  At redshift z = 4.618, GRB 220101A is the most distant gamma-ray burst (GRB)
detected by Fermi/LAT to date. It is also a very energetic event, with an
equivalent isotropic energy of $3.6\times10^{54}$ erg. We jointly analyzed the
Fermi/GBM and LAT observations of GRB 220101A with two independent approaches
and found a significant spectral break at sub-100 MeV energies during the
prompt emission. The fast variability of the emission suggests that this
spectral attenuation is caused by internal opacity to pair creation. Regardless
of the nature of the emission processes assumed in the spectral analysis, we
infer a moderate value for the jet Lorentz factor, $\Gamma\sim110$, and find
that all of the high-energy emission was produced above and near the
photosphere, at a distance of $\sim10^{14}$ cm from the central engine. We
compare these results with the four other LAT-detected GRBs with similar
properties.
