# Model Context Protocol (MCP) Core Architecture
The Model Context Protocol (MCP) is designed with a flexible and extensible architecture to facilitate seamless communication between Large Language Model (LLM) applications and various integrations. This section consolidates the core architectural components, communication patterns, and operational considerations of MCP.
## Overview of MCP Architecture
MCP operates on a client-server model, defining distinct roles for participants:
* **Hosts**: These are LLM applications, such as Claude Desktop or Integrated Development Environments (IDEs), which initiate connections.
* **Clients**: Residing within the host application, clients maintain one-to-one connections with servers.
* **Servers**: These entities are responsible for providing context, tools, and prompts to the clients.
This architecture allows for multiple clients within a host to connect to different server processes, enabling a distributed and modular system.
## Core Components
MCP's functionality is built upon two primary layers: the Protocol layer and the Transport layer.
### Protocol Layer
The protocol layer manages high-level communication patterns, including message framing, request/response linking, and handling of incoming and outgoing messages. Key classes within this layer include `Protocol`, `Client`, and `Server`.
* **Request Handling**: The protocol allows setting handlers for incoming requests, expecting a `Result` in response.
* **Notification Handling**: It also supports setting handlers for one-way incoming notifications.
* **Sending Messages**: Clients and servers can send requests (expecting a response) and notifications (one-way messages).
### Transport Layer
The transport layer is responsible for the actual communication between clients and servers. All MCP transports utilize [JSON-RPC 2.0](https://www.jsonrpc.org/) for message exchange. MCP supports multiple transport mechanisms:
1. **Stdio Transport**: This mechanism uses standard input/output streams for communication, making it ideal for local processes due to its efficiency for same-machine communication and simple process management.
2. **HTTP with SSE Transport**: This transport uses Server-Sent Events (SSE) for server-to-client messages and HTTP POST requests for client-to-server messages. It is suitable for scenarios requiring HTTP compatibility.
### Message Types
MCP defines four main types of messages exchanged between clients and servers:
1. **Requests**: Messages that expect a response from the receiving party. They typically include a `method` and optional `params`.
2. **Results**: Successful responses to a `Request`, containing the requested data.
3. **Errors**: Indicate that a `Request` failed. They include a `code`, a `message`, and optional `data`.
4. **Notifications**: One-way messages that do not expect a response. Like requests, they include a `method` and optional `params`.
## Connection Lifecycle
An MCP connection follows a defined lifecycle:
1. **Initialization**: The client initiates the connection by sending an `initialize` request, providing its protocol version and capabilities. The server responds with its own version and capabilities. The client then sends an `initialized` notification to acknowledge, after which the connection is ready for normal message exchange.
2. **Message Exchange**: After initialization, both request-response patterns (where either party can send requests and receive responses) and one-way notifications are supported.
3. **Termination**: Connections can be terminated cleanly via a `close()` method, through transport disconnection, or due to error conditions.
## Error Handling
MCP defines standard JSON-RPC error codes (e.g., `ParseError`, `InvalidRequest`, `MethodNotFound`, `InvalidParams`, `InternalError`). SDKs and applications can define custom error codes above -32000. Errors are propagated through error responses to requests, error events on transports, and protocol-level error handlers.
## Implementation Example
Implementing an MCP server involves instantiating a `Server` object, defining request handlers, and connecting it to a transport. For instance, a server can be configured to handle `ListResourcesRequestSchema` to return a list of available resources, then connected to a `StdioServerTransport` for local communication.
## Best Practices
Effective use of MCP involves adhering to several best practices:
### Transport Selection
* **Local Communication**: Use `stdio` transport for processes on the same machine due to its efficiency.
* **Remote Communication**: Use `SSE` for HTTP-compatible scenarios, considering security implications like authentication and authorization.
### Message Handling
* **Request Processing**: Validate inputs thoroughly, use type-safe schemas, handle errors gracefully, and implement timeouts for long operations.
* **Progress Reporting**: For long-running operations, use progress tokens to report incremental progress, including total progress when known.
* **Error Management**: Use appropriate error codes, provide helpful error messages, and ensure resources are cleaned up on errors.
## Security Considerations
Security is paramount in MCP implementations:
* **Transport Security**: Employ TLS for remote connections, validate connection origins, and implement authentication as needed.
* **Message Validation**: Validate all incoming messages, sanitize inputs, check message size limits, and verify the JSON-RPC format.
* **Resource Protection**: Implement access controls, validate resource paths, monitor resource usage, and rate limit requests to prevent abuse.
* **Error Handling**: Avoid leaking sensitive information in error messages, log security-relevant errors, implement proper cleanup, and handle Denial of Service (DoS) scenarios.
## Debugging and Monitoring
To ensure robust MCP applications:
* **Logging**: Log protocol events, track message flow, monitor performance, and record errors for diagnostics.
* **Diagnostics**: Implement health checks, monitor connection state, track resource usage, and profile performance.
* **Testing**: Thoroughly test different transports, verify error handling, check edge cases, and perform load testing on servers.
