LangChain

LangChain is an open-source framework for building apps on top of language models. It gives you standard building blocks, such as prompts, models, tools, retrievers and memory, plus integrations with hundreds of providers and databases. Its sibling project LangGraph is used to build agents as graphs with state, loops and human approval steps, which suits more complex workflows. Libraries are available for both Python and JavaScript. LangChain is free and open source, with LangSmith as the paid companion for tracing and testing. It is a common starting point, especially when you need lots of integrations.