Oversized untested core module

What it is

A module central to the application has grown large — many hundreds of lines with several responsibilities — and has little or no test coverage.

Why it matters

Size and untestedness compound. The file is where the important logic lives, it is the hardest to change safely, and its size is precisely what stopped anyone writing tests for it.

How to fix it

Extract the pure logic into functions you can test without setup, and cover those first. Splitting the file follows naturally once the seams are visible, and is much safer once the tests exist.