Fabricating full oxide garnet type Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO)-based solid-state batteries has posed challenges, particularly in cosintering cathode composites. In this research, we achieve high-performance cathode composites through ultrafast cosintering, facilitated by residual lithium as a sintering agent under an O₂ atmosphere. These composites demonstrate compatibility with various cathode materials including LiCoO₂ and LiNi_1/3Co_1/3Mn_1/3O₂ in an LLZTO-based composite. Significantly, our findings reveal that residual stress on the cathode active material plays a pivotal role in degradation during cycling. The rigid LLZTO framework constrains volume changes in the cathode material during (de)lithiation, leading to mechanical failure. This discovery challenges prior assumptions about the primary susceptibility of the cathode/electrolyte interface to electro-chemo-mechanical failure. Furthermore, stress release mechanisms are found to be influenced by the particle morphology of the cathode material, whether single crystalline LiCoO₂ or polycrystalline LiNi_1/3Co_1/3Mn_1/3O₂. These insights underscore the importance of managing residual stress and optimizing cathode material morphology for achieving stable performance in full oxide LLZTO-based solid-state batteries.