Andrew Brett focused on improving the reliability of incremental compilation in the sbt/zinc repository by addressing a bug in cycle registration when pipelining was disabled. He introduced explicit control over cycle registration through a new parameter, shouldRegisterCycle, in core methods such as mergeAndInvalidate and completeCycle. Working primarily in Java and Scala, Andrew expanded the test suite to validate compilation cycle counts across various pipelining configurations, ensuring correctness and reducing build flakiness. His work demonstrated a deep understanding of build tools and compiler internals, resulting in more predictable incremental compilation behavior and enhanced stability for the Zinc incremental compiler.