  Recent LHAASO observations of the prompt emission phase of the
brightest-of-all-time GRB 221009A imposes a stringent limit on the flux ratio
between the TeV and MeV emissions, $F_{\rm TeV}/F_{\rm MeV}\le 2\times10^{-5}$,
during the period $220 \operatorname{-}230\, {\rm s}$ after the trigger. bf
This period covers the peak of the main MeV burst and is just before the TeV
afterglow emerges. Within the framework of internal shocks, we study the
internal $\gamma\gamma$ absorption in GRB 221009A by generating a set of
synthetic bursts in a simulation that reproduces the observed feature of GRB
221009A. We find that the $\gamma\gamma$ absorption does not lead to an
exponential cutoff, but rather a power-law spectrum, consistent with previous
works. We further find that the attenuation due to $\gamma\gamma$ absorption
alone cannot explain the flux limit ratio of GRB 221009A, suggesting a low
ratio between synchrotron self-Compton (SSC) and synchrotron emission outputs.
This requires the magnetic field energy density to be much larger than the
synchrotron photon energy density so that the SSC flux is greatly suppressed.
This indicates that the jet composition of GRB 221009A is likely
Poynting-flux-dominated.
