X-ray-to-radio ratios (and α_rx) of the extended jets observed by Chandra reported in the literature. The data are from: Marshall et al. (2018), circles at z ≲ 2; McKeough et al. (2016), squares at 2 ≲ z ≲ 4.7 (given by the sum of their ‘detected’ components); Siemiginowska et al. (2003), Cheung et al. (2006), Simionescu et al. (2016), Schwartz et al. (2020), Worrall et al. (2020), Snios et al. (2021); and Connor et al. (2021), diamonds at z ≳ 3. If the same object is reported in more than one work, we consider the most recent. PSO J0309+27 (z = 6.1) is represented by a red star and PKS J0637−752 (z = 0.65) by an orange cross. The solid red line connecting the two sources is the evolution of the X-ray-to-radio ratio as a function of redshift based on the model reported in Fig. 5, where the extended X-ray emission is given by the sum of a synchrotron (dashed black line) and an IC/CMB component (dotted blue line). The solid green line represents the limit above which an extended X-ray jet with the same radio luminosity as the one found in PSO J0309+27 can be observed with a 100 ksec Chandra exposure.