  Classical novae are shock-powered multi-wavelength transients triggered by a
thermonuclear runaway on an accreting white dwarf. V1674 Her is the fastest
nova ever recorded (time to declined by two magnitudes is t_2=1.1 d) that
challenges our understanding of shock formation in novae. We investigate the
physical mechanisms behind nova emission from GeV gamma-rays to cm-band radio
using coordinated Fermi-LAT, NuSTAR, Swift and VLA observations supported by
optical photometry. Fermi-LAT detected short-lived (18 h) 0.1-100 GeV emission
from V1674 Her that appeared 6 h after the eruption began; this was at a level
of (1.6 +/- 0.4)x10^-6 photons cm^-2 s^-1. Eleven days later, simultaneous
NuSTAR and Swift X-ray observations revealed optically thin thermal plasma
shock-heated to kT_shock = 4 keV. The lack of a detectable 6.7 keV Fe K_alpha
emission suggests super-solar CNO abundances. The radio emission from V1674 Her
was consistent with thermal emission at early times and synchrotron at late
times. The radio spectrum steeply rising with frequency may be a result of
either free-free absorption of synchrotron and thermal emission by unshocked
outer regions of the nova shell or the Razin-Tsytovich effect attenuating
synchrotron emission in dense plasma. The development of the shock inside the
ejecta is unaffected by the extraordinarily rapid evolution and the
intermediate polar host of this nova.
