MCP Servers

Overview

This package implements the Model Context Protocol (MCP) server specification in Go, providing a framework for real-time communication between AI models and clients using Server-Sent Events (SSE). The implementation follows the standardized protocol for building AI-assisted applications with bidirectional communication capabilities.

Core Components

Server-Sent Events (SSE) Communication

• Real-time Communication: Robust SSE-based communication system that maintains persistent connections with clients
• Connection Management: Client registration, message broadcasting, and client cleanup mechanisms
• Event Handling: Event types for tools, prompts, and resources changes

JSON-RPC Implementation

• Request Processing: Complete JSON-RPC request processor for handling MCP protocol methods
• Response Formatting: Proper response formatting according to JSON-RPC specifications
• Error Handling: Comprehensive error handling with appropriate error codes

Tool Management

• Tool Registration: System to register custom tools with handlers
• Tool Execution: Mechanism to execute tool functions with proper timeout handling
• Result Handling: Flexible result handling supporting various return types (string, JSON, images)

Prompt System

• Prompt Registration: System for registering both static and dynamic prompts
• Argument Validation: Validation for required arguments and default values for optional ones
• Message Generation: Handlers that generate properly formatted conversation messages

Resource Management

• Resource Registration: System for managing and accessing external resources
• Content Delivery: Handlers for delivering resource content to clients on demand
• Resource Subscription: Mechanisms for clients to subscribe to resource updates

Protocol Features

• Initialization Sequence: Proper handshaking with capability negotiation
• Notification Handling: Support for both standard and client-specific notifications
• Message Routing: Intelligent routing of requests to appropriate handlers

Technical Highlights

Configuration System

• Flexible Configuration: Configuration system with sensible defaults and customization options
• CORS Support: Configurable CORS settings for cross-origin requests
• Server Information: Proper server identification and versioning

Client Session Management

• Session Tracking: Client session tracking with unique identifiers
• Connection Health: Ping/pong mechanism to maintain connection health
• Initialization State: Client initialization state tracking

Content Handling

• Multi-format Content: Support for text, code, and binary content
• MIME Type Support: Proper MIME type identification for various content types
• Audience Annotations: Content audience annotations for user/assistant targeting

Usage

Setting Up an MCP Server

To create and start an MCP server:

package main

import (
  "github.com/zeromicro/go-zero/core/conf"
  "github.com/zeromicro/go-zero/core/logx"
  "github.com/zeromicro/go-zero/mcp"
)

func main() {
  // Load configuration from YAML file
  var c mcp.McpConf
  conf.MustLoad("config.yaml", &c)

  // Optional: Disable stats logging
  logx.DisableStat()

  // Create MCP server
  server := mcp.NewMcpServer(c)

  // Register tools, prompts, and resources (examples below)

  // Start the server and ensure it's stopped on exit
  defer server.Stop()
  server.Start()
}

Sample configuration file (config.yaml):

name: mcp-server
host: localhost
port: 8080
mcp:
  name: my-mcp-server
  messageTimeout: 30s # Timeout for tool calls
  cors:
    - http://localhost:3000 # Optional CORS configuration

Registering Tools

Tools allow AI models to execute custom code through the MCP protocol.

Basic Tool Example:

// Register a simple echo tool
echoTool := mcp.Tool{
  Name:        "echo",
  Description: "Echoes back the message provided by the user",
  InputSchema: mcp.InputSchema{
    Properties: map[string]any{
      "message": map[string]any{
        "type":        "string",
        "description": "The message to echo back",
      },
      "prefix": map[string]any{
        "type":        "string",
        "description": "Optional prefix to add to the echoed message",
        "default":     "Echo: ",
      },
    },
    Required: []string{"message"},
  },
  Handler: func(ctx context.Context, params map[string]any) (any, error) {
    var req struct {
      Message string `json:"message"`
      Prefix  string `json:"prefix,optional"`
    }

    if err := mcp.ParseArguments(params, &req); err != nil {
      return nil, fmt.Errorf("failed to parse params: %w", err)
    }

    prefix := "Echo: "
    if len(req.Prefix) > 0 {
      prefix = req.Prefix
    }

    return prefix + req.Message, nil
  },
}

server.RegisterTool(echoTool)

