This ship is a General Contact Unit of the Rhapsode class, called And I Kid You Not (and I kid you not!). GCU are intended to carry out diplomatic missions towards civilizations equal or inferior to the Culture. It operates on behalf of the Contact section.
It is about 3km long, which makes it a rather small ship compared to the GSV's which are over 100km long and have billions of inhabitants. Anyway it has enough room for a large crew and many smaller ships like the one in foreground. This small one is the fast picket from the previous picture.