  We present observations of the T8 dwarf 2MASS 0415-0935 with JWST's NIRSpec
spectrograph using the G395H grating ($\sim$ 2.87 - 5.14 $\mu$m). We perform
the first atmospheric retrieval analysis at the maximum spectral resolution of
NIRSpec (R$\sim$2700) and combine the spectrum with previous observations to
study the 0.9-20 $\mu$m spectral energy distribution. We obtain precise
constraints on chemical abundances ($\sim$0.02 dex) for a number of species
which complicate our understanding of disequilibrium chemistry, particularly
for CO$_{2}$ and PH$_{3}$. Furthermore, we measure a $^{12}$CO/$^{13}$CO ratio
of $\sim 97^{+9}_{-8}$, making 2MASS 0415-0935 the coldest ($\sim 760$ K)
substellar object outside of our solar system with a measured
$^{12}$CO/$^{13}$CO ratio. This work shows promise for similar observations
with JWST to provide precise abundances of major chemical species as well as
isotopologues, allowing for new tests of our understanding of the formation and
atmospheres of substellar objects.
