Model Context Protocol (MCP) Implementation

This repository contains a reference implementation of the Model Context Protocol (MCP), a standardized client-server protocol designed for Large Language Model (LLM) applications.

⚠️ Pre-1.0 Release Notice

This is a pre-1.0 release (version 0.9.0) of the Mcp.Net libraries. While the core functionality is stable and usable, some features are still in development. The public API may undergo minor changes before the 1.0.0 release. See the Current Status section for details.

Overview

The Model Context Protocol enables seamless communication between LLM clients and context servers, allowing for dynamic tool execution, resource management, and prompt handling. This implementation follows the latest MCP specification (2024-11-05) and provides a solid foundation for building MCP-compatible applications.

Project Structure

• Mcp.Net.Core: Shared models, protocol abstractions, and attribute-based tool definitions
• Mcp.Net.Server: Server implementation with ASP.NET Core and SSE for streaming
• Mcp.Net.Client: Client implementation for consuming MCP services
• Mcp.Net.Examples.SimpleServer: Example server implementation with sample tools

Key Features

• JSON-RPC 2.0 messaging: Standards-based message format for API communication
• Dual transport support: Both Server-Sent Events (SSE) and Standard I/O (stdio) transport implementations
• Tool registration system: Attribute-based tool registration with automatic schema generation
• Tool discovery and invocation: Dynamic tool discovery and parameter validation
• Error handling: Comprehensive error handling and propagation
• Content types: Support for different content types in responses
• Capability negotiation: Protocol-compliant capability negotiation during initialization

Transport Layer Implementation

Server-Sent Events (SSE) Transport

The SSE transport implementation enables real-time streaming from server to client via HTTP:

1. Connection Establishment:

   • Client connects to the /sse endpoint
   • Server generates a unique session ID
   • Server establishes an SSE stream with appropriate headers (Content-Type: text/event-stream)
   • Server sends an endpoint event containing a message channel URL (/messages?sessionId={sessionId})

2. Message Processing:

   • Client sends JSON-RPC requests to the message endpoint URL via HTTP POST
   • Server returns 202 Accepted immediately as an acknowledgment
   • Server processes the request asynchronously
   • Server delivers the actual response via the SSE stream
   • Client correlates responses with requests using the JSON-RPC ID

3. Session Management:

   • SseConnectionManager tracks active SSE connections by session ID
   • Connections are automatically removed when closed
   • Each session maintains its own state

4. Error Handling:

   • Transport errors are propagated via event handlers
   • Connection issues are logged and reported to clients
   • Proper cleanup occurs when connections are closed

Standard I/O (stdio) Transport

The stdio transport implementation allows direct communication via standard input/output streams:

1. Connection Handling:

   • Reads raw bytes from stdin, processes them as JSON-RPC messages
   • Writes responses to stdout
   • Maintains a persistent buffer to handle partial messages

2. Message Processing:

   • Line-based protocol with newline-delimited JSON messages
   • Processes each complete line as a separate JSON-RPC message
   • Maintains a buffer for incomplete messages

3. Error Handling:

   • Captures and reports parsing errors
   • Gracefully handles EOF and stream closures
   • Proper cleanup on connection termination

Client Implementation

McpClient (SSE-based)

Provides a high-level client for communicating with MCP servers:

1. Connection Setup:

   • Establishes HTTP connection to server base URL
   • Connects to SSE endpoint for receiving server messages
   • Handles dynamic endpoint discovery via SSE endpoint event
   • Supports capability negotiation during initialization

2. Request-Response Handling:

   • Manages pending requests with a correlation system based on message IDs
   • Automatically routes responses to the appropriate waiting tasks
   • Provides type-safe methods for common operations (ListTools, CallTool)

3. Error Handling:

   • Robust error detection and propagation
   • Timeout handling for endpoint discovery
   • Automatic recovery from transient errors

StdioMcpClient

Provides a simplified client for stdio-based communication:

1. Stream Handling:

   • Uses standard input/output streams by default
   • Supports custom streams for testing
   • Maintains a continuous read loop on a background task

2. Message Parsing:

   • Line-based protocol with newline-delimited JSON
   • Buffer management for partial messages
   • Type-safe deserialization of response objects

3. Request Management:

   • Similar ID-based correlation mechanism as the SSE client
   • Support for both requests and notifications
   • Type-safe API for common operations

Connection Flow Analysis

Initialization Sequence

1. Client connects to server:

   • For SSE: HTTP connection to /sse endpoint
   • For stdio: Direct connection via standard I/O

2. Transport establishment:

   • For SSE: Server sends endpoint URL via SSE event
   • For stdio: Direct bidirectional communication is established

3. Protocol initialization:

   • Client sends initialize request with client info and capabilities
   • Server responds with server info and capabilities
   • Client sends notifications/initialized notification

