langchain/tools/vectorstore/tool.py/VectorStoreQATool/_run
class VectorStoreQATool:    def _run(
        self,
        query: str,
        run_manager: Optional[CallbackManagerForToolRun] = None,
    ) -> str:
        """Use the tool."""
        chain = RetrievalQA.from_chain_type(
            self.llm, retriever=self.vectorstore.as_retriever()
        )
        return chain.run(query)