<p>The effectiveness of plant growth-promoting microorganisms (PGPMs) in agriculture depends on their formulation into stable, viable products. This study evaluated the stability and viability of three PGPMs-<i>Trichoderma harzianum</i> I 20, <i>Pseudomonas fluorescens</i> P-187, and <i>Bacillus subtilis</i> SWRIs-using various carrier-based and encapsulation formulations. A factorial experiment was conducted with eight formulations, stored at room and cold temperatures for up to six months. Significant interactions between formulation type, storage temperature, and duration were observed (<i>p</i> &lt; 0.01). After six months at room temperature, the biochar-trehalose-glycerol formulation maintained the highest populations for all three microorganisms. At cold room temperature, this formulation also provided the greatest stability for <i>T. harzianum</i> I 20 (4.33 × 10⁸ CFU/g) and <i>P. fluorescens</i> P-187 (6 × 10⁷ CFU/g), while <i>B. subtilis</i> SWRIs showed the least population reduction in formulation 7 (talc-trehalose-glycerol) with 5 × 10⁸ CFU/g. Although talc and biochar showed comparable overall effectiveness as carriers, biochar generally promoted better microbial survival. Glycerol enhanced the preservation of all three species at cold room temperature, whereas trehalose was more effective for <i>P. fluorescens</i> P-187 and <i>T. harzianum</i> I 20 stability at room temperature. These findings highlight the critical role of carrier selection and osmotic stabilizer inclusion in developing robust bioinoculant formulations for different storage conditions and target microorganisms.</p>