Explain the MCP architecture: what are hosts, clients, and servers?

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Re-explain

MCP splits an integration into three roles: a host, one or more clients inside it, and servers that expose capabilities.

Host (Claude Desktop, IDE, agent)
├── Client A  <-->  Server A  (filesystem, local)
├── Client B  <-->  Server B  (GitHub, remote)
└── Client C  <-->  Server C  (database, local)

The host is the AI application the user sees. It holds the model conversation, renders the UI, and asks the user for permission.

A client is a connector the host creates, and each client talks to exactly one server. Ten servers means ten clients. Since the 2026-07-28 revision, every request a client sends carries its own protocol version and capabilities, so no session state sits between requests.

A server is a separate program offering capabilities to its client. It may run locally as a subprocess or remotely over HTTP.

That one-to-one pairing is the isolation boundary. A server sees only what its own client sends, never other servers or the full conversation.

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Likely follow-ups

Why does each client talk to exactly one server instead of one client serving them all? Tie it to isolation. A server sees only its own client's messages, never other servers or the full conversation, so the host controls what each gets.
What capabilities can a server expose, and who decides when each one gets used? Name tools, resources and prompts, and say which one the model, the host application or the user controls.
What changes in practice when a server runs remotely instead of as a local subprocess? Cover stdio versus HTTP transport, who deploys and updates the server, authorization for remote users, and network failures.
What risks appear when an MCP server is also a client of other MCP servers? Data and instructions now pass through a middle server, so the isolation weakens. Say where auth, trust and user consent get checked at each hop.
Why does the host, not the server, ask the user for permission? The host is the part the user sees and trusts. A server may be buggy or hostile and never sees the full conversation, so it can't be the gatekeeper.

What you can say

MCP splits an integration into three roles: the host, the clients inside it, and the servers that expose capabilities.
The host is the AI application the user sees, like Claude Desktop or an IDE, and it holds the model conversation, renders the UI and asks the user for permission.
The host creates one client per server, and each client talks to exactly one server, so ten servers means ten clients.
A server is a separate program offering capabilities to its client, running either locally as a subprocess or remotely over HTTP.
Since the 2026-07-28 revision, every request a client sends carries its own protocol version and capabilities, so no session state sits between requests.
That one-to-one pairing is the isolation boundary, so a server sees only what its own client sends, never other servers or the full conversation.

Weak answers to avoid

Mixes up host and client The host is the app the user sees. A client is a connector it creates for one server. Blur them and you can't say where permission checks and isolation live.
Has one client talking to every server That drops the one-to-one pairing the design rests on. Say ten servers means ten clients, and each server sees only its own client's traffic.
Assumes servers see the whole conversation A server gets only what its client sends, never other servers or the full chat. Missing this means you can't reason about privacy or data leaks between servers.
Treats every server as a hosted web API Servers can run locally as a subprocess or remotely over HTTP. Forgetting the local case skips common setups like filesystem and database servers.

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