Make the bug louder

The default move with a bug is to read the code until the mistake jumps out, or to paste it into an AI assistant and ask what is wrong. Both work on small, honest bugs. On the rest, the stronger move comes first: build something that makes the bug impossible to miss, and only then change the code.

Four ways to turn up the volume

• Catch more versions. A good test fails on many different wrong programs, not just the one you have.
• Test a property, not an example. "Decoding what I encoded gives it back" holds for every input, so you can try thousands of them.
• Shrink the input. Cut a 500-line failing file down to the 3 lines that still fail.
• Bisect the history. If version 12 worked and version 76 does not, the first bad version is found in about six checks, not sixty-four.

The variants lessons in this module grade your test, not your fix. The course hides several broken versions of a function; your test is run against each one and against the correct one. It must pass the correct version and catch enough of the broken ones. When it misses one, the result names the area that version gets wrong, not the bug itself. This is a cut-down form of mutation testing.