I'm Just a Shopkeeper, But I Saved the Dynasty

A man transmigrates to the Tang Dynasty and opens a small shop in the capital while waiting for his system to activate. When famine strikes, the emperor visits his shop in disguise — and the shopkeeper casually gives him a pesticide formula and a way to eat locusts. Soon after, he enters the court, solves a livestock crisis, develops gunpowder weapons to defeat half a million invaders, and uses vaccines to stop a deadly plague. The emperor makes him a top official. Thirty years later, he watches the golden age he built, completes his final mission, and quietly vanishes.