When learning about MCP (Model Context Protocol), almost everyone gets stuck on the same point: Who is who among the Host, Client, and Server? These three terms appear repeatedly in the documentation. Phrases like "the Host contains the Client" are particularly confusing at first glance. Yet, understanding their relationship is the key to grasping the MCP architecture—once you get this right, everything else follows smoothly, from tool discovery to permissions and communication.
This article explains the roles, responsibilities, and message flow of these three components in the most straightforward way possible. If you are still unclear about the relationship between MCP and Function Calling, we recommend reading "MCP vs Function Calling" first for a solid foundation.
The three roles in MCP are not three parallel parties; rather, they form a nested relationship where the "Host contains the Client," and the "Client connects to the Server."
Defining the Three Roles in One Sentence
Here is the simplest version first, with details following:
- Host (the host application): The AI application you are actually using, such as Claude Code, Cursor, or a specific desktop assistant. It is the primary entity interacting with both you and the large language model.
- Client: A "connector" component inside the Host responsible for establishing and maintaining connections with a specific Server. The Client resides within the Host; they are not two separate, parallel entities.
- Server (the server): An independent program that provides specific capabilities (tools, data, prompt templates), such as a file search Server, a database Server, or a GitHub Server.
One critical and easily misunderstood point: The Client is not the "user side," and the Server is not the "cloud." In MCP, the Client is an internal connector within the Host, while the Server often runs locally on your machine (as a subprocess). Do not apply the usual intuition that equates "client" with mobile apps and "server" with remote servers.
Expanded: What Exactly Does Each Component Do?
The Host: The Commander-in-Chief
The Host acts as the brain and front end of the entire system. Its responsibilities include:
- Interacting with users and conversing with large language models;
- Deciding which Servers to connect (based on what you have configured);
- Creating a corresponding Client for each Server it connects to;
- Aggregating all tools provided by the Servers and presenting them to the model. When the model decides to invoke a specific tool, the Host forwards that request via the appropriate Client;
- Controlling permissions and security: The Host has final say on whether to execute a tool call or require user confirmation before proceeding.
The Client: A One-to-One Connector
This is the role that requires the most clarification. A single Client connects to exactly one Server, acting as a dedicated channel between the Host and that specific Server. If a Host connects to three Servers simultaneously, it internally manages three Clients, each handling one connection. The Client is responsible for:
- Establishing and maintaining sessions with its corresponding Server;
- Sending and receiving messages according to the MCP protocol (based on JSON-RPC): querying the Server about "what tools are available," forwarding model invocation requests to the Server, and receiving results returned by the Server;
- Handling the protocol details specific to this connection.
Why include a Client layer instead of having the Host connect directly to Servers? Because it standardizes and isolates the mechanics of communicating with a single Server—each connection operates independently, so if one Server fails, others remain unaffected, keeping the Host's logic cleaner.
The Server: Capability Provider
The Server is where the actual work happens; it declares what capabilities it offers externally. MCP defines three primary categories of capabilities:
- Tools: Actions that can be invoked by models, such as "search files," "check orders," or "initiate API requests";
- Resources: Readable data, such as file contents or database records;
- Prompts: Predefined prompt templates available for the Host to invoke.
A Server typically focuses on a single domain (files, databases, a specific SaaS). It can run locally (communicating with the Host via standard input/output) or act as a remote service (communicating over HTTP). To learn how to build one from scratch, see Building an MCP Server from Scratch.
The Host manages global operations and permissions; each Client handles a dedicated connection to a Server; the Server focuses on providing capabilities.
How Messages Flow: The Full Journey of a Tool Call
Connect the three components to see how an actual tool invocation proceeds:
Make this clear: The Model never touches the Server directly, nor does the Host touch the Server directly; a corresponding Client always sits between them. This "Model ↔ Host ↔ Client ↔ Server" chain is the entire skeleton of MCP.
Why Split It Up: The Benefits of Three Layers
This seemingly circuitous layering delivers tangible decoupling:
- Server Reusability: GitHub writes one official Server, and any Host with Client capabilities (Claude Code, Cursor, etc.) can connect to it without each building its own integration—this is the core way MCP reduces integrations from M×N to M+N;
- Connection Isolation: Each Server has a dedicated Client that operates independently; if one Server fails, it does not drag down others;
- Centralized Permissions in Host: Security decisions are uniformly managed by the Host. The Server simply provides capabilities without worrying about whether it should be invoked, ensuring clear separation of duties;
- Unified Protocol: Clients and Servers speak a common JSON-RPC dialect that is language-agnostic, allowing anyone to implement it.
The payoff of layering: A Server encapsulated once can be reused across multiple Hosts, connections remain mutually isolated, and permissions are centralized in the Host.
Target Audience and Common Misconceptions
This guide is for developers just starting with MCP who are confused by the terminology, as well as users trying to understand exactly what "MCP configuration" means within Claude Code or Cursor. We clarify several frequent misconceptions:
- "Client is the user; Server is in the cloud." Incorrect. The Client is an internal component of the Host, and Servers often run locally on your machine. Do not apply standard web client/server intuition here.
- "Host and Client are two separate programs." No. The Client is a module inside the Host; a single Host can contain multiple Clients.
- "A Server must be a remote service." No. The most common local Server is simply a subprocess launched by the Host, communicating via standard input/output streams.
Frequently Asked Questions
Q: When I configure MCP in Cursor, which role am I configuring? A: You are specifying "which Servers the Host (Cursor) should connect to." Cursor acts as the Host and will automatically create a Client for each Server you configure to establish the connection. You only need to provide the startup command or address for the Server.
Q: How many Servers can one Host connect to? A: There is no hard technical limit, but connecting too many can cause your tool list to balloon, consume excessive context window space, and interfere with model selection. It is recommended that you attach only the necessary Servers for a specific task to keep things streamlined.
Q: Can Servers communicate directly with each other? A: In the standard MCP model, Servers do not communicate directly; all interaction is coordinated through the Host. Each Server functions as an independent capability unit, while orchestration is handled by the Host (and the AI model).
Summary
Remember this nested relationship among the three MCP roles to avoid confusion: the Host is your AI application (the commander that manages permissions); within the Host, a Client is configured for each Server (a one-to-one connector); and the Server is an independent capability provider offering tools, resources, and prompts. Messages always flow along the path "Model ↔ Host ↔ Client ↔ Server." This layered architecture enables servers to be reused across applications, ensures connections are isolated from one another, and centralizes permission control. Once you grasp this diagrammatic overview, everything else in MCP is merely detail.