> db.runCommand({isMaster: 1, $db: "admin"}) { "ok" : 0, "errmsg" : "Parsed command object contains duplicate top level key: $db", "code" : 9, "codeName" : "FailedToParse" }
There is no duplicate top level key $db, but I suspect the server is upgrading the OP_QUERY to OP_MSG and $db itself from the wire protocol