16. A computer-readable storage medium storing computer-executable instructions for causing a computing device to perform operations for updating language understanding classifier models, the operations comprising: determining a user request based on user input received at a computing device, the user request received via at least one of text input and voice input, the request for a functionality of a digital personal assistant running on the computing device; determining the user request does not match at least one of a plurality of pre-defined voice commands in an agent definition of the digital personal assistant; generating a confidence score by applying a plurality of available language understanding classifiers associated with the agent definition to the user request; upon determining that the confidence score is less than a threshold value: receiving using a graphical user interface of an end-user labeling tool (EULT) of the computing device, a user selection of at least one of the following: at least one intent of a plurality of available intents and at least one slot for the at least one intent, wherein: the at least one intent is associated with at least one action used to perform at least one function of a category of functions for a domain; and the at least one slot indicating a value used for performing the at least one action; generating a labeled data set by pairing the user voice request and the user selection; selecting a language understanding classifier from the plurality of available language understanding classifiers associated with the agent definition, the selecting based at least on the at least one intent selected by the user; and generating an updated language understanding classifier by training the selected language understanding classifier using the generated labeled data set.
16. A computer-readable storage medium storing computer-executable instructions for causing a computing device to perform operations for updating language understanding classifier models, the operations comprising: determining a user request based on user input received at a computing device, the user request received via at least one of text input and voice input, the request for a functionality of a digital personal assistant running on the computing device; determining the user request does not match at least one of a plurality of pre-defined voice commands in an agent definition of the digital personal assistant; generating a confidence score by applying a plurality of available language understanding classifiers associated with the agent definition to the user request; upon determining that the confidence score is less than a threshold value: receiving using a graphical user interface of an end-user labeling tool (EULT) of the computing device, a user selection of at least one of the following: at least one intent of a plurality of available intents and at least one slot for the at least one intent, wherein: the at least one intent is associated with at least one action used to perform at least one function of a category of functions for a domain; and the at least one slot indicating a value used for performing the at least one action; generating a labeled data set by pairing the user voice request and the user selection; selecting a language understanding classifier from the plurality of available language understanding classifiers associated with the agent definition, the selecting based at least on the at least one intent selected by the user; and generating an updated language understanding classifier by training the selected language understanding classifier using the generated labeled data set. 19. The computer-readable storage medium according to claim 16 , wherein the agent definition comprises at least one of a voice command definition (VCD) schema and a reactive agent definition (RAD) schema associated with the digital personal assistant.