Tool with Different Response Types:

// Tool returning JSON data
dataTool := mcp.Tool{
  Name:        "data.generate",
  Description: "Generates sample data in various formats",
  InputSchema: mcp.InputSchema{
    Properties: map[string]any{
      "format": map[string]any{
        "type":        "string",
        "description": "Format of data (json, text)",
        "enum":        []string{"json", "text"},
      },
    },
  },
  Handler: func(ctx context.Context, params map[string]any) (any, error) {
    var req struct {
      Format string `json:"format"`
    }

    if err := mcp.ParseArguments(params, &req); err != nil {
      return nil, fmt.Errorf("failed to parse params: %w", err)
    }

    if req.Format == "json" {
      // Return structured data
      return map[string]any{
        "items": []map[string]any{
          {"id": 1, "name": "Item 1"},
          {"id": 2, "name": "Item 2"},
        },
        "count": 2,
      }, nil
    }

    // Default to text
    return "Sample text data", nil
  },
}

server.RegisterTool(dataTool)

Image Generation Tool Example:

// Tool returning image content
imageTool := mcp.Tool{
  Name:        "image.generate",
  Description: "Generates a simple image",
  InputSchema: mcp.InputSchema{
    Properties: map[string]any{
      "type": map[string]any{
        "type":        "string",
        "description": "Type of image to generate",
        "default":     "placeholder",
      },
    },
  },
  Handler: func(ctx context.Context, params map[string]any) (any, error) {
    // Return image content directly
    return mcp.ImageContent{
      Data:     "base64EncodedImageData...", // Base64 encoded image data
      MimeType: "image/png",
    }, nil
  },
}

server.RegisterTool(imageTool)

Using ToolResult for Custom Outputs:

// Tool that returns a custom ToolResult type
customResultTool := mcp.Tool{
  Name:        "custom.result",
  Description: "Returns a custom formatted result",
  InputSchema: mcp.InputSchema{
    Properties: map[string]any{
      "resultType": map[string]any{
        "type": "string",
        "enum": []string{"text", "image"},
      },
    },
  },
  Handler: func(ctx context.Context, params map[string]any) (any, error) {
    var req struct {
      ResultType string `json:"resultType"`
    }

    if err := mcp.ParseArguments(params, &req); err != nil {
      return nil, fmt.Errorf("failed to parse params: %w", err)
    }

    if req.ResultType == "image" {
      return mcp.ToolResult{
        Type: mcp.ContentTypeImage,
        Content: map[string]any{
          "data":     "base64EncodedImageData...",
          "mimeType": "image/jpeg",
        },
      }, nil
    }

    // Default to text
    return mcp.ToolResult{
      Type:    mcp.ContentTypeText,
      Content: "This is a text result from ToolResult",
    }, nil
  },
}

server.RegisterTool(customResultTool)

Registering Prompts

Prompts are reusable conversation templates for AI models.

Static Prompt Example:

// Register a simple static prompt with placeholders
server.RegisterPrompt(mcp.Prompt{
  Name:        "hello",
  Description: "A simple hello prompt",
  Arguments: []mcp.PromptArgument{
    {
      Name:        "name",
      Description: "The name to greet",
      Required:    false,
    },
  },
  Content: "Say hello to {{name}} and introduce yourself as an AI assistant.",
})

Dynamic Prompt with Handler Function:

// Register a prompt with a dynamic handler function
server.RegisterPrompt(mcp.Prompt{
  Name:        "dynamic-prompt",
  Description: "A prompt that uses a handler to generate dynamic content",
  Arguments: []mcp.PromptArgument{
    {
      Name:        "username",
      Description: "User's name for personalized greeting",
      Required:    true,
    },
    {
      Name:        "topic",
      Description: "Topic of expertise",
      Required:    true,
    },
  },
  Handler: func(ctx context.Context, args map[string]string) ([]mcp.PromptMessage, error) {
    var req struct {
      Username string `json:"username"`
      Topic    string `json:"topic"`
    }

    if err := mcp.ParseArguments(args, &req); err != nil {
      return nil, fmt.Errorf("failed to parse args: %w", err)
    }

