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We cross-check the Hubble Space Telescope (HST) Cepheid/Type Ia supernova (SN Ia) distance ladder, which yields the most precise local H _0 , against early James Webb Space Telescope (JWST) subsamples (∼1/4 of the HST sample) from SH0ES and CCHP, calibrated only with NGC 4258. We find HST Cepheid distances agree well (∼1 σ ) with all combinations of methods, samples, and telescopes. The comparisons explicitly include the measurement uncertainty of each method in NGC 4258, an oft-neglected but dominant term. Mean differences are ∼0.03 mag, far smaller than the 0.18 mag “Hubble tension.” Combining all measures produces the strongest constraint yet on the linearity of HST Cepheid distances, 0.994 ±0.010, ruling out distance-dependent bias or offset as the source of the tension at ∼7 σ . However, current JWST subsamples produce large sampling differences in H _0 whose size and direction we can directly estimate from the full HST set. We show that Δ H _0 ∼ 2.5 km s ^−1 Mpc ^−1 between the CCHP JWST program and the full HST sample is entirely consistent with differences in sample selection. We combine all JWST samples into a new distance-limited set of 16 SNe Ia at D ≤ 25 Mpc. Using JWST Cepheids, JAGB, and tip of the red giant branch, we find 73.4 ± 2.1, 72.2 ± 2.2, and 72.1 ± 2.2 km s ^−1 Mpc ^−1 , respectively. Explicitly accounting for common supernovae, the three-method JWST result is H _0 = 72.6 ± 2.0, similar to H _0 = 72.8 expected from HST Cepheids in the same galaxies. The small JWST sample trivially lowers the Hubble tension significance due to small-sample statistics and is not yet competitive with the HST set (42 SNe Ia and 4 anchors), which yields 73.2 ± 0.9. Still, the joint JWST sample provides important cross-checks that the HST data pass.

We present phase-corrected photometric measurements of 88 Cepheid variables in the core of the Small Magellanic Cloud (SMC), the first sample obtained with the Hubble Space Telescope's (HST) Wide Field Camera 3, in the same homogeneous photometric system as past measurements of all Cepheids on the SH0ES distance ladder. We limit the sample to the inner core and model the geometry to reduce errors in prior studies due to the nontrivial depth of this cloud. Without crowding present in ground-based studies, we obtain an unprecedentedly low dispersion of 0.102 mag for a period–luminosity ( P – L ) relation in the SMC, approaching the width of the Cepheid instability strip. The new geometric distance to 15 late-type detached eclipsing binaries in the SMC offers a rare opportunity to improve the foundation of the distance ladder, increasing the number of calibrating galaxies from three to four. With the SMC as the only anchor, we find H _0 = 74.1 ± 2.1 km s ^−1 Mpc ^−1 . Combining these four geometric distances with our HST photometry of SMC Cepheids, we obtain H _0 = 73.17 ± 0.86 km s ^−1 Mpc ^−1 . By including the SMC in the distance ladder, we also double the range where the metallicity ([Fe/H]) dependence of the Cepheid P – L relation can be calibrated, and we find γ = −0.234 ± 0.052 mag dex ^−1 . Our local measurement of H _0 based on Cepheids and Type Ia supernovae shows a 5.8 σ tension with the value inferred from the cosmic microwave background assuming a Lambda cold dark matter (ΛCDM) cosmology, reinforcing the possibility of physics beyond ΛCDM.