CSTVIS
From Valve Developer Community
aldelr CSTVIS is a custom version of VVIS compiler written by Adam McKern. Similar in function to its Valve counterpart, CSTVIS creates a vismatrix data allowing sections of the world to not be drawn when they are not visible.
Although CSTVIS itself can be skipped during compile to create an unoptimized map, using the other two tools, CSTBSP and CSTRAD, is highly recommended for optimal results.

See also
- ammahls.com - the author's website.
- Map Compiling Theory