    // Create a user message
    userMessage := mcp.PromptMessage{
      Role: mcp.RoleUser,
      Content: mcp.TextContent{
        Text: fmt.Sprintf("Hello, I'm %s and I'd like to learn about %s.", req.Username, req.Topic),
      },
    }

    // Create an assistant response with current time
    currentTime := time.Now().Format(time.RFC1123)
    assistantMessage := mcp.PromptMessage{
      Role: mcp.RoleAssistant,
      Content: mcp.TextContent{
        Text: fmt.Sprintf("Hello %s! I'm an AI assistant and I'll help you learn about %s. The current time is %s.",
          req.Username, req.Topic, currentTime),
      },
    }

    // Return both messages as a conversation
    return []mcp.PromptMessage{userMessage, assistantMessage}, nil
  },
})

Multi-Message Prompt with Code Examples:

// Register a prompt that provides code examples in different programming languages
server.RegisterPrompt(mcp.Prompt{
  Name:        "code-example",
  Description: "Provides code examples in different programming languages",
  Arguments: []mcp.PromptArgument{
    {
      Name:        "language",
      Description: "Programming language for the example",
      Required:    true,
    },
    {
      Name:        "complexity",
      Description: "Complexity level (simple, medium, advanced)",
    },
  },
  Handler: func(ctx context.Context, args map[string]string) ([]mcp.PromptMessage, error) {
    var req struct {
      Language   string `json:"language"`
      Complexity string `json:"complexity,optional"`
    }

    if err := mcp.ParseArguments(args, &req); err != nil {
      return nil, fmt.Errorf("failed to parse args: %w", err)
    }

    // Validate language
    supportedLanguages := map[string]bool{"go": true, "python": true, "javascript": true, "rust": true}
    if !supportedLanguages[req.Language] {
      return nil, fmt.Errorf("unsupported language: %s", req.Language)
    }

    // Generate code example based on language and complexity
    var codeExample string

    switch req.Language {
    case "go":
      if req.Complexity == "simple" {
        codeExample = `
package main

import "fmt"

func main() {
  fmt.Println("Hello, World!")
}`
      } else {
        codeExample = `
package main

import (
  "fmt"
  "time"
)

func main() {
  now := time.Now()
  fmt.Printf("Hello, World! Current time is %s\n", now.Format(time.RFC3339))
}`
      }
    case "python":
      // Python example code
      if req.Complexity == "simple" {
        codeExample = `
def greet(name):
    return f"Hello, {name}!"

print(greet("World"))`
      } else {
        codeExample = `
import datetime

def greet(name, include_time=False):
    message = f"Hello, {name}!"
    if include_time:
        message += f" Current time is {datetime.datetime.now().isoformat()}"
    return message

print(greet("World", include_time=True))`
      }
    }

    // Create messages array according to MCP spec
    messages := []mcp.PromptMessage{
      {
        Role: mcp.RoleAssistant,
        Content: mcp.TextContent{
          Text: fmt.Sprintf("You are a helpful coding assistant specialized in %s programming.", req.Language),
        },
      },
      {
        Role: mcp.RoleUser,
        Content: mcp.TextContent{
          Text: fmt.Sprintf("Show me a %s example of a Hello World program in %s.", req.Complexity, req.Language),
        },
      },
      {
        Role: mcp.RoleAssistant,
        Content: mcp.TextContent{
          Text: fmt.Sprintf("Here's a %s example in %s:\n\n```%s%s\n```\n\nHow can I help you implement this?",
            req.Complexity, req.Language, req.Language, codeExample),
        },
      },
    }

    return messages, nil
  },
})

Registering Resources

Resources provide access to external content such as files or generated data.

