The concept of an AI assistant for task guidance is rapidly shifting from a science fiction staple to an impending reality. Such a system is inherently complex, requiring models for perceptual grounding, attention, and reasoning, an intuitive interface that adapts to the performer's needs, and the orchestration of data streams from many sensors. Moreover, all data acquired by the system must be readily available for post-hoc analysis to enable developers to understand performer behavior and quickly detect failures. We introduce TIM, the first end-to-end AI-enabled task guidance system in augmented reality which is capable of detecting both the user and scene as well as providing adaptable, just-in-time feedback. We discuss the system challenges and propose design solutions. We also demonstrate how TIM adapts to domain applications with varying needs, highlighting how the system components can be customized for each scenario.