I’m pretty senior, having been in the industry for some time, so I tend not to be asking the questions that will have a million examples out there

Me too, but this was C++ where there isn't a strong culture of making high quality libraries available for everything (because it doesn't have a proper package manager, at least until very recently), so you do end up having to reinvent the wheel a fair bit.

And sometimes you just need things a bit different to what other people have done. So even though there are a gazillion expression parsers out there (so the LLM understood it pretty well) there are hardly any that support 64-bit integers. But that's a small enough difference that it can deal with it.