It Receives the Screen — With None of It in the Client

Through part 3 the client knew what to show because we wrote it here. Now we do not.

Three lines

final read = await client.readResource('ui://desk');
final text = read.contents.first.text!;
final screen = jsonDecode(text) as Map<String, dynamic>;

ui://desk — 546 B, 6 lines, type "page", title "Desk"
nothing about this screen is written in the client

In a Flutter host this goes on to MCPUIRuntime.initialize(screen). The course sample stays plain Dart so anyone can run it, and confirms only that it arrived and that it is a screen.

What is absent

Things this file does not contain:

• a widget tree
• colours and font sizes
• button labels
• which button calls which tool

All of it is in the document the server gave. The client is plumbing that receives that document and hands it to a runtime.

So when the server changes the screen, the client does nothing. Just as server part 4 changed fontSize from 88 to 120 without stopping the server, this side does not rebuild.

546 bytes

One screen is 546 bytes. Six lines.

That the number looks small is the point. When moving a screen is cheap, moving it becomes the default. If every change means shipping an app, the screen belongs inside the app; if it means fetching one document, it belongs outside.