Basic Resource Example:

// Register a static resource
server.RegisterResource(mcp.Resource{
  Name:        "example-document",
  URI:         "file:///example/document.txt",
  Description: "An example document",
  MimeType:    "text/plain",
  Handler: func(ctx context.Context) (mcp.ResourceContent, error) {
    return mcp.ResourceContent{
      URI:      "file:///example/document.txt",
      MimeType: "text/plain",
      Text:     "This is an example document content.",
    }, nil
  },
})

Dynamic Resource with Code Example:

// Register a Go code resource with dynamic handler
server.RegisterResource(mcp.Resource{
  Name:        "go-example",
  URI:         "file:///project/src/main.go",
  Description: "A simple Go example with multiple files",
  MimeType:    "text/x-go",
  Handler: func(ctx context.Context) (mcp.ResourceContent, error) {
    // Return ResourceContent with all required fields
    return mcp.ResourceContent{
      URI:      "file:///project/src/main.go",
      MimeType: "text/x-go",
      Text:     "package main\n\nimport (\n\t\"fmt\"\n\t\"./greeting\"\n)\n\nfunc main() {\n\tfmt.Println(greeting.Hello(\"world\"))\n}",
    }, nil
  },
})

// Register a companion file for the above example
server.RegisterResource(mcp.Resource{
  Name:        "go-greeting",
  URI:         "file:///project/src/greeting/greeting.go",
  Description: "A greeting package for the Go example",
  MimeType:    "text/x-go",
  Handler: func(ctx context.Context) (mcp.ResourceContent, error) {
    return mcp.ResourceContent{
      URI:      "file:///project/src/greeting/greeting.go",
      MimeType: "text/x-go",
      Text:     "package greeting\n\nfunc Hello(name string) string {\n\treturn \"Hello, \" + name + \"!\"\n}",
    }, nil
  },
})

Binary Resource Example:

// Register a binary resource (like an image)
server.RegisterResource(mcp.Resource{
  Name:        "example-image",
  URI:         "file:///example/image.png",
  Description: "An example image",
  MimeType:    "image/png",
  Handler: func(ctx context.Context) (mcp.ResourceContent, error) {
    // Read image from file or generate it
    imageData := "base64EncodedImageData..." // Base64 encoded image data

    return mcp.ResourceContent{
      URI:      "file:///example/image.png",
      MimeType: "image/png",
      Blob:     imageData, // For binary data
    }, nil
  },
})

Using Resources in Prompts

You can embed resources in prompt responses to create rich interactions with proper MCP-compliant structure:

// Register a prompt that embeds a resource
server.RegisterPrompt(mcp.Prompt{
  Name:        "resource-example",
  Description: "A prompt that embeds a resource",
  Arguments: []mcp.PromptArgument{
    {
      Name:        "file_type",
      Description: "Type of file to show (rust or go)",
      Required:    true,
    },
  },
  Handler: func(ctx context.Context, args map[string]string) ([]mcp.PromptMessage, error) {
    var req struct {
      FileType string `json:"file_type"`
    }

    if err := mcp.ParseArguments(args, &req); err != nil {
      return nil, fmt.Errorf("failed to parse args: %w", err)
    }

    var resourceURI, mimeType, fileContent string
    if req.FileType == "rust" {
      resourceURI = "file:///project/src/main.rs"
      mimeType = "text/x-rust"
      fileContent = "fn main() {\n    println!(\"Hello world!\");\n}"
    } else {
      resourceURI = "file:///project/src/main.go"
      mimeType = "text/x-go"
      fileContent = "package main\n\nimport \"fmt\"\n\nfunc main() {\n\tfmt.Println(\"Hello, world!\")\n}"
    }

    // Create message with embedded resource using proper MCP format
    return []mcp.PromptMessage{
      {
        Role: mcp.RoleUser,
        Content: mcp.TextContent{
          Text: fmt.Sprintf("Can you explain this %s code?", req.FileType),
        },
      },
      {
        Role: mcp.RoleAssistant,
        Content: mcp.EmbeddedResource{
          Type: mcp.ContentTypeResource,
          Resource: struct {
            URI      string `json:"uri"`
            MimeType string `json:"mimeType"`
            Text     string `json:"text,omitempty"`
            Blob     string `json:"blob,omitempty"`
          }{
            URI:      resourceURI,
            MimeType: mimeType,
            Text:     fileContent,
          },
        },
      },
      {
        Role: mcp.RoleAssistant,
        Content: mcp.TextContent{
          Text: fmt.Sprintf("Above is a simple Hello World example in %s. Let me explain how it works.", req.FileType),
        },
      },
    }, nil
  },
})

