Sebastian Mendel enhanced build tooling and developer experience for TYPO3CMS-Reference-CoreApi by restructuring Makefiles, updating PHP version constraints, and introducing maintainability guardrails to streamline onboarding and reduce build friction. He focused on PHP and Makefile scripting to improve reliability, standardize processes, and provide actionable error messages for missing dependencies. In the phpDocumentor/guides repository, Sebastian optimized document lookup performance by implementing O(1) caching and refactored the caching layer for constant-time access. He also improved test-driven development practices by strengthening test safety and root-document handling, resulting in faster document rendering, more robust tests, and a clearer path for future optimizations.