  Due to its 1770 K equilibrium temperature, WASP-17b, a 1.99 $R_\mathrm{Jup}$,
0.486 $M_\mathrm{Jup}$ exoplanet, sits at the critical juncture between hot and
ultra-hot Jupiters. We present its 0.3-5 $\mu m$ transmission spectrum, with
newly obtained with Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3)
measurements, and, taking advantage of improved analysis techniques, reanalysed
HST Space Telescope Imaging Spectrograph (STIS) and Spitzer Space Telescope
Infrared Array Camera (IRAC) observations. We achieve a median precision of 132
ppm, with a mean of 272 ppm across the whole spectrum. We additionally make use
of Transiting Exoplanet Survey Satellite (TESS) and ground-based transit
observations to refine the orbital period of WASP-17b. To interpret the
observed atmosphere, we make use of free and equilibrium chemistry retrievals
using the POSEIDON and ATMO retrieval codes respectively. We detect absorption
due to H$_2$O at $>7 \sigma$, and find evidence of absorption due to CO$_2$ at
$>3 \sigma$. We see no evidence of previously detected Na and K absorption.
Across an extensive suite of retrieval configurations, we find the data favours
a bimodal solution with high or low metallicity modes as a result of poor
constraints in the optical, and demonstrate the importance of using multiple
statistics for model selection. Future James Webb Space Telescope (JWST) GTO
observations, combined with the presented transmission spectrum, will enable
precise constraints on WASP-17b's atmosphere.
