MCP Go 🚀

A Go implementation of the Model Context Protocol (MCP), enabling seamless integration between LLM applications and external data sources and tools.

package main

import (
    "context"
    "fmt"

    "github.com/mark3labs/mcp-go/mcp"
    "github.com/mark3labs/mcp-go/server"
)

func main() {
    // Create MCP server
    s := server.NewMCPServer(
        "Demo 🚀",
        "1.0.0",
    )

    // Add tool
    tool := mcp.NewTool("hello_world",
        mcp.WithDescription("Say hello to someone"),
        mcp.WithString("name",
            mcp.Required(),
            mcp.Description("Name of the person to greet"),
        ),
    )

    // Add tool handler
    s.AddTool(tool, helloHandler)

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

func helloHandler(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
    name, ok := request.Params.Arguments["name"].(string)
    if !ok {
        return mcp.NewToolResultError("name must be a string"), nil
    }

    return mcp.NewToolResultText(fmt.Sprintf("Hello, %s!", name)), nil
}

That's it!

MCP Go handles all the complex protocol details and server management, so you can focus on building great tools. It aims to be high-level and easy to use.

Key features:

• Fast: High-level interface means less code and faster development
• Simple: Build MCP servers with minimal boilerplate
• Complete*: MCP Go aims to provide a full implementation of the core MCP specification

(*emphasis on aims)

🚨 🚧 🏗️ MCP Go is under active development, as is the MCP specification itself. Core features are working but some advanced capabilities are still in progress.

Table of Contents

• Installation
• Quickstart
• What is MCP?
• Core Concepts
  • Server
  • Resources
  • Tools
  • Prompts
• Examples
• Contributing
  • Prerequisites
  • Installation
  • Testing
  • Opening a Pull Request

Installation

go get github.com/mark3labs/mcp-go

Quickstart

Let's create a simple MCP server that exposes a calculator tool and some data:

package main

import (
    "context"
    "fmt"

    "github.com/mark3labs/mcp-go/mcp"
    "github.com/mark3labs/mcp-go/server"
)

func main() {
    // Create a new MCP server
    s := server.NewMCPServer(
        "Calculator Demo",
        "1.0.0",
        server.WithResourceCapabilities(true, true),
        server.WithLogging(),
    )

    // Add a calculator tool
    calculatorTool := mcp.NewTool("calculate",
        mcp.WithDescription("Perform basic arithmetic operations"),
        mcp.WithString("operation",
            mcp.Required(),
            mcp.Description("The operation to perform (add, subtract, multiply, divide)"),
            mcp.Enum("add", "subtract", "multiply", "divide"),
        ),
        mcp.WithNumber("x",
            mcp.Required(),
            mcp.Description("First number"),
        ),
        mcp.WithNumber("y",
            mcp.Required(),
            mcp.Description("Second number"),
        ),
    )

    // Add the calculator handler
    s.AddTool(calculatorTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
        op := request.Params.Arguments["operation"].(string)
        x := request.Params.Arguments["x"].(float64)
        y := request.Params.Arguments["y"].(float64)

        var result float64
        switch op {
        case "add":
            result = x + y
        case "subtract":
            result = x - y
        case "multiply":
            result = x * y
        case "divide":
            if y == 0 {
                return mcp.NewToolResultError("Cannot divide by zero"), nil
            }
            result = x / y
        }

        return mcp.NewToolResultText(fmt.Sprintf("%.2f", result)), nil
    })

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

What is MCP?

The Model Context Protocol (MCP) lets you build servers that expose data and functionality to LLM applications in a secure, standardized way. Think of it like a web API, but specifically designed for LLM interactions. MCP servers can:

• Expose data through Resources (think of these sort of like GET endpoints; they are used to load information into the LLM's context)
• Provide functionality through Tools (sort of like POST endpoints; they are used to execute code or otherwise produce a side effect)
• Define interaction patterns through Prompts (reusable templates for LLM interactions)
• And more!

Core Concepts

Server

Show Server Examples

The server is your core interface to the MCP protocol. It handles connection management, protocol compliance, and message routing:

// Create a basic server
s := server.NewMCPServer(
    "My Server",  // Server name
    "1.0.0",     // Version
)

// Start the server using stdio
if err := server.ServeStdio(s); err != nil {
    log.Fatalf("Server error: %v", err)
}

Resources

Show Resource Examples

Resources are how you expose data to LLMs. They can be anything - files, API responses, database queries, system information, etc. Resources can be:

• Static (fixed URI)
• Dynamic (using URI templates)

Here's a simple example of a static resource:

