  Gamma-ray bursts (GRBs) are exceptionally bright electromagnetic events
occurring daily on the sky. The prompt emission is dominated by
X-/$\gamma$-rays. Since their discovery over 50 years ago, GRBs are primarily
studied through spectral and temporal measurements. The properties of the
emission jets and underlying processes are not well understood. A promising way
forward is the development of missions capable of characterising the linear
polarisation of the high-energy emission. For this reason, the SPHiNX mission
has been developed for a small-satellite platform. The polarisation properties
of incident high-energy radiation (50-600 keV) are determined by reconstructing
Compton scattering interactions in a segmented array of plastic and
Gd$_3$Al$_2$Ga$_3$O$_{12}$(Ce) (GAGG(Ce)) scintillators. During a two-year
mission, $\sim$200 GRBs will be observed, with $\sim$50 yielding measurements
where the polarisation fraction is determined with a relative error $\leq$10%.
This is a significant improvement compared to contemporary missions. This
performance, combined with the ability to reconstruct GRB localisation and
spectral properties, will allow discrimination between leading classes of
emission models.
