This paper describes the design of a Minimally Invasive Gesture Sensing Interface (MIGSI) for trumpet. The interface attaches effortlessly to any B-flat or C trumpet and requires no permanent modifications to the host-instrument. It was designed first and foremost with accessibility in mind an approach that is uncommon in augmented instrument design and seeks to strike a balance between minimal design and robust control. MIGSI uses sensor technology to capture gestural data such as valve displacement, hand tension, and instrument position, to offer extended control and expressivity to trumpet players. Several streams of continuous data are transmitted wirelessly from MIGSI to the receiving computer, where MIGSI Mapping application (a simple graphical user interface) parses the incoming data into individually accessible variables. It is our hope that MIGSI will be adopted by trumpet players and composers, and that over time a new body of repertoire for the augmented trumpet will emerge.