Model predictions for selected gas-phase ¹²C/¹³C ratios as a function of distance from the protostar taken at a simulation time of 10⁴ yr. The constant ratio of 68 is indicated by the horizontal dashed black line. The vertical dotted black line separates the spatial scales discussed in this paper (R < 300 au) from the larger scales, which are not largely discussed but shown for completeness.