Refactoring Without Tests: A Risky Business

Refactoring is one of the most valuable practices in modern software engineering. Done well, it keeps a codebase clean, comprehensible, and adaptable — reducing the technical debt that accumulates naturally over the lifecycle of any digital product. But there is a version of refactoring that is far less valuable, and considerably more dangerous: refactoring without an adequate test suite.
This article explores why untested refactoring is a significant risk, how that risk manifests in practice, and what teams can do to refactor safely — even when starting from a codebase with poor test coverage.

What Refactoring Is — and Is Not

Martin Fowler, whose work on the subject remains definitive, defines refactoring as ‘a disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behaviour.’ That final clause is the critical one.
Refactoring should not change what software does. It should change how it does it — improving readability, reducing duplication, simplifying logic, or enabling future extensibility. If a refactoring changes observable behaviour, it is not a refactoring; it is a modification, and it should be treated as one.
This distinction matters because it is the foundation of safe refactoring practice. If you know that a refactoring should not change behaviour, then you need a way to verify that it has not. Tests are that mechanism.

The Problem with Refactoring Without Tests

When there are no automated tests — or insufficient tests — to characterise the existing behaviour of a system, refactoring becomes an act of faith. The developer believes the behaviour has not changed, but they cannot verify it. This creates several compounding risks:

Silent Regressions

The most immediate risk is regression: a change that inadvertently breaks existing functionality. Without tests, regressions may not be detected until a user encounters them in production. In a complex system with many interdependencies, a seemingly localised change can have far-reaching consequences that are not immediately apparent.

Loss of Confidence

Teams without tests tend to become progressively more conservative about making changes. The codebase grows increasingly fragile not because of any single failure, but because nobody is willing to touch it for fear of breaking something. The irony is that this conservatism prevents the very improvements that would reduce fragility.

Accumulated Technical Debt

Refactoring is one of the primary tools for managing technical debt. Without it, debt accumulates. Without tests, refactoring cannot be done safely. The result is a kind of debt spiral: the codebase becomes harder to maintain, which makes testing harder to introduce, which makes refactoring riskier, which allows debt to compound further.

Impaired Onboarding and Knowledge Transfer

A codebase without tests is also a codebase without documentation of intended behaviour. New team members cannot interrogate the system by running tests and observing what they describe. They must instead rely on institutional knowledge, code comments, and inference — all of which are unreliable substitutes.

The Legacy Codebase Problem

It is all very well to advocate for tests, but many teams inherit codebases with little or no existing test coverage. The system works — after a fashion — but it has evolved organically, often without clear architectural boundaries, and writing meaningful tests for it is itself a substantial undertaking.

This is a genuine challenge, and it is one that requires a pragmatic rather than a purist response. A few principles apply:

• Do not attempt to test everything at once. Prioritise test coverage for the highest-risk areas: business-critical logic, complex integrations, and components that change frequently.
• Characterisation tests, sometimes called ‘golden master’ tests, can be a useful starting point. These capture the current — however imperfect — behaviour of the system, providing a safety net before any changes are made.
• Refactor incrementally. Small, contained changes with focused test coverage are far safer than large-scale restructurings. Each successful iteration builds confidence and coverage simultaneously.
• Treat test writing as part of the refactoring effort, not a precondition that must be achieved separately first.

A Responsible Approach

The responsible approach to refactoring can be summarised simply: never change code without understanding its current behaviour, and never rely solely on your own judgement to verify that behaviour has been preserved. Tests provide the verification layer that makes that confidence objective rather than subjective.
Where tests do not exist, the responsible path is to introduce them — strategically, incrementally, and in proportion to the risk of the changes being made — before proceeding with structural changes.

At COMMpla, we treat test coverage as a prerequisite for any substantive refactoring engagement. When we take on legacy systems, our first step is always an assessment of existing test infrastructure. The state of that infrastructure tells us as much about the risk profile of the project as any other factor.

Final Thoughts

Refactoring is not a luxury; it is a maintenance requirement for any codebase that needs to remain healthy over time. But refactoring without tests is not maintenance — it is a gamble. The risks are real, the costs of regressions are high, and the alternatives are available.

If your team is considering a refactoring effort on a codebase with limited test coverage, start with the tests. The investment will repay itself many times over.
To learn more about how COMMpla approaches legacy systems and technical debt reduction, visit commpla.com.