Sorption-enhanced reforming and gasification (SERG) offers a promising approach to intensify hydrogen production from carbonaceous feedstocks. However, conventional sorbents require substantial temperature increases for the endothermic CO2 release step and are prone to deactivation. This study introduces a new class of redox-activated sorbents capable of stable isothermal operation and tunable heats of reactions, thereby facilitating an efficient reactive separation scheme. Using plane-wave density functional theory (DFT) calculations of structures and free energies, we screened 1225 perovskite-structured sorbent candidates, subsequently subjecting selected candidates to experimental validation. These advanced sorbents showed reversible, isothermal carbonation, permitting isothermal SERG or iSERG. Up to 78% reversible carbonation of the A-site cation was demonstrated. Additionally, an effective descriptor, (ΔGabs + ΔGreg), was identified to expedite the optimization of sorbents, displaying a strong correlation with their CO2 capacity. The versatility of these sorbents was demonstrated in a fluidized bed for woody biomass gasification and a packed bed for biogas conversion, yielding hydrogen-enriched (76 vol.%) syngas from biomass and 95+% pure H2 from biogas. Furthermore, employing concentrated O2 in the regeneration step can facilitate integrated CO2 capture, leading to negative life-cycle CO2 emission.