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We extend the {\sl\,PARSEC} library of stellar evolutionary tracks bycomputing new models of massive stars, from 14\Msun to 350\Msun. The inputphysics is the same used in the {\sl\,PARSEC}~V1.1 version, but for themass-loss rate which is included by considering the most recent updates inliterature. We focus on low metallicity, $Z$=0.001 and $Z$=0.004, for which themetal poor dwarf irregular star forming galaxies, Sextans A, WLM and NCG6822,provide simple but powerful workbenches. The models reproduce fairly well theobserved CMDs but the stellar colour distributions indicate that the predictedblue loop is not hot enough in models with canonical extent of overshooting. Inthe framework of a mild extended mixing during central hydrogen burning, theonly way to reconcile the discrepancy is to enhance the overshooting at thebase of the convective envelope (EO) during the first dredge-UP. The mixingscales required to reproduce the observed loops, EO=2\HP or EO=4\HP, aredefinitely larger than those derived from, e.g., the observed location of theRGB bump in low mass stars. This effect, if confirmed, would imply a strongdependence of the mixing scale below the formal Schwarzschild border, on thestellar mass or luminosity. Reproducing the features of the observed CMDs withstandard values of envelope overshooting would require a metallicitysignificantly lower than the values measured in these galaxies. Otherquantities, such as the star formation rate and the initial mass function, areonly slightly sensitive to this effect. Future investigations will considerother metallicities and different mixing schemes.

New PARSEC evolutionary tracks of massive stars at low metallicity: testing canonical stellar evolution in nearby star forming dwarf galaxies