Lesson 7 of 30, Module 1: The Whole Idea. You understand what MCP is, why it exists, and you have FELT the problem it solves.
What you can do after this lesson
A protocol splits into a thin, protocol-aware client (comes free inside every host) and a capability-publishing server (the one piece a builder actually writes). Once you can name which of the three pieces is yours, you know exactly how much code a new integration costs, no matter which host, which SDK version, or which transport sits underneath.
Code in this lesson
Every block the lesson shows on screen, in the order it appears.
You -> Host -> Client -> Server -> Your Drive folder
Downloads
- Module 1 checkpoint (scribe-checkpoint-m1.zip): working code as the module leaves it, so a broken session costs you nothing
- The course GitHub repo: every checkpoint, the README and the full lesson list
Prefer to read?
The written version of this part of the course is on Substack: https://genaiunplugged.substack.com/p/inside-mcp-how-hosts-clients-and.
Lesson transcript
Only 3 parts
There are only 3 moving parts in MCP, and you are going to build exactly one of them.
Hello, and welcome to lesson 7 of the MCP Masterclass. Last lesson you watched one server feed 2 apps. Today we open the lid and name the parts. And we start with our map, as always.
Both app boxes are lit, the pipe is lit, and the folder is lit. The hole in the middle is still waiting for us.
Last lesson you installed a server into 2 apps and it worked. Well, today we open the lid and name the parts, because you are about to build one of them.
There are only 3 pieces in the whole arrangement, and exactly one of them belongs to you.
The host: the app you look at
So let us start with the host.
The host is the AI app you look at. Claude Desktop is a host, and so is ChatGPT, and so is Cursor, and so is Claude Code.
A host is nothing but the program a human being uses. It owns the window, the conversation, and the model itself.
The host also owns permission. When Claude wants to use one of your tools, the host is the thing that stops and asks you first.
Remember that last point, because I come back to it hard in module 3.
The client you never see
Now, the client.
Inside every host there is a client. You never see it, and you never install it.
A client is nothing but the part of the host that understands how to talk MCP. It speaks the protocol, so the remainder of the application never has to.
One client handles exactly one server. So if you attach 3 servers to Claude Desktop, the host runs 3 separate clients without ever mentioning it to you.
You will never write a client in this course. It comes free inside the host.
The server, which is yours
And now the server, which is yours.
A server is nothing but a small program that publishes a list of things it can do, and then performs them whenever it is asked.
That is it. That is the entire job description.
Your MCP server does not think. It does not talk to the model, and it does not decide anything. It publishes a list, waits, does the work, and hands back an answer.
The whole chain, both ways
So how do they all line up?
Here is the order, going from you all the way to your files.
You -> Host -> Client -> Server -> Your Drive folder
You. Then the host. Then the client. Then the server. And then your Drive folder.
Read it from left to right and it is a chain. You type into the host, and the host asks the client. The client speaks to the server, and the server touches your documents.
Then the answer comes back the same way, in reverse.
The pipe in between
And where does the pipe fit into this?
Between the client and the server there is a pipe.
For everything in modules 1 through 3, that pipe is called stdio. It is nothing but the ordinary way one program on your computer talks to another program that it started itself.
Your server runs on your machine. The host starts it, talks down the pipe, and shuts it down when you quit.
In module 4 we swap that pipe for a web address, and your server carries on working with 1 line changed. The rest of the chain never notices.
Which box do you build
So which piece do you build?
Look at the chain again. You build exactly one box in it.
The host is written by Anthropic, by Cursor, by Microsoft and by others. The client comes inside the host. The pipe comes with the protocol.
The server is yours. That one is you.
And this is the good news of the whole course. You write one small program, and you get every host for free.
That is the same point as lesson 6, said from the inside instead of from the outside.
Server sounds bigger than it is
Now there is a common mix up here, and I want to clear it before we go on.
The server sounds like the big clever piece, because the word server sounds important.
Your MCP server will be about 14 lines when we first write it in module 2. The clever part lives in the model, and the model belongs to the host.
Your job is to hand over the capability, and then get out of the way entirely.
Where the map stands
So where does our map stand at the end of this lesson?
Nothing new lit, and the names on the boxes now mean something. You, the host, the client inside it, the pipe, and the folder.
The empty box is the server. That is your box, and it stays dark for 5 more lessons.
Next lesson we will look at the 3 kinds of thing a server can offer.
Bye now, and I will see you in the next lesson.
