  Exceptionally bright gamma-ray burst (GRB) afterglows can reveal the angular
structure of their jets. GRB jets appear to have a narrow core (of half-opening
angle $\theta_c$), beyond which their kinetic energy drops as a power-law with
angle $\theta$ from the jet's symmetry axis, $E_{k,\rm
iso}(\theta)\propto[1+(\theta/\theta_c)^2]^{-a/2}$. The power-law index $a$
reflects the amount of mixing between the shocked jet and confining medium,
which depends on the jet's initial magnetization. Weakly magnetized jets
undergo significant mixing, leading to shallow ($a\lesssim2$) angular profiles.
We use the exquisite multi-waveband afterglow observations of GRB 221009A to
constrain the jet angular structure using a dynamical model that accounts for
both the forward and reverse shocks, for a power-law external density profile,
$n_{\rm{}ext}\propto{}R^{-k}$. Both the forward-shock emission, that dominates
the optical and X-ray flux, and the reverse-shock emission, that produces the
radio afterglow, require a jet with a narrow core ($\theta_c\approx0.021$) and
a shallow angular structure ($a\approx0.8$) expanding into a stellar wind
($k\approx2$). Moreover, these data appear to favor a small fraction
($\xi_e\approx10^{-2}$) of shock-heated electrons forming a power-law energy
distribution in both shocks.
