MCP Architecture: Components, Lifecycle, and Client-Server Tutorial
What Is Model Context Protocol Architecture?
The model context protocol (MCP) architecture defines a structured way to extend the capabilities of large language models (LLMs) beyond their training data. It introduces a standardized communication layer that allows LLMs to interact with external tools, systems, and data sources. MCP architecture enables dynamic and distributed integration of tools and information by acting as a bridge between the model and the outside world.
MCP architecture consists of several components: the MCP host (typically the AI-powered application), the MCP client (the communication interface), the MCP server (the source of tools and actions), and the context store (which manages contextual knowledge). These components can be deployed locally or remotely and can use different communication transports such as stdio or HTTP.
Read our Beginner’s Guide to MCP for a deeper dive into the model context protocol (MCP).
By using MCP, developers can expose tools to LLMs through structured interfaces. This allows LLMs not just to generate text but also to perform actions, fetch data, and respond to user input with real-time context. MCP supports richer agentic workflows, and is supported by AI development environments like Cursor, Claude Desktop, and VS Code.
In this article:
Key Components of MCP Architecture
Host
The MCP host is the AI-enabled application that integrates and runs the MCP system. It typically embeds the MCP client and acts as the user-facing interface. Examples include development environments like Cursor and Claude Desktop or productivity tools like Raycast. These applications manage the interaction between the user, the model, and the underlying MCP system. While “MCP host” and “MCP client” are sometimes used interchangeably, the host usually refers to the full application stack that includes the MCP client.
MCP Client
The MCP client is the communication layer within the host application. It serves as the interface between the language model and the external MCP servers. The client receives structured tool definitions from the server and forwards tool calls or queries from the model to the correct function implementations. It abstracts the transport protocol (e.g., STDIO or HTTP) and manages the back-and-forth message flow. Though implemented within host applications, the MCP client can also be separated from the host, allowing more flexible deployment architectures.
MCP Server
The MCP server provides the actual tools and data access points that the model can use. These tools may include operations like reading files, querying databases, or interacting with APIs. The server makes these tools available to the MCP client in a discoverable and callable format. Depending on the deployment, MCP servers may run locally as separate processes communicating via STDIO, or remotely via HTTP using Server-Sent Events (SSE) or streamable HTTP specifications. The flexibility of the transport layer allows developers to scale MCP servers across local machines or cloud platforms like Vercel and Cloudflare.
Context Store
The context store manages the persistent or transient context used by the model during interactions. It allows the MCP system to maintain state across tool calls or user sessions. For example, a context store might retain a list of previous user actions, loaded files, or current working directory information. Though not always explicitly discussed in implementation guides, the context store plays a critical role in enabling continuity and richer interactions within MCP-based applications.
MCP Architecture Layers
Data Layer
The data layer defines the core communication protocol used in MCP architecture, based on the JSON-RPC 2.0 specification. It standardizes the structure and meaning of messages exchanged between clients and servers, acting as the foundational contract that all MCP components follow.
This layer handles connection lifecycle events such as initialization, capability negotiation, and termination. It also specifies server-side features, including tools for AI-powered tasks, access to structured resources, and prompts used to guide interactions. On the client side, the protocol supports mechanisms that let the server request user input, sample outputs from the host language model, or log information to the client.
In addition to functional messaging, the data layer includes utility features for real-time communication needs, such as notifications and progress updates. These enable clients and servers to maintain responsive interactions, even during long-running operations.
Transport Layer
The transport layer is responsible for the actual data exchange between MCP clients and servers. It abstracts away the details of connection management, message framing, and secure transmission, allowing the data layer to function independently of the underlying transport method.
MCP supports two main transport mechanisms. The first is stdio, which uses standard input and output streams for local, high-performance communication between processes. The second is streamable HTTP, which enables remote communication using HTTP POST for requests and server-sent events for receiving streaming responses. This method supports various authentication strategies, including bearer tokens, API keys, and custom headers, with OAuth recommended for token acquisition.
By decoupling the transport mechanics from the protocol definition, MCP ensures that the same JSON-RPC 2.0 messages can be sent over different transport types. This enables developers to adapt deployment strategies without modifying the core logic of the application.
The MCP Lifecycle: How MCP Clients Interact with MCP Servers
1. Initialization
The initialization phase is the first required interaction between the MCP client and server. This step ensures that both sides are compatible and capable of communicating effectively. The client starts by sending an initialize request that includes the protocol version it supports, its available capabilities, and client implementation details. This request must not be part of a JSON-RPC batch, since no other messages are allowed before initialization is complete.
