How do you implement Test Driven Development in an existing legacy codebase?

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mahek70 23 Jan 2026

Implementing Test Driven Development (TDD) in a legacy codebase requires a gradual and controlled approach. Legacy systems often lack test coverage, have tight coupling, and contain undocumented dependencies, making direct adoption of TDD impractical.

To implement Test Driven Development in an existing legacy codebase, start by writing characterization tests to capture current behavior. Focus on high-risk or frequently changed modules and add tests at system boundaries like APIs or service layers. When fixing bugs or adding features, follow the Test Driven Development cycle—write a failing test first, implement minimal code, then refactor safely. Use mocking and dependency isolation to handle tightly coupled components. Over time, this approach improves code quality without requiring a full rewrite.

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