Simon Jakobi contributed to core language tooling in the agda/agda and haskell/haskell-language-server repositories, focusing on performance, type safety, and dependency management. He consolidated data processing by replacing multiple HMap traversals with a single HMap.mapMaybe pass, reducing runtime overhead in Agda. Simon also enhanced type safety by migrating DisplayForms to a non-empty List1 structure, strengthening compile-time guarantees in the type-checking pipeline. In the Haskell language server, he upgraded the unordered-containers dependency to enable new ghcide functionality and ensured reproducible builds. His work demonstrated depth in Haskell, functional programming, and library integration, emphasizing maintainability and robust abstractions.