Definition
In programming, a side effect is any code that changes state outside its local scope. Writing to a database is a side effect. So is calling an external API, and so is emitting an analytics event.
Why side effects matter here
When an AI agent refactors a function, it reasons about the function's inputs, outputs, and logic. The side effects in the middle are not part of that reasoning. They can be moved, dropped, or modified without breaking the function's contract. So they often are.