// Static resource example - exposing a README file
resource := mcp.NewResource(
    "docs://readme",
    "Project README",
    mcp.WithResourceDescription("The project's README file"), 
    mcp.WithMIMEType("text/markdown"),
    mcp.WithAnnotations([]mcp.Role{mcp.RoleAssistant}, 0.8),
)

// Add resource with its handler
s.AddResource(resource, func(ctx context.Context, request mcp.ReadResourceRequest) ([]interface{}, error) {
    content, err := os.ReadFile("README.md")
    if err != nil {
        return nil, err
    }
    
    return []interface{}{
        mcp.TextResourceContents{
            ResourceContents: mcp.ResourceContents{
                URI:      "docs://readme",
                MIMEType: "text/markdown",
            },
            Text: string(content),
        },
    }, nil
})

And here's an example of a dynamic resource using a template:

// Dynamic resource example - user profiles by ID
template := mcp.NewResourceTemplate(
    "users://{id}/profile",
    "User Profile",
    mcp.WithTemplateDescription("Returns user profile information"),
    mcp.WithTemplateMIMEType("application/json"),
    mcp.WithTemplateAnnotations([]mcp.Role{mcp.RoleAssistant, mcp.RoleUser}, 0.5),
)

// Add template with its handler
s.AddResourceTemplate(template, func(ctx context.Context, request mcp.ReadResourceRequest) ([]interface{}, error) {
    userID := request.Params.URI // Extract ID from the full URI
    
    profile, err := getUserProfile(userID)  // Your DB/API call here
    if err != nil {
        return nil, err
    }
    
    return []interface{}{
        mcp.TextResourceContents{
            ResourceContents: mcp.ResourceContents{
                URI:      fmt.Sprintf("users://%s/profile", userID),
                MIMEType: "application/json",
            },
            Text: profile,
        },
    }, nil
})

The examples are simple but demonstrate the core concepts. Resources can be much more sophisticated - serving multiple contents, using annotations, integrating with databases or external APIs, etc.

Tools

Show Tool Examples

Tools let LLMs take actions through your server. Unlike resources, tools are expected to perform computation and have side effects. They're similar to POST endpoints in a REST API.

Simple calculation example:

calculatorTool := mcp.NewTool("calculate",
    mcp.WithDescription("Perform basic arithmetic calculations"),
    mcp.WithString("operation",
        mcp.Required(),
        mcp.Description("The arithmetic operation to perform"),
        mcp.Enum("add", "subtract", "multiply", "divide"),
    ),
    mcp.WithNumber("x",
        mcp.Required(),
        mcp.Description("First number"),
    ),
    mcp.WithNumber("y",
        mcp.Required(),
        mcp.Description("Second number"),
    ),
)

s.AddTool(calculatorTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
    op := request.Params.Arguments["operation"].(string)
    x := request.Params.Arguments["x"].(float64)
    y := request.Params.Arguments["y"].(float64)

    var result float64
    switch op {
    case "add":
        result = x + y
    case "subtract":
        result = x - y
    case "multiply":
        result = x * y
    case "divide":
        if y == 0 {
            return mcp.NewToolResultError("Division by zero is not allowed"), nil
        }
        result = x / y
    }
    
    return mcp.FormatNumberResult(result), nil
})

HTTP request example:

httpTool := mcp.NewTool("http_request",
    mcp.WithDescription("Make HTTP requests to external APIs"),
    mcp.WithString("method",
        mcp.Required(),
        mcp.Description("HTTP method to use"),
        mcp.Enum("GET", "POST", "PUT", "DELETE"),
    ),
    mcp.WithString("url",
        mcp.Required(),
        mcp.Description("URL to send the request to"),
        mcp.Pattern("^https?://.*"),
    ),
    mcp.WithString("body",
        mcp.Description("Request body (for POST/PUT)"),
    ),
)

s.AddTool(httpTool, func(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
    method := request.Params.Arguments["method"].(string)
    url := request.Params.Arguments["url"].(string)
    body := ""
    if b, ok := request.Params.Arguments["body"].(string); ok {
        body = b
    }

    // Create and send request
    var req *http.Request
    var err error
    if body != "" {
        req, err = http.NewRequest(method, url, strings.NewReader(body))
    } else {
        req, err = http.NewRequest(method, url, nil)
    }
    if err != nil {
        return mcp.NewToolResultError(fmt.Sprintf("Failed to create request: %v", err)), nil
    }

    client := &http.Client{}
    resp, err := client.Do(req)
    if err != nil {
        return mcp.NewToolResultError(fmt.Sprintf("Request failed: %v", err)), nil
    }
    defer resp.Body.Close()

