--- title: "MCP meaning | Model Context Protocol | Len P. van der Hof" description: "MCP means Model Context Protocol: a contract for how a model calls tools. What it may see, what it may change, how the call is logged. A server without a contract is a hole in the repo." image: "https://lenvanderhof.com/media/generated/blog-hero-mcp-meaning-en.ac35227c0b1d.wide.webp" --- [AI Systems](https://lenvanderhof.com/en/blog/category/ai-systems/) Research guide # MCP meaning: Model Context Protocol, not a product The full form is the least interesting part. The permission boundary is the operating decision. Len P. van der HofPublished 30 August 20264 min read What the model may touch. Not a logo. Direct answer MCP means Model Context Protocol. It is a contract for how a language model calls tools: what it may see, what it may change, and how each call is logged. The official split is host, client, and server. A server exposes tools, resources, and prompts to a host: tools are model-controlled, resources application-controlled, prompts user-controlled. The official reference servers are filesystem, fetch, git, memory, time, sequential thinking, and everything. They are educational examples, not a production catalogue; the other former reference servers are archived. The MCP Registry is the catalogue. ReasonKit is first-party to this site, not an official reference. MCP is not a product you buy or the definition of an agent. A clean server with no ceiling is still a hole with nicer plumbing. ## Key takeaways - MCP = Model Context Protocol. The letters are a contract, not a brand. - A server exposes tools (actions), resources (reads), and prompts (templates) to a host. - Who triggers what matters: tools are model-controlled, resources application-controlled, prompts user-controlled. - Read three things on every server: visibility, mutation, and the log. - Reference servers teach the protocol. The registry lists what people publish. Neither is a ceiling. - This page is the meaning. The founder article is the longer operating note. In AI systems, **MCP means Model Context Protocol**: a contract for how a model calls tools. What it may see. What it may change. How the call is logged. That is the whole definition. The rest is collision control. Here MCP means the AI protocol, not Microsoft Certified Professional or the Burroughs Master Control Program. ## Not a product, not a contact app, not an agent MCP is not software you buy. It is not a CRM. It is not the worker. [An AI agent](https://lenvanderhof.com/en/blog/what-is-an-ai-agent/) still needs a charter. MCP is how that worker is allowed to touch the building. The official split is **host**, **client**, and **server**. The host is the application (Claude Desktop, Claude Code, VS Code, and others). The host opens one client per server. The server is a program that offers context. It may run on the same machine over stdio, or remotely over Streamable HTTP. The protocol moves context. It does not tell the host how to use the model. ## What a server actually does A server exposes three primitives. - **Tools** are actions, and they are model-controlled: the model decides when to call one. They can write. Send mail, edit a file, call an API, move money if you wired that. - **Resources** are reads, and they are application-controlled: the host decides what to pull in. A schema, a file, a log, a customer record. - **Prompts** are reusable templates, and they are user-controlled: a person invokes them. A template is not a permission. The control column is the part people skip. It is the difference between “a human chose to do this” and “the model chose, and nobody was asked.” The traffic is not one-way. **Elicitation** lets a server ask the client to collect more input from the user. Older MCP explainers also list roots and sampling. Both are deprecated as of protocol version 2026-07-28, and roots never enforced a security boundary. Do not build a new permission model around either. That is the whole job. A “known MCP server” is just a program that speaks this contract for one slice of the world: files, git, the clock, a knowledge-graph memory. Not everything a model can call is an MCP server. A REST API with an OpenAPI file is an API, and wrapping it in a function definition does not make it MCP. The [MCP Registry](https://registry.modelcontextprotocol.io/) is where published servers are listed. It is a catalogue, not a safety review. ## Reference servers, not a shopping list The MCP project keeps a small set of **reference implementations** on [GitHub](https://github.com/modelcontextprotocol/servers) to show the protocol and official SDKs. The README, read 2026-08-27, says they are educational, not production-ready. Evaluate your own ceiling. Current reference servers in that repository: - **Filesystem.** File operations inside a directory you allow. Read and write live here. Treat write as a gate. - **Fetch.** Pull web content into something a model can read. - **Git.** Read and change a repository. - **Memory.** A small knowledge-graph store the model can persist into. A graph is not a ceiling. - **Time.** Clock and timezone conversion. - **Sequential thinking.** A scratchpad of steps. No files, still a tool. - **Everything.** A test server that exposes all three primitives. Other former reference servers in that repository are **archived**. A third-party successor elsewhere does not become an official reference. Use the registry for the catalogue, then evaluate the server yourself. [ReasonKit](https://reasonkit.sh/) is first-party to this site, not an official reference. Same rule as any other server: visibility, mutation, log. ## Three reads Open the server. Official or homemade. Same card. 1. **Visibility.** What can it see? 2. **Mutation.** What can it change, send, spend, or delete? 3. **Log.** Can you replay the call after it goes wrong? Test the card: read one allowed file, attempt one denied read, make one dry-run write, then find all three attempts in the log. An unexpected success or a missing trace fails the review. If the permission lives in a Slack thread, you do not have a contract. Local servers may rely on process, filesystem, and sandbox boundaries; remote servers add transport authorization. Either way, least privilege is your operating design, not a property of the acronym. ## Two pages [MCP explained for founders](https://lenvanderhof.com/en/blog/mcp-explained-for-founders/) is the longer operating note: blast radius, Tuesday’s log, the fifteen-minute review. [Human in the loop meaning](https://lenvanderhof.com/en/blog/human-in-the-loop-meaning/) is who may approve the call once the contract allows it. This page is the dictionary entry plus what a server is. [STACK](https://lenvanderhof.com/glossary/stack/) is where those contracts live in a repository humans and agents share. [The Agentic Codebase](https://lenvanderhof.com/books/the-agentic-codebase/) is live. You do not need the hardcover to refuse an anonymous tool. ## Terminology - [MCP](https://lenvanderhof.com/glossary/mcp/) - [STACK](https://lenvanderhof.com/glossary/stack/) ## Sources 1. [MCP (glossary)](https://lenvanderhof.com/glossary/mcp/) 2. [MCP explained for founders](https://lenvanderhof.com/en/blog/mcp-explained-for-founders/) 3. [STACK](https://lenvanderhof.com/glossary/stack/) 4. [The Agentic Codebase](https://lenvanderhof.com/books/the-agentic-codebase/) 5. [MCP architecture](https://modelcontextprotocol.io/docs/learn/architecture) · MCP project 6. [MCP reference servers](https://github.com/modelcontextprotocol/servers) · MCP steering group 7. [MCP Registry](https://registry.modelcontextprotocol.io/) · MCP project ## 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 problem ## Further reading - [MCP](https://lenvanderhof.com/glossary/mcp/) - [MCP explained for founders](https://lenvanderhof.com/en/blog/mcp-explained-for-founders/) - [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 der Hof", "L.P. van der Hof", "Leendert Pieter van der 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