This book constitutes the refereed proceedings of the 12th International Conference on Rigorous State-Based Methods, ABZ 2026, held in Tokyo, Japan, during May 2026.The 15 full papers, 3 short papers and 2 ABZ 2026 case study papers presented in this book were carefully reviewed and selected from 25 submissions.The ABZ conference series is dedicated to the cross-fertilization of state-based and machine-based formal methods. Abstract State Machines (ASM), Alloy, B, TLA, VDM, and Z are examples of these methods. They share a common conceptual foundation and are widely used in both academia and industry for the rigorous design and analysis of hardware and software systems. The ABZ conferences aim to be a forum for the vital exchange of knowledge and experience among the research communities around different formal methods.
