Some fields can appear in the BSON in the response to any command, such as "ok". We even document a subset of them here: https://github.com/mongodb/mongo/blob/87d6ec40779e3750a123efd941e16d159bf938bb/src/mongo/idl/generic_argument.idl#L108. However, the parser from BSON to c++-representation of the reply object generated by IDL throws an error when it encounters these fields. We should teach the parser to allow for generic reply fields to be present when parsing.