I feel like I've opened quite a few issues lately, but this bug deserves a dedicated issue. I've been a heavy xTB user for over a year, and the C API is a basic requirement to interface with programming languages besides Fortran.

I got the program to compile on MSYS2, reproducing the .github/workflows case. All tests passed, except the C API test. It was also mentioned that the 6.7.1 release "excluded" C-API tests on Windows.

Also related: #1315 (comment). In this issue, I was trying to "improve speed" by linking with MKL on Windows. However, even with OpenBLAS, the whole library would not work at all. I didn't investigate runtime loading with LoadLibraryA, but this already reveals a lack of proper C API functionality for users. I've gotten DirectXShaderCompiler and FidelityFX integrated into my application, and they both host convenient, pre-compiled pairs of .dll + .lib. It ought to be just as easy as snatching the .dylib from the macOS Homebrew installation.

This is of particularly high importance to my use case. Unlike the other issues I've opened, which are about speed or undesired file writing ("nice to haves"), in this issue the entire thing does not work at all.