Henry Schreiner

Have you activated all verbosity options? `SKBUILD_BUILD_VERBOSE=1` and `SKBUILD_LOGGING_LEVEL=DEBUG` are the envvar versions. Comparing a working and failing build would be very interesting! Using `#include "../"` is somewhat poor form,...

> appropriate way of setting that? It's a C define, not a CMake define. It needs to be set on building any file that includes Python.h.

I could be wrong, but it seems like the broken one has a `//?/` prefix: ``` -- Disabling Python GIL -- Found Python: \\?\C:\Users\runneradmin\AppData\Local\uv\cache\builds-v0\.tmp3yRqOT\Scripts\python.exe (found suitable version "3.13.0", minimum required...

Played around on a windows box; looks like `//?/` still doesn't care about the type of slash, and `\\?\` is equivalent, but it does break `..` handling (as it is...

Could you try this branch of pybind11 without the `..`'s? `"pybind11 @ git+https://github.com/henryiii/pybind11@hernyiii/fix/upheader"` ?

I wasn't able to easily get `python3.13t` to get a `\\?\` prefixed path in pyvenv.cfg trying it locally with the nuget package. I did run into some bugs with `uv...

I do see `/external:I "//?/C:/Users/runneradmin/AppData/Local/uv/builds-v0/.tmpdZmHW4/Lib/site- packages/pybind11/include"` in the compiler invocation, so it seems like it should work unless MSVC doesn't support long paths.

That might be the case - it's possible (from things like https://developercommunity.visualstudio.com/t/allow-building-running-and-debugging-a-net-applica/351628) that MSVC may not support long paths.

I'm going to go ahead and make a scikit-build-core release with a couple of other fixes, don't think this is a scikit-build-core issue (and can always fix and release if...

It's in the base of the virtual environments. `cat $VIRTUAL_ENV/pyvenv.cfg` basically. `python -c 'import sys; from pathlib import Path; print(Path(sys.prefix).joinpath("pyvenv.cfg").read_text())'` would print it in a cross-platform way, I believe.