  We present a combined strong and weak gravitational-lensing analysis of the
massive galaxy cluster MACS J1423.8+2404 ($z=0.545$, MACS J1423 hereafter), one
of the most dynamically relaxed and massive cool-core clusters discovered in
the MAssive Cluster Survey at $z>0.5$. We combine high-resolution imaging from
the Hubble Space Telescope (HST) in the F606W, F814W, and F160W pass-bands with
spectroscopic observations taken as part of the KALEIDOSCOPE survey with the
Multi-Unit Spectroscopic Explorer mounted on the Very Large Telescope. Our
strong lensing analysis of the mass distribution in the cluster core is
constrained by four multiple-image systems (17 individual images) within
redshift range $1.779<z<2.840$. Our weak-lensing analysis of the cluster
outskirts, confined to the HST field of view, is based on a background galaxy
catalogue with a density of 57 gal.arcmin$^{-2}$. We measure a projected mass
of M($\textrm{R}<200$ kpc) = (1.6 $\pm$ 0.05) $\times$ 10$^{14}$ M$_{\rm\odot}$
from our strong-lensing model, and a projected mass of M($\textrm{R}<640$ kpc)
= (6.6 $\pm$ 0.6) $\times$ 10$^{14}$ M$_{\rm\odot}$ when combining with our the
weak-lensing constraints. Our analysis of the cluster mass distribution yields
no evidence of substructures, confirming the dynamically relaxed state of MACS
J1423. Our work sets the stage for future analysis of MACS J1423 in the
upcoming Canadian Near Infrared Imager and Stiltless Spectrograph Unbiased
Cluster Survey for the James Webb Space Telescope.
