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JWST will launch during HST cycle 28 and begin an unprecedented era in atmospheric characterization for all types of exoplanets. Atmospheric spectroscopy of warm rocky planets comma sub-Neptune comma and Jupiter-mass planets will be obtained through a robust set of guaranteed transiting planet observations (the JWST ERS and GTO programs). To accurately model and interpret observations of these planets. atmospheres comma we must understand the high-energy SED of their host starsdoublePoint FUV and NUV-driven photochemistry shapes an atmosphere.s molecular abundances and the formation of hazes comma EUV irradiation can erode a planet.s gaseous envelope comma and flares can affect long term stability. A number of recent surveys have used HST.s UV capabilities to characterize the energetic irradiance spectra across a range of stellar masses comma ages comma and activity levels. While these surveys have proven invaluable for predicting photochemical tracers and evolution of an exoplanet.s atmosphere comma they have also shown scatter in behavior and irradiance properties for stars of similar type. As a result comma direct UV observations remain the gold standard for understanding the effects of the stellar irradiance on a specific exoplanet. We propose to obtain temporally-resolved UV (1150-3200 Ang) stellar spectroscopy of the 13 JWST guaranteed time targets with no UV characterization data in the HST archive. Our proposed observations will provide the UV context necessary to determine the likelihood of atmospheric formation and retention comma the identification and interpretation of atmospheric chemistry comma and the impacts of stellar activity on the exoplanet atmospheric stability.