Feder is a tool that enables users to visualize approximate nearest neighbor search (ANNS) algorithms, specifically indexes like IVF_FLAT and HNSW. It helps users understand the structure of different indexes, how data are organized using each type of index, and how parameter configuration influences the indexing structure. Feder currently supports the HNSW from hnswlib but plans to support more indexes in the future. The tool is built with JavaScript and Python, allowing users to visualize index structures and search processes under IPython Notebook or as an HTML file for web service use.