This study examined the effects of adding CUB alongside HCN sources from fresh cassava diets on HCN reduction, gas production, and in vitro digestibility. A completely randomized design (CRD) with a 2 × 2 × 3 + 1 factorial approach was used, where Factor A was the HCN source [fresh cassava root (FCR) or leaf (FCL)], Factor B was the HCN concentration (300 and 600 mg/kg dry matter (DM)), and Factor C was the bacterial supplement [no-CUB, E. faecium KKU-BF7 (CUB1), and E. gallinarum KKU-BC15 (CUB2)]. Statistical analysis was performed using the PROC GLM procedure in SAS. No interaction was observed among the main factors on gas kinetics and cumulative gas (p > 0.05). The addition of CUB1 or CUB2 enhanced cumulative gas production compared to the no-CUB group (p = 0.04). Cyanide degradation efficiency was high when FCR was included at a high HCN level. At 12 h post-incubation, HCN degradation efficiency was higher in the CUB2 and CUB1 groups, reaching 98.44–99.07% compared to the no-CUB group. The higher HCN level increased in vitro acid detergent fiber digestibility (IVADFD) (p = 0.01) by 7.20% compared to the low HCN level, and CUB2 further improved IVADFD. Compared to the FCL-fed group, FCR supplementation increased total VFA concentration (p = 0.03) and propionic acid (C3) concentration (p = 0.04). The addition of CUB2 further enhanced propionic acid concentration by 8.97% compared to no-CUB supplementation (p = 0.04). These results indicate that supplementing E. gallinarum KKU-BC15 at the highest HCN levels in FCR boosts HCN degradation efficiency, fiber digestibility, total VFA, and C3 concentration.