Keep an agent from tidying load-bearing legacy code

You inherited a 15-year-old Java monolith with no docs and few tests. It is full of code that looks wrong but makes up for something elsewhere: a sleep before a call, a field set twice, a catch that swallows one exception type. You want a coding agent to help modernize it, but the agent cannot tell a bug from a deliberate workaround.

How do you put the agent to work without breaking what holds the system up? Decide what it does first, what it may change, and what you check.