Hello World - MCP Server in Go

Introduction

This example demonstrates how to build a minimal Model Context Protocol (MCP) server in Go, using the mcp-go SDK. MCP is a lightweight JSON‑RPC‑based protocol that lets language model clients discover and invoke external tools in a structured way.

With just a few lines of Go code, you can:

1. Define an MCP server with a human‑readable name and version.
2. Declare one or more tools (commands) with typed parameters and descriptions.
3. Register handler functions that execute your custom logic when a client calls a tool.
4. Expose the server over standard I/O (stdin/stdout) so any MCP‑compatible client (like Cursor, Claude Desktop, or other IDE extensions) can launch and communicate with it without extra setup.

How It Works

1. Server Initialization

   mcpServer := server.NewMCPServer(
     "Hello World MCP Server",
     "1.0.0",
     server.WithToolCapabilities(false),
   )
   

   • Creates an MCP server named “Hello World MCP Server” at version 1.0.0.
   • Disables automatic tool capability announcements (so only your explicit tools appear).

2. Tool Definition

   helloWorldTool := mcp.NewTool("hello_world",
     mcp.WithDescription("Greet someone"),
     mcp.WithString("name",
      mcp.Required(),
      mcp.Description("Person’s name"),
     ),
   )
   

   • Defines a hello_world tool that accepts a single required string parameter name.
   • Descriptions help clients display usage information and generate prompts.

3. Handler Registration

   mcpServer.AddTool(helloWorldTool, func(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
     name, ok := req.Params.Arguments["name"].(string)
   
     if !ok {
      return nil, errors.New("name must be a string")
     }
   
     return mcp.NewToolResultText(fmt.Sprintf("Hello, %s!", name)), nil
   })
   

   • Associates the hello_world tool with a Go function.
   • Extracts the name argument, validates it, and returns a greeting.
   • Errors are surfaced back to the client as JSON‑RPC errors.

4. Serving Over Stdio

   if err := server.ServeStdio(mcpServer); err != nil {
    fmt.Printf("Server error: %v\n", err)
   }
   

   • Starts an event loop reading JSON‑RPC requests from stdin and writing responses to stdout.
   • Allows any MCP client to launch the binary directly and speak MCP over pipes.

Getting Started

1. Install Dependencies

   go mod tidy
   

2. Build the Server

   go build -o hello-mcp ./src/main.go
   

3. Connect from an MCP Client

   • In your client’s configuration, point to the hello-mcp executable.
   • The client will automatically discover the hello_world tool schema and let you invoke it.

   Example:

   {
     "mcpServers": {
       "demo-go-mcp": {
         "command": "path-to-server/hello-mcp"
       }
     }
   }
   

   or, use my MCP client made in Go

Enjoy building more complex workflows by adding additional tools, parameters, and transports (e.g., HTTP/SSE) as needed!