I Had 93% Test Coverage. Then I Ran Mutation Testing.

📰 Dev.to · jghiringhelli

Learn how mutation testing can reveal hidden bugs in your code despite high test coverage and how to close the gap between line coverage and mutation score

intermediate Published 19 Mar 2026
Action Steps
  1. Run mutation testing on your code to identify gaps in test coverage
  2. Analyze the mutation score to detect potential bugs
  3. Compare line coverage and mutation score to identify areas for improvement
  4. Write additional tests to close the gap between line coverage and mutation score
  5. Refactor code to make it more testable and reduce the mutation score gap
Who Needs to Know This

Developers and QA engineers can benefit from understanding the difference between line coverage and mutation score to improve their testing strategies and reduce bugs in their code

Key Insight

💡 A high line coverage percentage doesn't guarantee bug-free code, as mutation testing can reveal gaps in detection

Share This
🚨 93% test coverage isn't enough! 🚨 Learn how mutation testing can reveal hidden bugs and improve your code quality

Key Takeaways

Learn how mutation testing can reveal hidden bugs in your code despite high test coverage and how to close the gap between line coverage and mutation score

Full Article

Line coverage measures execution. Mutation score measures detection. A 34-point gap between them is where bugs survive CI. If your numbers are already close, be proud. Here is what the gap means and how to close it.
Read full article → ← Back to Reads

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