Ch 2 — Architecture: Hosts, Clients & Servers

The three-layer model, security isolation, 1:N topology, and how it all fits together
IndexUnder the Hood →
High Level
-
Click play or press Space to begin...
Step- / 8
AThe Three-Layer ModelHost → Client → Server
1
laptop
HostThe AI application
you interact with
creates
swap_horiz
ClientProtocol connector
inside the host
connects
dns
ServerExternal process
exposing capabilities
2
arrow_downward What does the Host actually do?
BThe Host: Orchestrator & GatekeeperClaude Desktop, Cursor, ChatGPT, your custom app
security
SecurityControls consent,
approves tool calls
+
account_tree
LifecycleCreates/destroys
client instances
+
merge
AggregationMerges context from
multiple servers
3
arrow_downward The 1:N topology — one client per server
C1:N TopologyOne host, many clients, many servers
laptop
HostSingle AI app
instance
spawns
swap_horiz
Client 1→ GitHub
MCP Server
+
4
swap_horiz
Client 2→ Postgres
MCP Server
+
swap_horiz
Client 3→ Slack
MCP Server
5
arrow_downward Security isolation: servers can't see each other
DSecurity IsolationEach server lives in its own sandbox
visibility_off
No Cross-TalkGitHub server can't
see Postgres data
+
shield
Host ControlsOnly host decides
what to share
+
gavel
ConsentUser approves
sensitive actions
6
arrow_downward What does a server actually expose?
EServer CapabilitiesTools, Resources, Prompts — and who controls each
build
ToolsModel-controlled
LLM decides to call
+
description
ResourcesApp-controlled
host picks context
+
7
chat
PromptsUser-controlled
human selects
8
arrow_downward Real-world example: Cursor with multiple MCP servers
FReal-World Example: CursorHow a real host manages multiple MCP servers
edit_note
Cursor (Host)Reads config, spawns
clients on startup
stdio
code
GitHub ServerLocal process
search, PR, issues
HTTP
cloud
Remote ServerCloud-hosted
company API
1
Detail