Authors: Ali Rahmati (Space Sciences Laboratory, University of California Berkeley), Davin Larson (Space Sciences Laboratory, University of California Berkeley), Rebecca Jolitz (Space Sciences Laboratory, University of California Berkeley)

We describe the design, development, testing, calibration, and performance of the SupraThermal Ion Sensor (STIS), an energetic particle solid-state detector onboard the SOLAR-1 (Space weather Observations at L1 to Advance Readiness – 1) previously named SWFO-L1 (Space Weather Follow On-Lagrange 1) spacecraft, which launched in September 2025. The STIS instrument comprises two solid-state telescopes, one for ions and one for electrons, each featuring 80°x60° baffled collimators and containing a dual-pixel silicon detector for high dynamic range flux measurements, followed by a thick silicon detector for measuring higher energy particles and providing anti-coincidence penetrating particle shielding. STIS is designed for real-time, continuous measurement of energetic ions (25-6000 keV) and electrons (25-250 keV) to provide early warning of space weather events. Ion measurements characterize solar ejecta, including Coronal Mass Ejections (CMEs), co-rotating interaction regions, and interplanetary shocks, and facilitate early detection of Solar Energetic Particle (SEP) events, particularly those associated with strong CMEs. Electron measurements offer more advanced warning of SEP events due to their higher velocity and enable background contamination removal from ion data.