What is steam_api.dll ? The steam_api.dll file is a dynamic link library created by Valve Corporation. It acts as the bridge between a game and the Steam client’s features (achievements, cloud saves, DRM, multiplayer matchmaking). In Call of Duty: Modern Warfare 2 (2009), this file is critical because MW2 was one of the first major AAA titles to require Steam for multiplayer (via IWNET, which routed through Steam).