  We discuss implications that can be obtained by searches for neutrinos from
the brightest gamma-ray burst, GRB 221009A. We derive constraints on GRB model
parameters such as the cosmic-ray loading factor and dissipation radius, taking
into account both neutrino spectra and effective areas. The results are strong
enough to constrain proton acceleration near the photosphere, and we find that
the single burst limits are comparable to those from stacking analysis.
Quasithermal neutrinos from subphotospheres and ultrahigh-energy neutrinos from
external shocks are not yet constrained. We show that GeV-TeV neutrinos
originating from neutron collisions are detectable, and urge dedicated analysis
on these neutrinos with DeepCore and IceCube as well as ORCA and KM3NeT.
