Serena is more about text editing and better code search tooling while lumen is (a) chunking code with tree sitter and (b) storing embeddings (vectors) generated by an embedding model (ideally ones which are for code) which you then can search against. Effectively it‘s RAG for code made available as an MCP server.

This reduces tool calls (and thus saves times and tokens) because instead of „trying“ / „guessing“ names repeatedly, tools like claude code typically get useful search results on the first try.

Claude for example may search for „dbal“ via regex, but the function name is „sql“ - semantic search will find that while for regex, claude would try 3 additional guesses before it actually finds what its looking for. Hope this helps!