Worked on the facebookresearch/faiss repository to deliver observability enhancements for the InvertedListsIterator component. Developed a backward-compatible feature in C++ that introduced virtual callback methods, enabling users to monitor distance computations and heap changes during vector processing. The implementation used object-oriented programming principles and included a branch-prediction-friendly guard to ensure zero overhead for non-user code. Unit testing was applied to validate the new callbacks, which were designed to fire within both L2 and IP branches of the iterate_codes loop. This work improved the flexibility and debuggability of distance calculations while maintaining compatibility with existing user workflows.