LangChainKnowledgeBase works great when you already have a LangChain-based retrieval setup that uses vectorstores, but when you want something more basic, like a TFIDFRetriever, you cannot use it because of an instance check.

Looking at the source of VectorStoreRetriever, it doesn't override the invoke method, which is the only one the LangChainKnowledgeBase uses (here specifically).

After some manual testing for confirmation, I believe it would be best to relax the check to BaseRetriever instead, which defines/implements¹ the required invoke method. What do you think? :)