Quake3World

using the _celshade feature causes any object in the skybox to flicker black. I've tried every trick I know, like using planes instead of brushes, etc. But there is always a black flicker behind each texture. Anyone know how to turn off or reduce the celshade effect in a skybox?

Don't use the _celshader key in the worldspawn. It's a better solution to func_group all parts of your map you wish to apply the celshader and put the _celshader key into the func_group.
In this way you can also use diffrent celshaders, with diffrent colors or offset values, in your map.

Thanks for the response. I haven't tested it but I'm sure it would work. I forgot you could do that with func_groups and misc_models. My only concern here is reaching the 256 entity limit.

Do you know if func_groups are considered entities ingame, or like lights, are they just used for compiling and then discarded once in the game?

They are discarded.