Active fluids are ubiquitous in biology—their ability to convert free energy supplies into work is a valuable property that can be leveraged to create novel and functional materials, such as living droplets. However, characterizing active fluids poses serious computational and experimental hurdles, primarily due to their existence outside of equilibrium, making classical thermodynamic descriptions inapplicable. Here, we propose a minimalist fluid model that mimics active biological fluids through simple, stochastic particle interconversion events. This model comprises two size-asymmetric particle types, modeled using shifted Lennard-Jones potentials, in an equimolar ratio within a molecular dynamics simulation utilizing a Brownian integrator. Interconversion-driven activity is implemented through a Monte Carlo process that interconverts particle types and sizes with a tunable probability. By systematically varying the interconversion probability and system temperature, we explore the diverse phase behaviors of active fluids, identify non-equilibrium mixing regimes, and pinpoint the emergence of activity-induced spinodal architectures. Our active fluid model thus uncovers the essential physical principles by which generic interconversion reactions can regulate phase composition and morphology in a system exhibiting liquid–liquid phase separation—offering design rules for active soft matter systems.