  Decades of astrophysical observations have convincingly shown that soft X-ray
(SXR; ~0.1--10 keV) emission provides unique diagnostics for the high
temperature plasmas observed in solar flares and active regions. SXR
observations critical for constraining models of energy release in these
phenomena can be provided using instruments that have already been flown on
sounding rockets and CubeSats, including miniaturized high-resolution
photon-counting spectrometers and a novel diffractive spectral imager. These
instruments have relatively low cost and high TRL, and would complement a wide
range of mission concepts. In this white paper, we detail the scientific
background and open questions motivating these instruments, the measurements
required, and the instruments themselves that will make groundbreaking progress
in answering these questions.
