Does 100% Code Coverage Mean Tested?

A green badge. A passing pipeline. A dashboard that says 100%. For engineering teams, few numbers feel as reassuring as complete code coverage. But that confidence, it turns out, may be one of the most expensive illusions in software development.

Code coverage tools track which lines of code were executed during testing. What they cannot tell you is whether any of those lines actually worked correctly. A test can run a payment processing function, trigger every line, and still completely miss a decimal point error that charges customers 29% instead of 2.9%. The coverage report stays green. The customer gets a shocking bill.

The deeper problem is that coverage targets create perverse incentives. Engineers optimize for the number, not the evidence.

A more meaningful approach starts before a single test is written: defining what the system must do, what it must never do, and what the consequences of failure would be. That inventory, not a percentage, is the foundation of software quality.