No it didn't make a giant plan of every detail. It made a plan of the core concepts and then when it was in implementation mode it kept checking the excel file to get more info. It took around ~30 mins in implementation mode to build it.

I was impressed because the prompt didn't ask it to do that. It doesn't normally add tests for me without asking, YMMV.

Ah, I see.

Did it build a test suite for the Excel side? A fuzzer or such?

It's the cross-concern interactions that still get me.

80% of what I think about these days when writing software is how to test more exhaustively without build times being absolute shit (and not necessarily actually being exhaustive anyway).