What was the first game ever released that ran on the Source engine?

The first game ever released that ran on the Source engine was Counter-Strike: Source. It was the successor to the immensely popular game Counter-Strike, which initially started as a modification for Half-Life. Counter-Strike: Source brought the iconic gameplay of Counter-Strike to the next level with improved graphics, physics, and overall game performance. Being the first game to showcase the capabilities of the Source engine, it set the standard for future Source engine games and solidified the engine's reputation for delivering immersive and realistic gaming experiences.