Small planets on close-in orbits tend to exhibit envelope mass fractions of either effectively zero or up to a few percent depending on their size and orbital period. Models of thermally-driven atmospheric mass loss and of terrestrial planet formation in a gas-poor environment make distinct predictions regarding
the location of this rocky/non-rocky transition in period-radius space. Here we present the confrmation of TOI-1235 b (P = 3:44 days, rp = 1:738+0:087
-0:076 R⊕), a planet whose size and period are intermediate between the competing model predictions thus making the system an important test case
for emergence models of the rocky/non-rocky transition around early M dwarfs (Rs = 0:630 ± 0:015 RΘ, Ms = 0:640 ± 0:016 MΘ). We confirm the TESS planet discovery using reconnaissance spectroscopy, ground-based photometry, high-resolution imaging, and a set of 38 precise radial-velocities from HARPS-N and HIRES. We measure a planet mass of 6:91+0:75-0:85 M⊕, which implies an iron core mass fraction of 20+15-12% in the absence of a gaseous envelope. The bulk composition of TOI-1235 b is therefore consistent with being Earth-like and we constrain a H/He envelope mass fraction to be < 0:5% at 90% confidence. Our results are consistent with model predictions from thermally-driven
atmospheric mass loss but not with gas-poor formation, suggesting that the former class of processes remain effocient at sculpting close-in planets around early M dwarfs. Our RV analysis also reveals a strong periodicity close to the first harmonic of the photometrically-determined stellar rotation period
that we treat as stellar activity, despite other lines of evidence favoring a planetary origin (P = 21:8+0:9-0:8 days, mp sin i = 13:0+3:8-5:3 M⊕) that cannot be firmly ruled out by our data.