The timing of rainy-season onset and cessation remains essential for characterising seasonal rainfall variability and supporting climate-informed planning in West Africa. This study examines the sensitivity of rainy-season onset diagnostics to the choice of definition and assesses variations in common cessation dates, rainy-season duration, and interannual variability within the bimodal rainfall regime of Lamto, Côte d’Ivoire, using daily rainfall data from 1964 to 2023. Time series, Mann–Kendall statistics, and Sen’s slope were used to analyse interannual variability and long-term trends in onset, cessation, and rainy-season duration. To assess methodological sensitivity, onset dates under four definitions (AGRHYMET, Stern, Sivakumar, and Omotosho) were compared for timing, dispersion, agreement, and false starts. The results showed important differences among these methods in the timing of the onset of the first rainy season (March-July) and indicated that onset dates ranged from 19 March (AGRHYMET) to 20 April (Stern), with Omotosho and Sivakumar reporting intermediate dates (30 March and 8 April) within that range. The recorded mean cessation date is 24 July (between July 14 and August 16), and the season length falls between 95 and 127 days, depending on the method of onset. In the second rainy season (September to November), the methods showed better agreement, with mean onset dates between 11 and 17 September and a cessation around 25 November. Thus, its duration was shorter, about 70 to 75 days, with more consistency across methods. The analysis of long-term trends reveals no significant changes in the first rainy season’s onset and cessation, while there is a weak tendency towards later onset, with no statistically significant trends for the second rainy season. Furthermore, the analysis of sensitivity and agreement across rainy seasons shows a clear seasonal difference: the first rainy season showed greater variability, weaker agreement, and a higher likelihood of false starts among methods, whereas the second season appeared more stable and consistent regardless of the definition used. Therefore, these results highlight the importance of selecting appropriate onset definitions for impact assessment, especially for the first rainy season, and suggest that the second rainy season provides a more reliable onset signal across methods. Future work should focus on understanding the mechanisms underlying seasonal transitions and improving understanding of onset behaviour in West Africa’s transitional climate zones.