Modern agriculture relies on synthetic agrochemicals, but regulatory pressure, environmental costs, and supply chain volatility are accelerating interest in microbial-based agrochemicals (MBAs) as renewable inputs for sustainable intensification. This review frames MBAs through a circular economy lens, highlighting agricultural byproducts as fermentation feedstocks and formulation carriers to reduce cost and footprint while enabling scale-up. We compare cell-in products, which require viable microorganisms to survive processing, storage, and field stresses and to establish in target niches, versus cell-free products, which depend on maintaining the activity and residence time of metabolites, enzymes, and extracts despite degradation, dilution, and adsorption. We summarize submerged and solid-state fermentation and downstream routes to biomass or clarified bioactive fractions and link formulation families (liquids, solids, granules, encapsulated/controlled-release, immobilized) to delivery routes (seed, soil, foliar, fertigation). We highlight critical quality attributes, release criteria, and validation metrics connecting process control to field performance, then outline barriers (feedstock heterogeneity, batch variability, compatibility, context dependence) and priorities for standardization, mechanism-based stabilization/controlled release, and multienvironment validation.