In this talk, we will walk through different scalability challenges various modern CI setups encounter today: caching, distributed computing, reproducibility, telemetry... and how advanced build tools such as Bazel, Buck2, Pants,... addressed them at scale at big enterprises. We will then drill down into the common Remote Build Execution protocol that these tools all have in common. Finally, we shall reflect upon the current state of the build tool ecosystem and highlight key areas to improve upon.