The sessions spec does not mention endSessions in the exceptions section (https://github.com/mongodb/specifications/blob/master/source/sessions/driver-sessions.rst#exceptions-to-sending-the-session-id-to-the-server-on-all-commands) so the assumption is that endSessions should be used in a session. However, the command is used in many drivers to end all open sessions when the client is disconnected and creating an implicit session for it would open up another session on the server, so I think it should be included in the exceptions.