"isMaster" is no longer in API Version 1, so you can't call it with apiStrict. MongoClients configured with apiStrict won't be able to connect until DRIVERS-1633. After that they'll call "hello" instead of "isMaster" if they're configured with API parameters. I've updated the legacy mongo shell so it can connect with --apiVersion 1 --apiStrict. Remove isMaster from any lists we have of API Version 1 features. Note that this only affects you if you're using apiStrict, and you're using a development version of a driver or shell and/or you're explicitly calling isMaster by name.