According to the sessions spec, estimatedDocumentCount should not support an explicit session parameter: https://github.com/mongodb/specifications/blob/master/source/sessions/driver-sessions.rst#new-collection-methods-that-take-an-explicit-session

However, I am able to pass one in by appending it to the options bson_t and I've confirmed via command monitoring that the id for the session I specified is being used.

The docs section on sessions also makes it seem like you can use this method in a transaction: http://mongoc.org/libmongoc/current/mongoc_collection_estimated_document_count.html