We describe a smartphone application that helps people with Autism Spectrum Disorder (ASD) cope with anxiety attacks. Our prototype provides a one-touch interface for indicating a panic level. The device's response - to instruct, soothe, and/or contact carers - is sensitive to the user's context, consisting of time, location, ambient noise, and nearby friends. Formative evaluation unearths a critical challenge to building assistive technologies for ASD sufferers: can regimented interfaces foster flexible behaviour? Our observations suggest that a delicate balance of design goals is required for a viable assistive technology.