<p>All-solid-state batteries offer high-energy-density and eco-friendly energy storage but face commercial hurdles due to dendrite formation, especially with lithium metal anodes. Here we report that dendrite formation in Li/Li₇La₃Zr₂O₁₂/Li batteries occurs via two distinct mechanisms, using non-invasive solid-state nuclear magnetic resonance and magnetic resonance imaging. Tracer-exchange nuclear magnetic resonance shows non-uniform Li plating at electrode–electrolyte interfaces and local Li⁺ reduction at Li₇La₃Zr₂O₁₂ grain boundaries. In situ magnetic resonance imaging reveals rapid dendrite formation via non-uniform Li plating, followed by sluggish bulk dendrite nucleation from Li⁺ reduction, with an intervening period of stalled growth. Formation of amorphous dendrites and subsequent crystallization, the defect chemistry of solid electrolytes and battery operating conditions play a critical role in shaping the complex interplay between the two mechanisms. Overall, this work deepens our understanding of dendrite formation in solid-state Li batteries and provides comprehensive insight that might be valuable for mitigating dendrite-related challenges.</p>