Nix builds things in sandbox (no network access, no file system access etc) this can make creating derivation for application that has build with side effects a

Nix builds things in sandbox (no network access, no file system access etc) this can make creating derivation for application that has build with side effects a frustrating experience, but that brings reproducibility (or at least gets us really close there) so generally this is not a concern. As for splitting build into steps. I am not entirely sure what are you trying to do, but you should not need to do that. Each package in Nix has a hash, which is generated from things like hash of the source code, of the dependencies, compile flags, system architecture etc. This means that you could have multiple versions of the same package in /nix/store with different compile options or different dependencies. When you need a given dependency you know all the information to generate the hash and can easily know if you need to build it or you can use cached version. This is what makes binary cache a pretty much plug and play and don't need to worry what files to cache or whether you should split build into stages.