<![CDATA[General Background: Indonesia entered a major criminal law transition on 2 January 2026 through the effective implementation of the National Criminal Code (Law No. 1 of 2023) and the 2025 Criminal Procedure Code (Law No. 20 of 2025). Specific Background: This transition raises juridical issues concerning cases involving acts committed before 2 January 2026 while judicial proceedings remain ongoing, particularly regarding the application of provisions that are more favorable to defendants. Knowledge Gap: Previous studies have predominantly examined the formation, legal policy, and substantive aspects of the National Criminal Code, while empirical examination of transitional provisions through actual post-implementation judicial decisions remains limited. Aims: This study analyzes the normative construction of lex favor reo under Articles 2, 3, and 618 of the National Criminal Code and the procedural transition under Article 361 of the 2025 Criminal Procedure Code, as well as their application in narcotics cassation cases. Results: Using normative legal research with statutory and case approaches, the study qualitatively analyzes legislation, Supreme Court Circular Letter No. 1 of 2026, and relevant narcotics cassation decisions from January to early March 2026. The findings show that the Supreme Court distinguishes substantive and procedural criminal law: substantive provisions follow lex favor reo, whereas procedural provisions depend on the stage of defendant examination. Novelty: The study uses actual cassation decisions from the initial months of the transition to examine judicial reasoning in practice. Implications: Potential disparities in lower courts, uneven institutional readiness, and uncertainty concerning the commencement of defendant examination require continued judicial training and compilation of transitional jurisprudence. Highlights: Substantive and procedural criminal law follow different transitional rules. The Supreme Court generally maintained this distinction in narcotics cassation decisions. Transitional implementation still presents interpretive and judicial consistency challenges. Keywords: Principle of Retroactivity, National Criminal Code, 2025 Code of Criminal Procedure, Transition Period, Lex Favor Reo]]>
