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 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 is the architecture article. This page is the definition.
The Agentic Codebase is live. You do not need the hardcover to put the rules in one place.