<![CDATA[Fraud detection in financial transactions remains a major challenge because fraudulent activities are extremely rare—often described as finding a “needle in a haystack”— and must be detected in real time. This study presents a hybrid feature engineering framework that integrates lightweight sequential indicators with Hidden Markov Model (HMM)-based behavioural features to improve accuracy and interpretability. Using the PaySim dataset containing 2.77 million transactions (0.2965% fraud), we extracted 22 sequential and 14 HMMbased features, from which 28 highly discriminative variables were retained. To address class imbalance, a batch-wise SMOTETomek approach was applied, expanding 1.94 million clean samples to 3.86 million balanced samples. Experimental results show that HMM-based features alone yield moderate performance (ROC AUC = 0.778, F2 = 0.051), but the combined ensemble of tuned XGBoost and LightGBM achieves superior accuracy (ROC AUC = 0.9983, F2 = 0.8431, MCC = 0.827). SHAP analysis identifies HMM-derived entropy and state likelihoods, together with transaction amount dynamics, as key predictors. The results demonstrate that optimized feature engineering plays a crucial role in achieving accurate, scalable, and interpretable fraud detection.]]>
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