The Steamworks SDK is incompatible with some open source licenses, which may affect your ability to distribute open source software via Steam.
Keep in mind that according to the Steam Distribution Agreement you warrant and represent that you have all necessary rights to distribute the game via Steam. If your application contains third party open source code that is incompatible with the Steamworks SDK, then YOU MUST NOT DISTRIBUTE YOUR APPLICATION VIA STEAM
Which Open Source licenses are compatible with the Steamworks SDK?
In general, permissive licenses that do not put any requirements on you to redistribute your modifications under an open source license work fine. Common permissive and acceptable licenses includes MIT License, BSD 3-clause and 4-clause, Apache 2.0 and WTFPL.
Which Open Source Licenses are problematic for shipping on Steam?
Generally, any license that has a so-called “copyleft” element will be problematic when combining code with the Steamworks SDK. The best-known example is GPL.
But I saw a GPL-licensed application on Steam!
This can happen if the author of the code that is GPL-licensed has given the permission to do so. The author can of course always (a) decide to grant Valve a different license than the author grants everyone else or (b) decide that what the Steamworks SDK does is just a communication with a service that does not invoke the copyleft requirement of the GPL.
I am using code that is (Insert Acronym Here) licensed. Can I use it?
We don’t know and don’t have the resources to do a due diligence review of your code base. Ultimately, you have to make the decision whether you can warrant to us that you have all necessary rights to your code base. It depends on the license and code in question.
If in doubt, consult a lawyer, or ask the author whose open source code you plan to use.