R drawclipbrushes

r_drawclipbrushes is a CVAR used to display clip brushes (invisible walls) in the map. These brushes can block both players and NPC's.

Usage

#	Description
0	Default value, clip brushes are not rendered.
1	Only shows clip brushes' outlines. These outlines can be seen through walls.
2*	Shows clip brushes as properly rendered brushes in the level.

*This option is only available on Source Engine 2007 and newer.

Clip brushes are colored to show what they block:

Color	Description
Red	Blocks both players and NPC's.
Pink	Blocks players only.
Purple	Blocks NPC's only.

The CVAR is often used for routing as you might need to see if there are any clip brushes blocking up a path or any that you could abuse to get to unintended places. Despite not being a cheat-protected command, you can not use this in a speedrun.