Players control Half-Life protagonist Gordon Freeman as he travels up a coastal cliff to destroy a Combine weapon in a monastery.

Lost Coast was the first Valve game with a commentary mode, in which the developers explain elements of design as the player progresses through the level.

It received a generally positive reception, and there was consensus among reviewers that the new features included in Lost Coast should be integrated into future games released by Valve.

An Eastern Orthodox architectural style was deliberately chosen for the monastery, as buildings of this type "are very colorful and have a large variety of materials" and are "often lit naturally, with extremes of darkness and brightness," providing an ideal showcase for the HDR lighting effects.

[a] Valve also thought a monastery would help provide a starker contrast between old human architecture and futuristic Combine technology inside it.

[5] The final version of Valve's high-dynamic-range technology adds a significant number of lighting effects to the engine, aimed at making the game appear more realistic.

Bloom shading was introduced, blurring bright edges in the game world and emulating a camera's overexposure to light.

[15] Valve announced on May 30, 2007, that Lost Coast, along with Half-Life 2: Deathmatch, would be made available for free to owners of ATI Radeon cards.

[16] It was later released without charge to Nvidia graphics card owners along with Half-Life 2: Deathmatch, Peggle Extreme, and the first eleven levels of Portal.

1UP.com enjoyed the amount of detail, including the graphics, puzzles, and intelligent enemies, saying, "Valve just packed more atmosphere into a tiny snippet than most shooters muster, period."

The review also praised the level's commentary system, calling it an informative addition, and enjoyed the interesting and insightful comments made by some of its creators.

1UP.com and UGO both considered it short;[19][20] Shawn Elliott of 1UP.com described it as "a lickety-split run through postcard-pretty tide pools, up cliffs, and into a church turned Combine outpost".

[20] The gameplay also disappointed Brad Shoemaker of GameSpot, who found it too similar to Half-Life 2 and so "isn't going to set the world on fire".