If a test fails on CI and nobody can reproduce it, did it really fail? 🧘 Nix gives me a build that is a function of its inputs via the extensional model. The same derivation, produces the same store path, and if I am lucky, the same bytes. What Nix does not give me is the same run.1 A test suite with a race in it may pass on my laptop, fail once on CI, and when I rebuild it to look, it passes again. A run is the sequence of events that happen in a process to produce a result. A build is a run of a derivation. ↩