Elysia is an open-source agentic platform that uses a decision tree architecture to dynamically select and execute tools for searching and reasoning over vector databases (Weaviate). The core is a `Tree` class that maintains conversation history, an environment of retrieved objects, and a decision loop: at each step, an LLM (via DSPy) chooses which tool to call based on the current state and user goal. Built-in tools include hybrid/semantic query, aggregate, summarization, visualisation, and linear regression; custom tools can be added via a `@tool` decorator or by subclassing `Tool`. The system preprocesses Weaviate collections to generate LLM-created summaries, field mappings, and statistics, enabling adaptive query formulation. The platform includes a full-stack web application: a FastAPI backend with WebSocket endpoints for real-time query execution, and a built Next.js frontend (served as static files) offering a UI for conversations, collection management, tree visualisation (React Flow), and feedback collection. The backend manages multi-user and multi-conversation state via `UserManager`/`TreeManager`, persists trees to Weaviate, encrypts API keys, and supports feedback-driven few-shot learning. A CLI entry point (`elysia start`) launches the API server.