The glass knife replaces the crowbar as the thief's means of breaking through windows. It attaches to a pane of glass and silently cuts a small hole in the pane, allowing him to reach his arm into the window and unlock it without causing any sound. This is especially useful when robbing houses while the tenants are still home.
To use the glass knife, the player must interact with the window, then hold down the action key and move the arm of the cutter in repeated circles. An outline will appear partway through the action, but the hole isn't officially cut until the interface closes and a popping sound is heard.
Several houses and businesses on Richie R. Street and in the Industrial Area have sensors on their windows. When a glass knife is used on them, an alarm will sound and the police will be called. If the building has window shutters they will immediately close, preventing the thief from opening the window he just unlocked.