  We show that, contrary to common belief, internal shocks can arise in an
accelerating radiation-dominated jet if it is confined even weakly to a
converging opening angle because the acceleration declines. The
radiation-dominated internal shock (RDIS) enables a very efficient yet highly
nonthermal emission by Fermi-like photon acceleration, keeping the
electron-positron ($e^{\pm}$) pair photosphere and inertia up to a high Lorentz
factor >1000. In gamma-ray bursts (GRBs), a weak confinement would persist
beyond the progenitor star or surrounding matter because of the fast cocoon
accompanying the breakout jet. The simplest model predicts few high-energy
cosmic rays and neutrinos, and a correlation between the early afterglow and
the GeV-TeV prompt emission. The central engine allows a less fine-tuned baryon
load than previously thought, even including pure-leptonic unmagnetized
outflows.
