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The 100% Coverage Lie

intermediate9 min90 pts

Your team asked AI to write tests for the payment service. The coverage report shows 100% line coverage and all tests pass. Your tech lead is skeptical and drops this in Slack:

“Coverage is green but I don't trust these tests. Can you review the test file and tell me if these tests actually catch real bugs?”

What you're being evaluated on

Circular testing detection - Tests that assert mocks return what they were told to return prove nothing.

Missing coverage - Happy paths only. Where are the edge cases, error paths, and boundary values?

Test quality over quantity - Naming, assertions, and async handling that silently pass without testing anything.

3 hints available (using hints reduces your score)

About this scenario

An AI generated a test suite with 100% code coverage, but the tests assert that mocks return their own setup values and prove nothing about real behavior. Practice spotting circular tests, vacuous assertions, missing error scenarios, and test data too uniform to catch bugs. Learn why high coverage from AI-generated tests does not mean correctness.

intermediate · 90 points · 9 min · AI Code Review

What you will practice

Circular testing detection - Tests that assert mocks return what they were told to return prove nothing.
Missing coverage - Happy paths only. Where are the edge cases, error paths, and boundary values?
Test quality over quantity - Naming, assertions, and async handling that silently pass without testing anything.

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