LangChain
An open-source framework for building applications that use language models — particularly RAG pipelines and AI agents. Popular partly because it provides abstractions for common patterns (retrieval, tool use, chains of prompts) that would otherwise require significant custom code. Criticized for adding complexity and making debugging harder than just writing the code directly. Useful as a starting point for understanding patterns; often replaced with custom implementations as teams mature.
In practice
You're building a RAG application with Claude and want pre-built components for document loading, chunking, embedding, retrieval, and prompt chaining rather than coding each piece from scratch. LangChain is the toolkit that provides those building blocks. It's popular for quickly prototyping complex Claude-powered pipelines.
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