The Terminology Problem The AI agent ecosystem has a vocabulary collision. "Tool" means one thing in LangChain, another in AutoGPT, and something else entirely in Claude's function-calling docs. "Skill" and "agent" are similarly overloaded—an "agent" might be a simple prompt wrapper or a fully autonomous system that books flights and deploys code. "MCP" arrived in late 2024 and added yet another term to the mix. This matters architecturally. When layers are conflated, testing becomes harder, reuse drops, and swapping a model means rewriting half the system. A function that orchestrates 15 steps gets called a "tool." A prompt that strings together API calls gets called an "agent." The result is codebases where nothing is composable. A 4-layer mental model resolves most of the confusion—similar to how the OSI model gave networking a shared vocabulary, or how MVC clarified web application structure. It's not a rigid specification, but a framework for making architectural discussions more productive.…