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Show HN: OpenFable – Open-source RAG engine using tree-structured indexeshttps://github.com/alainbrown/openfableHi HN, I built OpenFable, an open-source retrieval engine that implements the FABLE algorithm (https://arxiv.org/abs/2601.18116) for RAG pipelines. I'm using it in another project and thought that others might benefit. Most RAG systems chunk documents into flat segments and retrieve by vector similarity. This works for simple lookups but breaks when answers span multiple sections, when relevant content is buried in a subsection, or when you need to control how many tokens you're sending to an LLM. OpenFable takes a different approach: when you ingest a document, it uses an LLM to identify discourse boundaries (not fixed-size windows), then builds a hierarchical tree, root, sections, subsections, leaf chunks, with embeddings at every level. Retrieval combines two paths: 1. LLM-guided path: the LLM reasons about which documents and subtrees are relevant from summaries 2. Vector path: similarity search with structure-aware score propagation through the tree Results from both paths are fusApr 8, 2026 1:05 PM
