Solar thermochemical gasification is a promising solution for the clean production of low-emission synthetic fuels. It offers the possibility to upgrade various biomasses and waste feedstocks and further provides an efficient way to sustainably store solar energy into high-value and energy-intensive chemical fuels. In this work, a novel continuously-fed solar steam gasifier was studied using beechwood and solid recovered fuels (SRF) particles. Solar-only and hybrid solar/autothermal gasification experiments were performed at high temperatures to assess the performance of the reactor and its flexibility in converting various types of feedstocks. The hybrid operation was considered to increase the solar reactor temperature when the solar power input is not sufficient thanks to partial feedstock oxy-combustion. The hybrid solar process is thus a sustainable alternative option outperforming the conventional gasification processes for syngas production. Wood and waste particles solar conversion was successfully achieved, yielding high-quality syngas and suitable reactor performance, with Cold Gas Efficiencies (CGE) up to 1.04 and 1.13 respectively during the allothermal operation. The hybrid process allowed operating with a lower solar power input, but the H2 and CO yields noticeably declined. SRF gasification experiments suffered furthermore from ash melting/agglomeration issues and injection instabilities that undermined the continuity of the process. This study demonstrated the solar reactor flexibility in converting both biomass and waste feedstocks into syngas performed in continuous feeding operation. The experimental outcomes showed the feasibility of operating the reactor in both allothermal (solar-only) and hybrid allothermal/autothermal (combined solar and oxy-combustion heating) for continuous syngas production with high yields and energy conversion efficiencies.