How do you prevent coding agents from aggressively over-engineering simple bug fixes?

I asked an agent to patch a simple null-pointer exception in a login handler. Instead of a quick three-line fix, it spun up an entire abstract factory pattern, created custom error classes, and restructured the whole authentication pipeline.

For anyone deploying AI agents on day to day bug fixes how do you keep the scope minimal and prevent them from turning a tiny patch into a massive refactor?

What prompts or rules do you use to force AI tools to write the simplest possible fix instead of over-complicating things?