Sandbox Mode is exclusive to the original Killing Floor.

It allows you to modify the waves in a Survival match, including what Specimen can appear and if you will face a Boss at the end or not. You can also set your Starting Cash and how much Trader time you get between waves.

You can also set the Game Length to be anywhere between zero and fifteen waves. If you set it to zero, you will automatically start a fight against the Boss after the game starts.

Playing in Sandbox Mode prevents you from earning achievements or progressing in your Perk.