  The prompt GRB emission is thought to arise from electrons accelerated in
internal shocks propagating within a highly relativistic outflow. The launch of
Fermi offers the prospect of observations with unprecedented sensitivity in
high-energy (>100 MeV) gamma-rays. The aim is to explore the predictions for HE
emission from internal shocks, taking into account both dynamical and radiative
aspects, and to deduce how HE observations constrain the properties of the
relativistic outflow. The emission is modeled by combining a time-dependent
radiative code with a dynamical code giving the evolution of the physical
conditions in the shocked regions.Synthetic lightcurves and spectra are
compared to observations. The HE emission deviates significantly from
analytical estimates, which tend to overpredict the IC component, when the time
dependence and full cross-sections are included. The exploration of the
parameter space favors the case where the dominant process in the BATSE range
is synchrotron emission. The HE component becomes stronger for weaker magnetic
fields. The HE lightcurve can display a prolonged pulse duration due to IC
emission, or even a delayed peak compared to the BATSE range.Alternatively,
having dominant IC emission in the BATSE range requires most electrons to be
accelerated into a steep power-law distribution and implies strong 2nd order IC
scattering. In this case, the BATSE and HE lightcurves are very similar. The
combined dynamical and radiative approach allows a firm appraisal of GRB HE
prompt emission. A diagnostic procedure is presented to identify from
observations the dominant emission process and derive constrains on the bulk
Lorentz factor, particle density and magnetic field of the outflow.
