Hosts, Clients & Servers

Topology

flowchart TB
subgraph Host["Host (AI application)"]
  C1[Client 1]
  C2[Client 2]
  C3[Client 3]
end
C1 -- "1:1 (stdio)" --> S1["Server A (local)"]
C2 -- "1:1 (stdio)" --> S2["Server B (local)"]
C3 -- "1:1 (Streamable HTTP)" --> S3["Server C (remote)"]

MCP topology

Responsibilities

Host

The AI application. Creates and manages one client per server, enforces security policy, permissions, and user consent, aggregates context from multiple servers, and owns the user interface and conversation state.

Client

Connector inside the host. Maintains a dedicated 1:1 connection with exactly one server, attaches protocol version and capability metadata to requests, routes messages, and maintains the security boundary between servers.

Server

Exposes tools, resources, and prompts via MCP primitives. Operates independently with a focused responsibility, never sees the whole conversation, and can be local (stdio subprocess) or remote (Streamable HTTP).

Transports

Transport	Where	How
stdio	Local	Client launches the server as a subprocess; newline-delimited JSON-RPC over stdin/stdout; server may log to stderr
Streamable HTTP	Remote	Client POSTs each JSON-RPC message to a single MCP endpoint; the server replies with plain JSON or a request-scoped SSE stream

Key exam points

A server cannot read the conversation or see other servers’ data — the host enforces isolation by design
The host, not the server, is responsible for user consent
Each client talks to exactly one server; a host runs many clients
stdio = local subprocess; Streamable HTTP = remote (and is where OAuth-based authorization applies)

Check your knowledge

📝 Check your knowledge

1. Which participant enforces user consent and security policy?

2. What is the relationship between a client and a server?

3. Which transport would a locally installed filesystem server typically use?

4. On the stdio transport, where should a server write its log output?

5. Can a server read the host's full conversation history?