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We present a new optical transmission spectrum of the hot Jupiter WASP-79b.We observed three transits with the STIS instrument mounted on HST, spanning0.3 - 1.0 um. Combining these transits with previous observations, we constructa complete 0.3 - 5.0 um transmission spectrum of WASP-79b. Both HST andground-based observations show decreasing transit depths towards bluewavelengths, contrary to expectations from Rayleigh scattering or hazes. Weinfer atmospheric and stellar properties from the full near-UV to infraredtransmission spectrum of WASP-79b using three independent retrieval codes, allof which yield consistent results. Our retrievals confirm previous detectionsof H$_{2}$O (at 4.0$\sigma$ confidence), while providing moderate evidence ofH$^{-}$ bound-free opacity (3.3$\sigma$) and strong evidence of stellarcontamination from unocculted faculae (4.7$\sigma$). The retrieved H$_{2}$Oabundance ($\sim$ 1$\%$) suggests a super-stellar atmospheric metallicity,though stellar or sub-stellar abundances remain consistent with presentobservations (O/H = 0.3 - 34$\times$ stellar). All three retrieval codes obtaina precise H$^{-}$ abundance constraint: log(X$_{\rm{H^{-}}}$) $\approx$ -8.0$\pm$ 0.7. The potential presence of H$^{-}$ suggests that JWST observationsmay be sensitive to ionic chemistry in the atmosphere of WASP-79b. The inferredfaculae are $\sim$ 500 K hotter than the stellar photosphere, covering $\sim$15$\%$ of the stellar surface. Our analysis underscores the importance ofobserving UV - optical transmission spectra in order to disentangle theinfluence of unocculted stellar heterogeneities from planetary transmissionspectra.

HST PanCET Program: A Complete Near-UV to Infrared Transmission Spectrum for the Hot Jupiter WASP-79b