--- title: "What is STACK | Agent OS architecture | Len P. van der Hof" description: "STACK is a five-layer architecture for repositories where humans and agents share work: Structure, Toolchain, Agent configuration, Connection, Knowledge and quality. It is not a vendor AI-stack slide." image: "https://lenvanderhof.com/media/generated/blog-hero-stack-meaning-en.10a6aedba061.wide.webp" --- [AI Systems](https://lenvanderhof.com/en/blog/category/ai-systems/) Research guide # What is STACK? Five layers, not a vendor diagram The agent OS should be versioned like the app. If the rules live in five tools, you have debt, not a model problem. Len P. van der HofPublished 5 September 20261 min read Layers with owners. Not a slide titled stack. Direct answer STACK is a five-layer architecture for repositories where humans and agents share work: Structure, Toolchain, Agent configuration, Connection, Knowledge and quality. Structure is the map an agent can navigate on minute one. Toolchain is the commands it may run. Agent configuration is versioned standing context. Connection is MCP with contracts and least privilege. Knowledge and quality is memory, evals, and CI, so a model upgrade does not silently lower the bar. It is not a vendor 'AI stack' diagram. The book that owns the name is The Agentic Codebase, which is live. ## Key takeaways - STACK is five named layers with owners. Not a product pack. - Demo quality is not repo quality. - This page is the definition. The architecture article versions the OS. - MCP belongs in Connection, with a contract, or it does not belong. STACK is a word vendors print on a slide and operators need as an architecture. **What is STACK?** Five layers for repositories where humans and agents share work: Structure, Toolchain, Agent configuration, Connection, Knowledge and quality. The agent operating system should be versioned like the application. It is not a vendor “AI stack” diagram. It is not a pile of tools. If prompts, MCP servers, and skills live outside the repo, or live in five places inside it, you have agentic repo debt. The disease is ownership, not a weak model. ## The five names **Structure.** The map an agent can navigate on minute one. **Toolchain.** The commands it may run. **Agent configuration.** Versioned standing context. Not a prompt in a chat. **Connection.** [MCP](https://lenvanderhof.com/en/blog/mcp-meaning/) with contracts and least privilege. **Knowledge and quality.** Memory, evals, and CI, so a model upgrade does not silently lower the bar. ## Two pages [STACK: version the agent OS like you version the app](https://lenvanderhof.com/en/blog/stack-version-the-agent-os/) is the architecture article. This page is the definition. [The Agentic Codebase](https://lenvanderhof.com/books/the-agentic-codebase/) is live. You do not need the hardcover to put the rules in one place. ## Terminology - [STACK](https://lenvanderhof.com/glossary/stack/) - [MCP](https://lenvanderhof.com/glossary/mcp/) ## Sources 1. [STACK (glossary)](https://lenvanderhof.com/glossary/stack/) 2. [STACK: version the agent OS](https://lenvanderhof.com/en/blog/stack-version-the-agent-os/) 3. [MCP meaning](https://lenvanderhof.com/en/blog/mcp-meaning/) 4. [The Agentic Codebase](https://lenvanderhof.com/books/the-agentic-codebase/) ## Related essays What is an AI agent?MCP explained for foundersThe Agentic Codebase: introductionVersion the agent OSAn agentic workflow is not an automationLLM integration is an operating-design problemMCP meaning: Model Context Protocol, not a product ## Further reading - [STACK](https://lenvanderhof.com/glossary/stack/) - [Version the agent OS](https://lenvanderhof.com/en/blog/stack-version-the-agent-os/) - [The Agentic Codebase](https://lenvanderhof.com/books/the-agentic-codebase/) About the author ## [Len P. van der Hof](https://lenvanderhof.com/en/authors/len-p-van-der-hof/) Entrepreneur, AI Innovator and Venture Builder Len P. van der Hof builds practical AI systems, digital ventures and evidence-informed tools for founders. ```json { "@context": "https://schema.org", "@graph": [ { "@type": "Person", "@id": "https://lenvanderhof.com/#person", "name": "Len P. van der Hof", "alternateName": [ "Len van 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