Multiple File Resources Example

// Register a prompt that demonstrates embedding multiple resource files
server.RegisterPrompt(mcp.Prompt{
  Name:        "go-code-example",
  Description: "A prompt that correctly embeds multiple resource files",
  Arguments: []mcp.PromptArgument{
    {
      Name:        "format",
      Description: "How to format the code display",
    },
  },
  Handler: func(ctx context.Context, args map[string]string) ([]mcp.PromptMessage, error) {
    var req struct {
      Format string `json:"format,optional"`
    }

    if err := mcp.ParseArguments(args, &req); err != nil {
      return nil, fmt.Errorf("failed to parse args: %w", err)
    }

    // Get the Go code for multiple files
    var mainGoText string = "package main\n\nimport (\n\t\"fmt\"\n\t\"./greeting\"\n)\n\nfunc main() {\n\tfmt.Println(greeting.Hello(\"world\"))\n}"
    var greetingGoText string = "package greeting\n\nfunc Hello(name string) string {\n\treturn \"Hello, \" + name + \"!\"\n}"

    // Create message with properly formatted embedded resource per MCP spec
    messages := []mcp.PromptMessage{
      {
        Role: mcp.RoleUser,
        Content: mcp.TextContent{
          Text: "Show me a simple Go example with proper imports.",
        },
      },
      {
        Role: mcp.RoleAssistant,
        Content: mcp.TextContent{
          Text: "Here's a simple Go example project:",
        },
      },
      {
        Role: mcp.RoleAssistant,
        Content: mcp.EmbeddedResource{
          Type: mcp.ContentTypeResource,
          Resource: struct {
            URI      string `json:"uri"`
            MimeType string `json:"mimeType"`
            Text     string `json:"text,omitempty"`
            Blob     string `json:"blob,omitempty"`
          }{
            URI:      "file:///project/src/main.go",
            MimeType: "text/x-go",
            Text:     mainGoText,
          },
        },
      },
    }

    // Add explanation and additional file if requested
    if req.Format == "with_explanation" {
      messages = append(messages, mcp.PromptMessage{
        Role: mcp.RoleAssistant,
        Content: mcp.TextContent{
          Text: "This example demonstrates a simple Go application with modular structure. The main.go file imports from a local 'greeting' package that provides the Hello function.",
        },
      })

      // Also show the greeting.go file with correct resource format
      messages = append(messages, mcp.PromptMessage{
        Role: mcp.RoleAssistant,
        Content: mcp.EmbeddedResource{
          Type: mcp.ContentTypeResource,
          Resource: struct {
            URI      string `json:"uri"`
            MimeType string `json:"mimeType"`
            Text     string `json:"text,omitempty"`
            Blob     string `json:"blob,omitempty"`
          }{
            URI:      "file:///project/src/greeting/greeting.go",
            MimeType: "text/x-go",
            Text:     greetingGoText,
          },
        },
      })
    }

    return messages, nil
  },
})

Complete Application Example

Here’s a complete example demonstrating all the components:

package main

import (
  "context"
  "fmt"
  "log"
  "time"

  "github.com/zeromicro/go-zero/core/conf"
  "github.com/zeromicro/go-zero/core/logx"
  "github.com/zeromicro/go-zero/mcp"
)

func main() {
  // Load configuration
  var c mcp.McpConf
  if err := conf.Load("config.yaml", &c); err != nil {
    log.Fatalf("Failed to load config: %v", err)
  }

  // Set up logging
  logx.DisableStat()

  // Create MCP server
  server := mcp.NewMcpServer(c)
  defer server.Stop()

