements are no longer achievable due to changes in the game, making it impossible for me to get the 100% stat on a game that I have actually completed.
Is it against Valve's policies/rules to use an achievement "cheat"?
Ideally I'm looking for some official documentation. (Chances are I won't actually use anything, but I'd like to know the possible ramifications if I did)
I have seen this quote from a volunteer moderator on the steam forums, but it is only regarding VAC (The in-game anti-cheat), Ideally I'd like to know about any other possible issues.
Valve Anti-Cheat does not ban people who use steam achievement managers, nor will he be manually banned. I guess you could report him as a suspected cheater.
Best Answer
Yes, it is against the Steam Subscriber Agreement.
It is against Steam Subscriber Agreement which you agreed by creating a steam account, buying a game or buying/selling something from/on the Steam Community Market.
Steam Achievement Manager counts as a bot or cheat (TF2 Wiki considers it as hacking), if not just an unauthorized third-party software, therefore is against the Steam Subscriber Agreement.
If you use this software, your account may be terminated:
In past, the TF2 "Announcer" took the items that were obtained through hacked achievements back for one week, which shows that Valve doesn't accept them. This also shows that it is not authorized by Valve.