1. A method for operating an intelligent automated assistant, comprising: at an electronic device comprising a processor and memory storing instructions for execution by the processor: receiving a user request, the user request including at least a speech input received from the user; processing the user request to obtain a representation of user intent, wherein the representation of user intent associates the user request with a task flow operationalizing a requested task, and the task flow is operable to invoke a plurality of services each supporting functions according to a respective plurality of service parameters; based on the representation of user intent, identifying one or more relevant task parameters from a plurality of task parameters of the task flow; and selectively invoking a subset of the plurality of services during execution of the task flow, wherein the selectively invoked subset of the plurality of services support functions according to the identified one or more relevant task parameters.