4. Capability verification:

   • Client examines server capabilities
   • Client verifies required capabilities are supported

Tool Invocation Flow

1. Tool discovery:

   • Client sends tools/list request
   • Server responds with available tools and schemas

2. Tool invocation:

   • Client sends tools/call request with tool name and arguments
   • Server validates arguments against schema
   • Server executes tool logic
   • Server returns result via transport
   • Client processes and deserializes response

Connection Termination

1. Client-initiated termination:

   • Client calls Dispose() which cancels operations
   • For SSE: HTTP connection is closed
   • For stdio: No specific close action needed

2. Server-initiated termination:

   • For SSE: HTTP response completes or server closes connection
   • For stdio: If stdin is closed, client detects EOF
   • Both trigger OnClose events for cleanup

Known Considerations

1. Error Handling Robustness:

   • Both transports have comprehensive error handling
   • Exception propagation is consistent across transports
   • Proper cleanup occurs in all error scenarios

2. Message Correlation:

   • ID-based correlation system is reliable in both transports
   • Timeouts are properly handled
   • Orphaned requests are cleaned up

3. Transport Differences:

   • SSE transport is more suitable for web-based applications
   • stdio transport is ideal for CLI tools and local processes
   • Both implement the same ITransport interface for consistency

Getting Started

Prerequisites

• .NET 9.0 SDK or later

Installation via NuGet

# Install the client package dotnet add package Mcp.Net.Client --version 0.9.0 # Install the server package dotnet add package Mcp.Net.Server --version 0.9.0 # Install the core package (normally not needed directly) dotnet add package Mcp.Net.Core --version 0.9.0

Build and Run from Source

1. Clone the repository
2. Build the solution: dotnet build
3. Start the server with SSE transport (default): dotnet run --project Mcp.Net.Examples.SimpleServer Or with stdio transport: dotnet run --project Mcp.Net.Examples.SimpleServer -- --stdio
4. Run the client (in another terminal if using SSE): dotnet run --project Mcp.Net.Client

Example Usage

The Mcp.Net.Examples.SimpleServer project demonstrates how to set up a server with sample tools like Google Search and Web Scraping, while the Mcp.Net.Client provides classes for connecting to the server, listing available tools, and calling them.

Server-side Example

// Define a tool using attributes [McpTool("Calculator", "Provides mathematical operations")] public class CalculatorTool { [McpTool("add", "Add two numbers")] public CallToolResult Add( [McpParameter(true, "First number")] double a, [McpParameter(true, "Second number")] double b ) { return new CallToolResult { Content = new[] { new TextContent { Text = $"The sum of {a} and {b} is {a + b}" } }, IsError = false, }; } } // In Program.cs, register tools from assembly mcpServer.RegisterToolsFromAssembly(Assembly.GetExecutingAssembly(), app.Services);

Client-side Example

using var client = new McpClient("http://localhost:5000", "ExampleClient", "0.9.0"); // Initialize the connection await client.Initialize(); // List available tools var tools = await client.ListTools(); // Call a tool var result = await client.CallTool("add", new { a = 5, b = 3 }); var content = result.Content.FirstOrDefault() as TextContent; Console.WriteLine(content?.Text); // "The sum of 5 and 3 is 8"

Protocol Implementation

The implementation follows the MCP specification (2024-11-05) with these core methods:

• initialize: Establishes connection and exchanges capabilities
• tools/list: Discovers available tools on the server
• tools/call: Invokes specific tools with arguments

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

Current Status

This implementation is currently at version 0.9.0, which signifies a pre-1.0 release with the following status:

Fully Implemented Features

• ✅ Core JSON-RPC message exchange
• ✅ Dual transport support (SSE and stdio)
• ✅ Tool registration and discovery
• ✅ Tool invocation with parameter validation
• ✅ Basic error handling and propagation
• ✅ Text-based content responses
• ✅ Client connection and initialization flow

Partially Implemented Features

• ⚠️ Resource management (API defined but not fully implemented)
• ⚠️ Prompt management (API defined but not fully implemented)
• ⚠️ Advanced content types (Image, Resource, Embedded)
• ⚠️ XML documentation (present on key interfaces but not complete)

Planned for 1.0.0

• Complete resource management implementation
• Complete prompt management implementation
• Finalize all content type implementations
• Comprehensive XML documentation
• Enhanced test coverage
• Additional example implementations
• API stabilization (no more breaking changes)

We welcome feedback and contributions to help reach a stable 1.0.0 release.

License

This project is licensed under the MIT License - see the LICENSE file for details.

NuGet Packaging and Publishing

This project is configured for publishing to NuGet.org. For detailed instructions on building and publishing the NuGet packages, see NuGetPublishingSteps.md.