  Early observations of transient explosions can provide vital clues to their
progenitor origins. In this paper we present the nearby Type Iax (02cx-like)
supernova (SN), SN 2020udy that was discovered within hours ($\sim$7 hr) of
estimated first light. An extensive dataset of ultra-violet, optical, and
near-infrared observations was obtained, covering out to $\sim$150 d after
explosion. SN 2020udy peaked at -17.86$\pm$0.43 mag in the r band and evolved
similarly to other 'luminous' SNe Iax, such as SNe 2005hk and 2012Z. Its
well-sampled early light curve allows strict limits on companion interaction to
be placed. Main-sequence companion stars with masses of 2 and 6 M$_\odot$ are
ruled out at all viewing angles, while a helium-star companion is allowed from
a narrow range of angles (140-180$^\circ$ away from the companion). The spectra
and light curves of SN2020udy are in good agreement with those of the 'N5def'
deflagration model of a near Chandrasekhar-mass carbon-oxygen white dwarf.
However, as has been seen in previous studies of similar luminosity events, SN
2020udy evolves slower than the model. Broad-band linear polarisation
measurements taken at and after peak are consistent with no polarisation, in
agreement with the predictions of the companion-star configuration from the
early light curve measurements. The host galaxy environment is low metallicity
and is consistent with a young stellar population. Overall, we find the most
plausible explosion scenario to be the incomplete disruption of a CO white
dwarf near the Chandrasekhar-mass limit, with a helium-star companion.
