Tomasz Valeev contributed to the JetBrainsRuntime repository by enhancing both API design and benchmarking reliability using Core Java and TreeMap. He implemented default min and max methods in the Java Comparator interface, streamlining common comparison operations and reducing boilerplate, while ensuring backward compatibility through comprehensive unit testing. Additionally, he stabilized the TreeMapUpdate benchmark by introducing a comparator-aware map-operation strategy, replacing tailMap with subMap and conditionally applying headMap or tailMap to prevent IllegalArgumentException. These changes improved the reliability of performance regression detection and reduced CI flakiness, reflecting a thoughtful approach to both feature development and maintenance.