A slight deviation from the SDAM spec, but in parsing an ismaster reply, libmongoc always considers the existence of the field "msg" to indicate the server is a mongos.

But the SDAM spec is explicit in that the value should be "isdbgrid".

I don't believe there is any current bug that could surface from this, as I'm not aware of servers ever reporting "msg" with another value. But it could foreseeably change (e.g. as suggested in SERVER-3353).