Going from 340m in Rishikesh to 2,800m in Auli in one day is a massive jump. To avoid AMS (Altitude Mountain Sickness), ask your Rishikesh to Auli taxi service driver to stop for a long lunch in Karnaprayag. This mid-way break at 800m helps your body adjust. Trust a driver who prioritizes your health over a "fast" arrival time—it makes your first night in the snow much more comfortable.