Haystack by deepset

Haystack is deepset's open-source framework for building production-ready LLM applications, RAG pipelines, and search systems. With a component-based architecture, Haystack makes it easy to assemble sophisticated AI pipelines combining document retrieval, LLM generation, agents, and structured output, all with a clean Python API. The framework supports 30+ LLM providers, all major vector databases, and built-in evaluation tools for measuring RAG quality. Haystack is used by thousands of developers building everything from simple Q&A systems to complex multi-agent workflows.