Chainlit

Chainlit is an open-source Python package that lets developers wrap any LLM, agent, or RAG pipeline in a polished chat UI without writing a line of frontend code. Decorate a Python function with @cl.on_message, return a response, and you get a streaming chat interface with multi-step reasoning visualization, file uploads, message history, and feedback collection out of the box.

It sits in the same niche as Gradio and Streamlit, but is purpose-built for conversational and agentic workloads rather than generic ML demos. Chainlit integrates natively with LangChain, LlamaIndex, OpenAI, Mistral, Haystack, and Semantic Kernel, and ships first-class support for enterprise auth (OAuth, header-based, password), data persistence layers, and human-in-the-loop feedback. It's the de-facto choice for teams who want a working internal chatbot demo on day one and a path to production by week two.

The framework itself is free and Apache-licensed; the optional Literal AI observability/analytics platform from the same team is the commercial upsell. Caveats: the UI is opinionated (you get the Chainlit look, with limited theming), and deep customization usually means dropping down to a custom React frontend talking to the Chainlit backend over websockets.