LlamaIndex and LangChain are two open-source libraries that can be used to build applications that leverage the power of large language models (LLMs). LlamaIndex provides a simple interface between LLMs and external data sources, while LangChain provides a framework for building and managing LLM-powered applications. In summary, LlamaIndex is primarily focused on being an intelligent storage mechanism, while LangChain is a tool to bring multiple tools together. At the same time, you can build agents using only LlamaIndex or LlamaIndex as a LangChain agent tool.