Open-source Ruby gem created and maintained by Andrei Bondarev. Described as the most popular Ruby solution to build LLM-powered applications. Vendor-agnostic: common interface over multiple LLM providers (OpenAI, Google, Mistral) and vector databases — 'pick your poison'. Supports general LLM interfacing, building agents (with tool/function-calling orchestration via open-API-spec JSON tool signatures), and RAG systems (add_text + similarity_search + ask for naive RAG). 'Batteries-included' in the Rails tradition. Has a Discord community. Demo uses Langchain::Assistant with an LLM instance and an array of tool objects (each tool inherits from Langchain::Tool::Base and points to an open-API-style JSON describing its functions).