> ## Documentation Index
> Fetch the complete documentation index at: https://docs.0mcp.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Hosts, clients, and servers in MCP

> Understand the roles of MCP hosts, clients, and servers, how they connect, and where control and security responsibilities belong.

The [Model Context Protocol](https://modelcontextprotocol.io/docs/learn/architecture) (MCP) uses three main participants: **hosts**, **clients**, and **servers**.

A host is the AI application you use. It creates an MCP client for each server connection. An MCP server exposes selected capabilities from an external system.

```text theme={null}
User
  |
MCP host
  |
  +-- MCP client A <--> MCP server A <--> External system A
  |
  +-- MCP client B <--> MCP server B <--> External system B
```

This separation lets one AI application connect to multiple systems while keeping each connection isolated.

## The three participants at a glance

| Participant | What it is                                 | Main responsibility                                                 |
| ----------- | ------------------------------------------ | ------------------------------------------------------------------- |
| **Host**    | The AI application the user interacts with | Coordinates the AI experience, permissions, and connected servers   |
| **Client**  | A protocol component created by the host   | Maintains one connection and exchanges MCP messages with one server |
| **Server**  | A program or service that implements MCP   | Exposes tools, resources, prompts, or other supported capabilities  |

The API, database, file system, or SaaS product behind an MCP server is an **external system**. It does not need to support MCP when the server translates between MCP and the system's existing interface.

## What is an MCP host?

An **MCP host** is the application that brings the user, AI model, and MCP connections together.

Examples include an AI assistant, coding environment, desktop application, or internal agent platform. A host commonly:

* Creates and manages MCP clients
* Decides which servers can connect
* Controls what conversation context a server receives
* Combines approved context from multiple servers
* Presents consent and confirmation interfaces
* Applies application-level security policies

The host owns the overall user experience. Connecting a server should not automatically give that server access to the full conversation or to other connected servers.

## What is an MCP client?

An **MCP client** is the protocol component inside the host that communicates with one MCP server.

The client:

* Opens and maintains the connection
* Negotiates the protocol version and capabilities
* Sends requests and notifications
* Receives responses and server notifications
* Handles connection errors and shutdown

One host can run many clients. Each client normally has a one-to-one relationship with a server.

<Note>
  The host and client are different roles even when one application implements both. The host coordinates the complete AI experience; the client manages a specific MCP connection.
</Note>

## What is an MCP server?

An **MCP server** is a program that makes selected data, actions, or workflows available through MCP.

A server can expose:

* [Tools](/learn/core-concepts/tools) that a model can call
* [Resources](/learn/core-concepts/resources) that an application can read
* [Prompts](/learn/core-concepts/prompts) that a user can select

It can run as a local process or as a remote service. A server should expose a focused set of capabilities and enforce authorization when it accesses the underlying system.

## How they work together

A typical connection follows this sequence:

1. The user enables or selects an MCP server in the host.
2. The host creates an MCP client for that server.
3. The client connects through an MCP [transport](/learn/core-concepts/transports).
4. The client and server negotiate supported capabilities.
5. The client discovers the server's available features.
6. The host decides how those features appear to the user and model.
7. The client and server exchange requests, results, and notifications.

For example, a coding assistant may use one client to connect to a local file server and another to connect to a hosted issue-tracking server.

## Where control belongs

| Decision                                      | Responsible participant       |
| --------------------------------------------- | ----------------------------- |
| Which servers are connected                   | Host                          |
| What context a server receives                | Host                          |
| How MCP messages are exchanged                | Client and server             |
| Which capabilities are exposed                | Server                        |
| Whether an underlying operation is authorized | Server and external system    |
| Whether a sensitive operation proceeds        | Host, user, and server policy |

MCP standardizes communication. It does not replace authentication, authorization, validation, or user consent.

## Common points of confusion

### Is the host the AI model?

No. The host is the application around the model. It manages the conversation, connections, permissions, and model access.

### Is an MCP client a user-facing application?

Not necessarily. The client is usually an internal component of the host.

### Is every API an MCP server?

No. An API becomes available through MCP only when an MCP server exposes selected API operations as MCP capabilities.

### Can servers communicate with each other?

MCP normally connects each server to its own client. The host may coordinate results across connections, but servers should not assume direct access to one another.

## Key takeaway

**The host manages the AI experience, a client manages one MCP connection, and a server provides focused capabilities from an external system.**