For anyone having issues with CMD length, PIP, or flash-attn building in general, try PowerShellPython

[leomaxwell973]

This is a project I've focused on flash-attn and xformers specifically for windows users.
I've gone from barley possible installs for flash-attn to installing seamlessly over pip after finishing it.
it's mainly designed to address the cmd length issues for flash-attn/xformers but can help with pip issues and general build env issues as well or just give an alternative shell to throw at it if your errors are more general/niche.

https://github.com/leomaxwell973/PowerShellPython/