Modafinil promotes wakefulness primarily through dopamine reuptake inhibition and downstream effects on orexin and histamine pathways. Caffeine blocks adenosine receptors. Because their mechanisms do not fully overlap, many users report that a small amount of caffeine on modafinil feels cleaner and more focused than either alone.
The problem is dose creep. Modafinil already raises baseline alertness, so the effective caffeine threshold drops. A second or third coffee that would normally feel routine can produce jitteriness, tachycardia, sweating or a sharp crash. The combination also increases the risk of headaches, dehydration and disrupted sleep if taken late.
If you choose to combine them, start with a fraction of your normal caffeine intake and keep it early in the day. Hydration and food become more important. For many people, the best stack is not more stimulants but better timing: modafinil for the deep work window and caffeine only for a short top up, if at all.