MCP Architecture - Model Context Protocol

Client-Server Architecture

The Model Context Protocol (MCP) follows a client-server architecture that enables AI models to connect with external data sources and tools. This architecture consists of two main components:

MCP Architecture Diagram showing clients, servers, and transport layer interactions

MCP Clients

MCP clients are applications that use AI models and need to access external data or functionality. Examples include:

AI-powered chat interfaces (like Claude Desktop)
Integrated development environments (IDEs)
Custom applications that incorporate LLMs

Clients connect to MCP servers to retrieve context, execute tools, and access data needed by the AI model.

MCP Servers

MCP servers expose data sources, tools, and functionality to MCP clients. Examples include:

File system servers (for accessing local files)
Database connectors (for querying databases)
API integrations (for accessing web services)
Custom tool implementations (for specialized functionality)

Servers handle requests from clients, process them, and return results back to the client.

Communication Flow

The communication between MCP clients and servers follows a structured flow:

Connection Establishment: The client initiates a connection to one or more MCP servers.
Tool Discovery: The client queries the server for available tools and capabilities using the listTools method.
Tool Invocation: The client requests the server to perform an action using the callTool method, passing the necessary parameters.
Response Processing: The server executes the requested action and returns the results to the client.
Model Integration: The client incorporates the server's response into the AI model's context, allowing the model to use the information in generating its response.

This structured communication flow ensures that AI models can access external data and functionality in a consistent and secure manner.

MCP Technical Components

Protocol Layer

The protocol layer handles message framing, request/response linking, and high-level communication patterns between clients and servers. It defines the structure of messages and the sequence of operations.

Transport Layer

The transport layer deals with the actual communication between clients and servers. MCP supports multiple transport mechanisms, including stdio for local processes and HTTP over SSE (Server-Sent Events) for remote connections.

Tool Specifications

Tool specifications define the capabilities that MCP servers expose to clients. Each tool includes a name, description, parameter schema, and return type, allowing clients to understand how to use the tool.

Security Model

The security model ensures that MCP connections are secure and that data is protected. It includes features like server-controlled access to resources and clear system boundaries.

Transport Mechanisms

MCP supports different transport mechanisms for communication between clients and servers:

Stdio Servers

Stdio servers run as subprocesses of the client application, enabling local communication through standard input/output streams. This approach is suitable for accessing local resources like the file system or for running tools directly on the client machine.

Stdio servers are easy to set up and secure for local operations but are limited to running on the same machine as the client.

HTTP over SSE Servers

HTTP over Server-Sent Events (SSE) enables remote communication between clients and servers. This approach allows MCP servers to run on different machines or in cloud environments, making it suitable for distributed architectures.

HTTP/SSE servers provide greater flexibility for deployment but require additional security considerations for remote access.

Tool Implementation

MCP tools are the functional units that MCP servers expose to clients. Each tool has specific characteristics:

Name: A unique identifier for the tool.
Description: A human-readable explanation of what the tool does.
Parameters: A schema defining the inputs the tool expects, usually in JSON Schema format.
Return Value: The type and structure of data the tool returns.

Tools can range from simple utilities like file readers to complex integrations with external APIs or custom business logic.

// Example tool definition
{
  "name": "readFile",
  "description": "Reads the contents of a file",
  "parameters": {
    "type": "object",
    "properties": {
      "path": {
        "type": "string",
        "description": "Path to the file to read"
      }
    },
    "required": ["path"]
  },
  "returnType": {
    "type": "string",
    "description": "Contents of the file"
  }
}

Implementation Libraries

Anthropic and the MCP community provide libraries to simplify the implementation of MCP clients and servers:

Client Libraries

Client libraries handle the complexity of connecting to MCP servers, discovering tools, and making tool calls. They are available for different programming languages and environments.

TypeScript/JavaScript MCP Client
Python MCP Client
Java MCP Client (in collaboration with Spring AI)
C# MCP Client

Server Libraries

Server libraries simplify the creation of MCP servers by handling protocol implementation, tool registration, and request processing. They allow developers to focus on implementing the tool functionality.

TypeScript/JavaScript MCP Server
Python MCP Server
Java MCP Server
C# MCP Server

Security Architecture

Security is a fundamental aspect of the MCP architecture, with several key principles:

Server-Controlled Access: MCP servers maintain control over their resources, determining what data is accessible to clients.
No API Key Sharing: There's no need to share API keys with LLM providers, as the MCP server handles authentication with external services.
Clear System Boundaries: The client-server architecture establishes clear boundaries between systems, limiting exposure of sensitive information.
Local Processing: For sensitive operations, MCP can use local stdio servers to ensure data doesn't leave the user's machine.

As MCP evolves, additional security features are being developed, including enterprise-grade authentication for remote servers and more fine-grained access controls.