Safe Refactoring with AI Agents

An agent can quietly change behavior instead of just structure during a refactor — tests and small steps prevent that.

What refactoring actually means

Refactoring means improving code's internal structure without changing its externally visible behavior. The term was popularized by Martin Fowler's book of the same name. The "without changing behavior" part is the crucial bit — and it's exactly what gets lost with AI agents if nobody insists on it.

Why agents slip up here

While restructuring, an agent often spots things that look "wrong" to it — an odd condition, a duplicate check. Without a clear instruction, it fixes that along the way instead of just restructuring. A pure refactor quietly turns into a behavior change.

The three key safeguards

Tests first: before restructuring, a test suite must exist that locks in current behavior. Without tests you can't verify whether anything changed.
Small steps: one change per step, run tests after each step, commit each green step separately.
Demand behavior preservation explicitly: say "refactor without changing behavior" instead of just "make this code better."

Typical traps

The agent silently changes a public interface, deletes code that looks dead to it but is actually called dynamically, or mixes refactoring with feature requests in one pass.

EXAMPLE

Prompt: 'Extract the validation logic from processOrder() into its own function, validateOrder(). Behavior must not change. After extracting, run the existing test suite and stop to report if any test fails before continuing.'

🛠️ EXERCISE — TRY IT YOURSELF

Take a function from one of your own projects that does too much at once, and have an agent refactor it in small, test-backed steps.

Check first whether a test covers the function. If not, have a test written first that locks in current behavior.
Ask the agent explicitly: 'Extract [part X] into its own function, behavior must not change. Run tests after each step.'
Read the diff before committing — look specifically for changes that go beyond pure restructuring.

✅ SELF-CHECK

☐ Are all tests still green after the refactor — and had that already been checked beforehand?
☐ Does the diff contain any line that doesn't just move code, but changes a condition, a value, or an error message?
☐ Was everything committed in one shot, or could individual steps be traced separately?

QUICK QUIZ

What's the single most important prerequisite before letting an AI agent do a significant refactor on existing code?