Not to sound like a shill but Nix solves this very elegantly: tests are just a special kind of build (usually with a synthetic output like "echo ok > $out") and can therefore be "cached" like any other build. But because build recipes are deterministic, and Nix uses sandboxed, pure build environments, "caching" is just a special case of "uploading build artifacts". Meaning your cache = build artifact server ("binary store"), and it's shared between everyone (CI, build farm, dev laptops, ..), again because build recipes are deterministic. Meaning: if your laptop (or VM) is the same architecture as your CI you can just run your CI steps locally and push that to the shared store, and your CI will now automatically see the tests have already been run and not run them again. And vice-versa. Across all branches and all collaborators who share that same cache.<p>This is all deterministic and doesn't require any human in the loop: the test result is hashed by all of the inputs, so you never accidentally spuriously pass a test locally which shouldn't have.<p>It requires almost no setup at all. There are SaaS which make this basically turnkey (cachix, nixbuild.net, garnix, ..). Getting your app to build in Nix in the first place, though? Years of tears.<p>Nix lives in this group of tools where "easy things are hard, hard things are easy", and this is a classic example.