RAG & Knowledge
ontosphere
Edit RDF knowledge graphs in your browser — no backend needed.
★ 247 starsNOASSERTION
View guide →A self-maintaining AI second brain for Obsidian and Claude Code.
second-brain-os is an open-source rag & knowledge project by undefined-ui (undefined-ui/second-brain-os on GitHub). It has 888 GitHub stars and 138 forks, is released under the MIT license, and was last updated on GitHub on 29 September 2026.
Decide what you want the project to do before installing anything.
Grab the source and skim the README for setup requirements.
Install dependencies and add any API keys or settings it needs.
Launch it and follow the example workflow from the docs.
Exposes its capabilities to AI agents over the Model Context Protocol.
Works inside your Obsidian vault and notes.
Structures information as a queryable graph.
Works offline with on-device storage.
Bring notes from Obsidian, Notion, and more.
Own your second brain completely.
Download the source and move into the project folder.
git clone https://github.com/undefined-ui/second-brain-os.git
cd second-brain-osStart a tiny local web server in the project folder.
python3 -m http.server 8000Visit the local server in your browser.
# open http://localhost:8000It is an open-source project at undefined-ui/second-brain-os with 138 forks. Check the repo’s contributors and recent commits to judge how active maintenance is.
Yes. It is MIT licensed, so you can use it commercially with attribution.
Locally or on your own server, depending on how you deploy it.
Most support OpenAI, Anthropic, Ollama, and other providers via config.
Yes — MCP support lets agents search your knowledge base directly.
Edit RDF knowledge graphs in your browser — no backend needed.
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Free and open — grab the source, star the repo, and start creating.