Minor - P4
> 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