Built a .NET invoicing library with Claude Code over about a week. Zero dependencies, VAT and GST arithmetic, JSON and CSV export, the usual. By the third phase I had 202 tests passing on two target frameworks and green CI on three operating systems.
It would also have been wrong by 3p on any UK invoice with three items at £3.99.
Not wrong in some pathological edge case. Wrong on a completely normal invoice, at the most common price point in British retail, in a way that ends up in someone's VAT return.
I found it because I got suspicious of my own green checkmark.
Why agent-written tests lie to you
