IIRC ... in a interview about King Crimson, Levin stated that Fripp comes up will his basslines, and he just plays them. Levin went onto say that Fripp is also a very talented bassist, so he has no problem with it, as Fripp comes up with some challenging bass lines.
I'd be interested to see a citation for that. It's not that I don't believe Fripp wrote some of the King Crimson basslines -- I know for a fact he wrote most of the lines that Boz Burrell played in the 1972 version of Crimson, and I would bet that he wrote most of Tony's lines on the Discipline album since the music on that is all based on a very specific concept -- but there are also some basslines in King Cimson tunes that just reek of Tony's aesthetic; it would be very surprising to learn that Tony didn't come up with the basslines to, for example, "People" or "Sartori In Tangiers" or "The Howling" or "One Time" or "Sleepless" etc.