--- title: "Version the agent OS like the app (STACK) | Len P. van der Hof | Len P. van der Hof" description: "When agents demo well and then rot the repo, the disease is agentic repo debt. STACK is five layers: structure, toolchain, agent configuration, connection, knowledge and quality." image: "https://lenvanderhof.com/media/generated/blog-hero-stack-os-en-v2.2e85cab10797.wide.webp" --- [AI Systems](https://lenvanderhof.com/en/blog/category/ai-systems/) Research guide # STACK: version the agent OS like you version the app Rules in five tools. MCP without contracts. Skills nobody owns. That is not a model problem. Len P. van der HofPublished 23 August 20262 min read Five layers. One cage. Version the agent OS like the app. Direct answer STACK is a five-layer architecture 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. If prompts, MCP servers, and skills live outside the repo, or live in five places inside it, you have agentic repo debt. The fix is ownership and a single source of rules, not a smarter model. ## Key takeaways - Demo quality is not repo quality. - Five layers. One owner per layer. Rules in one place. - MCP belongs in Connection, with a contract, or it does not belong. - This is the architecture article. The book intro is a different URL. Agents demo beautifully. Then the repository starts to smell. Prompts in four files. The same rule in a README, a host config, and someone’s head. An MCP server nobody listed. That smell has a name: agentic repo debt. ## The disease A weak model is rarely the cause. The cause is an agent operating system that is not an operating system. It is a pile. You would not ship application code that way. You version the app. You review the app. You know who owns a package. The agent half of the repo deserves the same disrespect for folklore. ## Five layers [STACK](https://lenvanderhof.com/frameworks/stack/) is the map I use: 1. **Structure** — where agent artifacts live, and nowhere else. 2. **Toolchain** — the hosts and runners, pinned, not “whatever the laptop has.” 3. **Agent configuration** — charters, prompts, skills, with owners. 4. **Connection** — [MCP](https://lenvanderhof.com/glossary/mcp/) and other tool contracts. Visibility, mutation, log. 5. **Knowledge and quality** — what the agent may treat as true, and the tests that fail a bad change. If a change cannot be named as a layer, it does not belong in the PR description. “Tweaked the agent” is not a commit message. ## Version it Treat prompts, skills, and server contracts as code. - They live in the repo. - They have reviewers. - They have a rollback. - The [override doctrine](https://lenvanderhof.com/glossary/override-doctrine/) is a file, not a Slack memory. When a tool can send, spend, or delete, the first ten calls have a human name next to them. After that, the log is the review. No log, no tool. ## Where this sits [The Agentic Codebase](https://lenvanderhof.com/books/the-agentic-codebase/) is the book. The [introduction](https://lenvanderhof.com/en/blog/the-agentic-codebase-introduction/) is the launch article. This page is the architecture you install when the demo worked and the repo did not. [MCP explained for founders](https://lenvanderhof.com/en/blog/mcp-explained-for-founders/) is the connection-layer companion. Read that before you attach a server that can mail a customer. If you do one thing this week: pick a single directory as the agent OS, move the stray prompts into it, and put a name on the CODEOWNERS line. Folklore ends at the first owner. ## Terminology - [STACK](https://lenvanderhof.com/glossary/stack/) - [MCP](https://lenvanderhof.com/glossary/mcp/) - [Override doctrine](https://lenvanderhof.com/glossary/override-doctrine/) ## Sources 1. [STACK framework](https://lenvanderhof.com/frameworks/stack/) 2. [The Agentic Codebase](https://lenvanderhof.com/books/the-agentic-codebase/) 3. [MCP explained for founders](https://lenvanderhof.com/en/blog/mcp-explained-for-founders/) ## Related essays What is an AI agent?MCP explained for foundersThe Agentic Codebase: introductionAn agentic workflow is not an automationLLM integration is an operating-design problem ## Further reading - [STACK](https://lenvanderhof.com/frameworks/stack/) - [The Agentic Codebase](https://lenvanderhof.com/books/the-agentic-codebase/) - [MCP explained for founders](https://lenvanderhof.com/en/blog/mcp-explained-for-founders/) 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 der Hof", "L.P. van der Hof", "Leendert Pieter van 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