Systems and methods for asserting the initialization status of a virtualized system. An example method comprises: specifying, by a virtualization manager, configuration settings of a virtual machine instance, wherein the configuration settings comprise a callback address and an identifier of the virtual machine instance; causing a host computer system to boot the virtual machine instance using the configuration settings; receiving, at the callback address, a notification comprising the identifier of the virtual machine; and determining, in view of the notification, an initialization status of the virtual machine instance.