    // Return response
    respBody, err := io.ReadAll(resp.Body)
    if err != nil {
        return mcp.NewToolResultError(fmt.Sprintf("Failed to read response: %v", err)), nil
    }

    return mcp.NewToolResultText(fmt.Sprintf("Status: %d\nBody: %s", resp.StatusCode, string(respBody))), nil
})

Tools can be used for any kind of computation or side effect:

• Database queries
• File operations
• External API calls
• Calculations
• System operations

Each tool should:

• Have a clear description
• Validate inputs
• Handle errors gracefully
• Return structured responses
• Use appropriate result types

Prompts

Show Prompt Examples

Prompts are reusable templates that help LLMs interact with your server effectively. They're like "best practices" encoded into your server. Here are some examples:

// Simple greeting prompt
s.AddPrompt(mcp.NewPrompt("greeting",
    mcp.WithPromptDescription("A friendly greeting prompt"),
    mcp.WithArgument("name",
        mcp.ArgumentDescription("Name of the person to greet"),
    ),
), func(ctx context.Context, request mcp.GetPromptRequest) (*mcp.GetPromptResult, error) {
    name := request.Params.Arguments["name"].(string)
    if name == "" {
        name = "friend"
    }
    
    return mcp.NewGetPromptResult(
        "A friendly greeting",
        []mcp.PromptMessage{
            mcp.NewPromptMessage(
                mcp.RoleAssistant,
                mcp.NewTextContent(fmt.Sprintf("Hello, %s! How can I help you today?", name)),
            ),
        },
    ), nil
})

// Code review prompt with embedded resource
s.AddPrompt(mcp.NewPrompt("code_review",
    mcp.WithPromptDescription("Code review assistance"),
    mcp.WithArgument("pr_number",
        mcp.ArgumentDescription("Pull request number to review"),
        mcp.RequiredArgument(),
    ),
), func(ctx context.Context, request mcp.GetPromptRequest) (*mcp.GetPromptResult, error) {
    prNumber := request.Params.Arguments["pr_number"].(string)
    if prNumber == "" {
        return nil, fmt.Errorf("pr_number is required")
    }
    
    return mcp.NewGetPromptResult(
        "Code review assistance",
        []mcp.PromptMessage{
            mcp.NewPromptMessage(
                mcp.RoleSystem,
                mcp.NewTextContent("You are a helpful code reviewer. Review the changes and provide constructive feedback."),
            ),
            mcp.NewPromptMessage(
                mcp.RoleAssistant,
                mcp.NewEmbeddedResource(mcp.ResourceContents{
                    URI: fmt.Sprintf("git://pulls/%s/diff", prNumber),
                    MIMEType: "text/x-diff",
                }),
            ),
        },
    ), nil
})

// Database query builder prompt
s.AddPrompt(mcp.NewPrompt("query_builder",
    mcp.WithPromptDescription("SQL query builder assistance"),
    mcp.WithArgument("table",
        mcp.ArgumentDescription("Name of the table to query"),
        mcp.RequiredArgument(),
    ),
), func(ctx context.Context, request mcp.GetPromptRequest) (*mcp.GetPromptResult, error) {
    tableName := request.Params.Arguments["table"].(string)
    if tableName == "" {
        return nil, fmt.Errorf("table name is required")
    }
    
    return mcp.NewGetPromptResult(
        "SQL query builder assistance",
        []mcp.PromptMessage{
            mcp.NewPromptMessage(
                mcp.RoleSystem,
                mcp.NewTextContent("You are a SQL expert. Help construct efficient and safe queries."),
            ),
            mcp.NewPromptMessage(
                mcp.RoleAssistant,
                mcp.NewEmbeddedResource(mcp.ResourceContents{
                    URI: fmt.Sprintf("db://schema/%s", tableName),
                    MIMEType: "application/json",
                }),
            ),
        },
    ), nil
})

Prompts can include:

• System instructions
• Required arguments
• Embedded resources
• Multiple messages
• Different content types (text, images, etc.)
• Custom URI schemes

Examples

For examples, see the examples/ directory.

Contributing

Open Developer Guide

Prerequisites

Go version >= 1.23

Installation

Create a fork of this repository, then clone it:

git clone https://github.com/mark3labs/mcp-go.git
cd mcp-go

Testing

Please make sure to test any new functionality. Your tests should be simple and atomic and anticipate change rather than cement complex patterns.

Run tests from the root directory:

go test -v './...'

Opening a Pull Request

Fork the repository and create a new branch:

git checkout -b my-branch

Make your changes and commit them:

git add . && git commit -m "My changes"

Push your changes to your fork:

git push origin my-branch

Feel free to reach out in a GitHub issue or discussion if you have any questions!