The server responds with its own supported protocol version, advertised capabilities (such as access to tools, resources, logging, and prompts), and optional instructions for the client. If the protocol versions are incompatible and neither side can adjust, the client should disconnect.
Once initialization completes successfully, the client must send an initialized notification to signal readiness. Until this message is received, the server should not send requests other than logging or pings. Similarly, the client should avoid sending any tool requests during this phase. Capability negotiation is part of this handshake, defining which optional features (like file system roots, sampling, logging, or resource subscriptions) will be enabled in the session.
2. Operation
After initialization, the protocol enters the operation phase. This is the core phase of the lifecycle, where the client and server exchange structured messages according to the capabilities they agreed upon. Each side must respect the negotiated protocol version and only use supported features.
Common operations in this phase include tool execution, resource access, prompt retrieval, and communication of progress updates. Both parties should handle messages reliably and maintain a stable connection. Timeouts are strongly recommended during this phase to avoid stalled requests. If a response is not received within a predefined time window, the sender should cancel the request.
Middleware and SDKs should support configurable timeout settings, and implementations may choose to extend timeouts if progress notifications are being received. However, a maximum timeout should always be enforced. Error handling during operation includes cases like unsupported requests, version mismatches, and capability negotiation failures.
3. Shutdown
The shutdown phase ends the lifecycle with a clean disconnection. There is no specific shutdown message defined by the protocol. Instead, shutdown is signaled using the transport mechanism.
For stdio-based connections, the client typically closes its input stream to the server, waits for the server to exit, and escalates to SIGTERM or SIGKILL if necessary. The server may also initiate shutdown by closing its output stream and terminating.
In HTTP-based transports, the client or server closes the connection to indicate termination. There is no special protocol message required; connection closure is sufficient. To ensure resource safety and connection stability, both sides should treat timeouts and shutdown handling as critical parts of the implementation.
Related content: Read our guide to MCP tools.
Tutorial: MCP Client-Server Interaction
This tutorial provides a step-by-step walkthrough of how an MCP client and server communicate using JSON-RPC 2.0. It focuses on the data layer protocol and demonstrates how key operations (initialization, tool discovery, tool execution, and notifications) unfold in a real application lifecycle.
Instructions in this tutorial are adapted from the official MCP specification.
1. Initialization: Establishing the Connection
The interaction begins with a lifecycle handshake. The MCP client initiates communication by sending an initialize request that declares its protocol version, supported features, and identifying metadata.
{
"jsonrpc": "2.0",
"id": 1,
"method": "initialize",
"params": {
"protocolVersion": "2025-06-18",
"capabilities": {
"elicitation": {}
},
"clientInfo": {
"name": "example-client",
"version": "1.0.0"
}
}
}The server responds with its own supported features, such as available tools and resources. This negotiation ensures both sides agree on capabilities and protocol compatibility. Once initialization completes, the client sends a notifications/initialized message to signal readiness.
This step sets up the foundation for all future communication. It defines the scope of what each side can do and prevents incompatible operations during the session.
2. Tool Discovery: Listing Available Functions
After initialization, the client discovers server capabilities by requesting the tool list:
{
"jsonrpc": "2.0",
"id": 2,
"method": "tools/list"
}The server replies with metadata for each tool it offers. Each tool is described by:
name: A unique identifier used for invoking the tooltitle: A display name for UI purposesdescription: A textual explanation of the tool’s purposeinputSchema: A JSON Schema specifying valid input parameters
This allows clients to build UI affordances or enable LLMs to select tools dynamically, based on capabilities exposed during the discovery phase.
3. Tool Execution: Performing Server-Side Actions
To use a tool, the client issues a tools/call request that specifies the tool name and input arguments:
{
"jsonrpc": "2.0",
"id": 3,
"method": "tools/call",
"params": {
"name": "temperature_today",
"arguments": {
"location": "Paris",
"units": "metric"
}
}
}The server processes the call, executes the tool logic, and returns a structured response. The result is delivered in a content array, which may include plain text, images, or other types of content depending on the use case.
This pattern supports dynamic behavior where the client or an AI model can invoke functions at runtime and incorporate results directly into the user experience.
4. Real-Time Updates: Receiving Server Notifications
MCP supports server-initiated notifications that inform clients about changes. For example, if the list of tools changes, the server sends a notifications/tools/list_changed message:
{
"jsonrpc": "2.0",
"method": "notifications/tools/list_changed"
}Since this is a notification (no id), no response is expected. Upon receiving it, the client typically re-issues a tools/list request to refresh its registry.
This event-driven approach keeps clients in sync with the server’s evolving state, avoiding polling and ensuring that UI or AI systems have the most accurate tool availability at all times.