  // Register a simple echo tool
  echoTool := mcp.Tool{
    Name:        "echo",
    Description: "Echoes back the message provided by the user",
    InputSchema: mcp.InputSchema{
      Properties: map[string]any{
        "message": map[string]any{
          "type":        "string",
          "description": "The message to echo back",
        },
        "prefix": map[string]any{
          "type":        "string",
          "description": "Optional prefix to add to the echoed message",
          "default":     "Echo: ",
        },
      },
      Required: []string{"message"},
    },
    Handler: func(ctx context.Context, params map[string]any) (any, error) {
      var req struct {
        Message string `json:"message"`
        Prefix  string `json:"prefix,optional"`
      }

      if err := mcp.ParseArguments(params, &req); err != nil {
        return nil, fmt.Errorf("failed to parse args: %w", err)
      }

      prefix := "Echo: "
      if len(req.Prefix) > 0 {
        prefix = req.Prefix
      }

      return prefix + req.Message, nil
    },
  }
  server.RegisterTool(echoTool)

  // Register a static prompt
  server.RegisterPrompt(mcp.Prompt{
    Name:        "greeting",
    Description: "A simple greeting prompt",
    Arguments: []mcp.PromptArgument{
      {
        Name:        "name",
        Description: "The name to greet",
        Required:    true,
      },
    },
    Content: "Hello {{name}}! How can I assist you today?",
  })

  // Register a dynamic prompt
  server.RegisterPrompt(mcp.Prompt{
    Name:        "dynamic-prompt",
    Description: "A prompt that uses a handler to generate dynamic content",
    Arguments: []mcp.PromptArgument{
      {
        Name:        "username",
        Description: "User's name for personalized greeting",
        Required:    true,
      },
      {
        Name:        "topic",
        Description: "Topic of expertise",
        Required:    true,
      },
    },
    Handler: func(ctx context.Context, args map[string]string) ([]mcp.PromptMessage, error) {
      var req struct {
        Username string `json:"username"`
        Topic    string `json:"topic"`
      }

      if err := mcp.ParseArguments(args, &req); err != nil {
        return nil, fmt.Errorf("failed to parse args: %w", err)
      }

      // Create messages with current time
      currentTime := time.Now().Format(time.RFC1123)
      return []mcp.PromptMessage{
        {
          Role: mcp.RoleUser,
          Content: mcp.TextContent{
            Text: fmt.Sprintf("Hello, I'm %s and I'd like to learn about %s.", req.Username, req.Topic),
          },
        },
        {
          Role: mcp.RoleAssistant,
          Content: mcp.TextContent{
            Text: fmt.Sprintf("Hello %s! I'm an AI assistant and I'll help you learn about %s. The current time is %s.",
              req.Username, req.Topic, currentTime),
          },
        },
      }, nil
    },
  })

  // Register a resource
  server.RegisterResource(mcp.Resource{
    Name:        "example-doc",
    URI:         "file:///example/doc.txt",
    Description: "An example document",
    MimeType:    "text/plain",
    Handler: func(ctx context.Context) (mcp.ResourceContent, error) {
      return mcp.ResourceContent{
        URI:      "file:///example/doc.txt",
        MimeType: "text/plain",
        Text:     "This is the content of the example document.",
      }, nil
    },
  })

  // Start the server
  fmt.Printf("Starting MCP server on %s:%d\n", c.Host, c.Port)
  server.Start()
}

Error Handling

The MCP implementation provides comprehensive error handling:

• Tool execution errors are properly reported back to clients
• Missing or invalid parameters are detected and reported with appropriate error codes
• Resource and prompt lookup failures are handled gracefully
• Timeout handling for long-running tool executions using context
• Panic recovery to prevent server crashes

Advanced Features

• Annotations: Add audience and priority metadata to content
• Content Types: Support for text, images, audio, and other content formats
• Embedded Resources: Include file resources directly in prompt responses
• Context Awareness: All handlers receive context.Context for timeout and cancellation support
• Progress Tokens: Support for tracking progress of long-running operations
• Customizable Timeouts: Configure execution timeouts for tools and operations

Performance Considerations

• Tool execution runs with configurable timeouts to prevent blocking
• Efficient client tracking and cleanup to prevent resource leaks
• Proper concurrency handling with mutex protection for shared resources
• Buffered message channels to prevent blocking on client message delivery