31. 12. 2025 Alessandro Taufer Development, DevOps

What tests can tell you about your codebase

Tests are often treated as a safety net: something that catches bugs before they reach users. While that’s true, it’s only part of the story. A test suite is also a mirror. If you look closely, it tells you a lot about the structure, health, and long-term maintainability of your codebase.

If you know how to read it, your tests can guide you toward understanding your system better and toward improving it.

So, let’s take a look at what your tests can unintentionally reveal about your code.

Change Sensitivity

One of the clearest signals tests give you is how tightly coupled your code is.

If a small, localized change in production code causes dozens of seemingly unrelated tests to fail, that’s rarely a testing problem. It’s a design problem. It usually means that components know too much about each other, or that internal details are leaking across boundaries.

In a well-structured system, most changes should have a limited blast radius. You expect a few tests to fail: those that directly exercise the modified behavior, not half the suite.

Heavy mocking is another warning sign. Mocking itself isn’t bad, but when tests are full of mocks for deep dependency chains, it often means:

Components are tightly intertwined, making isolation difficult
Responsibilities aren’t clearly separated, so tests need to recreate context

In many cases, heavy mocking is just compensating for design that’s hard to test. It’s a signal that boundaries could be clearer and the code simplified.

The Distribution of Test Types Tells a Story

Beyond individual test quality, the distribution of test types across your codebase reveals how your system is shaped and where complexity lives. Looking at the balance between unit, integration, and end-to-end tests is like reading a topographical map of your software.

Unit-heavy codebases usually indicate clear boundaries and well-defined responsibilities. When most logic can be verified with small, fast tests, it suggests that components are loosely coupled and behavior can be reasoned about in isolation.

Integration-heavy test suites often show that important behavior only emerges when components interact, making isolation harder. Relying too much on these tests can indicate blurred boundaries or tightly interwoven responsibilities.

End-to-end–dominated suites typically signal high system complexity or a lack of trust in lower-level tests. They offer coverage but are slow, brittle, and less helpful at pinpointing failures, often highlighting fragile interfaces or scattered business rules.

The proportion between these test types is just as revealing. Few unit tests can mean logic is trapped in frameworks or infrastructure. Few integration tests may suggest overconfidence in mocks. Few end-to-end tests can indicate blind spots in real-world workflows.

When the balance feels off, it’s rarely a testing problem alone: it’s usually the architecture quietly asking for attention.

Gaps in Tests Reveal Knowledge Gaps

Even the absence of tests can reveal something important about your code. Untested areas are rarely random. They tend to cluster where knowledge is fragile:

Outdated, unmaintained legacy code
Features rushed out the door under deadline pressure
Parts of the system that are informally ‘owned’ by a single team member, making the team dependent on them

Tests do more than confirm that code works: they preserve context, intent, and decisions that would otherwise fade. When tests are missing, it’s often not just coverage that’s absent, but shared understanding itself.

Conclusions

So, at the end of this article, I hope you’ll look at your test suite a little differently. Not just as a checklist for coverage or a safety net against regressions, but as a source of insight into your system’s design, its weak spots, and its strengths.

Next time, instead of asking “Do we have enough tests?”, we might reframe the conversation and ask a better question: “What are our tests telling us?”

Interested in further reading? You might find this article useful!