  We analyze three scenarios to address the challenge of ultrafast gamma-ray
variability reported from active galactic nuclei. We focus on the energy
requirements imposed by these scenarios: (i) external cloud in the jet, (ii)
relativistic blob propagating through the jet material, and (iii) production of
high-energy gamma rays in the magnetosphere gaps. We show that while the first
two scenarios are not constrained by the flare luminosity, there is a robust
upper limit on the luminosity of flares generated in the black hole
magnetosphere. This limit depends weakly on the mass of the central black hole
and is determined by the accretion disk magnetization, viewing angle, and the
pair multiplicity. For the most favorable values of these parameters, the
luminosity for 5-minute flares is limited by $2\times10^{43}\rm\,erg\,s^{-1}$,
which excludes a black hole magnetosphere origin of the flare detected from
IC310. In the scopes of scenarios (i) and (ii), the jet power, which is
required to explain the IC310 flare, exceeds the jet power estimated based on
the radio data. To resolve this discrepancy in the framework of the scenario
(ii), it is sufficient to assume that the relativistic blobs are not
distributed isotropically in the jet reference frame. A realization of scenario
(i) demands that the jet power during the flare exceeds by a factor $10^2$ the
power of the radio jet relevant to a timescale of $10^8$ years.
