  The gamma-ray emission offers a powerful diagnostic tool to probe jets and
their surroundings in flat spectrum radio quasars (FSRQ). In particular,
sources emitting at high energies (>10 GeV) give us the strongest constraints.
This motivates us to start a systematic study of flares with bright emission
above 10 GeV, examining archival data of Fermi-LAT gamma-ray telescope. At the
same time, we began to trigger Target of Opportunity observations to the Swift
observatory at the occurrence of high-energy flares, obtaining a wide coverage
of the spectral energy distributions for several FSRQs during flares. Among the
others we investigate the SED of a peculiar flare of 3C 454.3, showing a
remarkable hard gamma-ray spectrum, quite different from the brightest flares
of this source, and a bright flare of CTA 102. We modeled the SED in the
framework of the one--zone leptonic model, using also archival optical
spectroscopic data to derive the luminosity of the broad lines and thus
estimate the disk luminosity, from which the structural parameters of the FSRQ
nucleus can be inferred. The model allowed us to evaluate the magnetic field
intensity in the blazar zone, and to locate the emitting region of gamma rays
in the particular case in which gamma-ray spectra show neither absorption from
the BLR, nor the Klein-Nishina curvature expected in leptonic models assuming
the BLR as source of seed photons for the External Compton. For FSRQs bright
above 10 GeV, we where able to identify short periods lasting less than 1 day
characterized by high rate of high energy gamma rays, and hard gamma-ray
spectra. We discussed the observed spectra and variability timescales in terms
of injection and cooling of energetic particles, arguing that these flares
could be triggered by magnetic reconnections events or turbulence in the flow.
