Using esriGeometryEnvelope as the geometry type when submitting a rectangular polygon request to a custom geoprocessing (GP) tool causes the tool to stop responding with a 400 Bad Request error, “Unable to complete operation.“ This occurs in Portal for ArcGIS 10.3 Web AppBuilder for ArcGIS, when setup to use a polygon feature set as an input. However, other polygon shapes run successfully because the shapes use the esriGeometryPolygon geometry type.