MCP package

The mcp package provides an SDK for writing model context protocol clients and servers. It is a work-in-progress. As of writing, it is a prototype to explore the design space of client/server transport and binding.

Installation

The mcp package is currently internal and cannot be imported using go get.

Quickstart

Here's an example that creates a client that talks to an MCP server running as a sidecar process:

package main

import (
	"context"
	"log"
	"os/exec"

	"golang.org/x/tools/internal/mcp"
)

func main() {
	ctx := context.Background()
	// Create a new client, with no features.
	client := mcp.NewClient("mcp-client", "v1.0.0", nil)
	// Connect to a server over stdin/stdout
	transport := mcp.NewCommandTransport(exec.Command("myserver"))
	if err := client.Connect(ctx, transport, nil); err != nil {
		log.Fatal(err)
	}
	// Call a tool on the server.
	if content, err := client.CallTool(ctx, "greet", map[string]any{"name": "you"}); err != nil {
		log.Printf("CallTool returns error: %v", err)
	} else {
		log.Printf("CallTool returns: %v", content)
	}
}

Here is an example of the corresponding server, connected over stdin/stdout:

package main

import (
	"context"

	"golang.org/x/tools/internal/mcp"
)

type HiParams struct {
	Name string `json:"name"`
}

func SayHi(ctx context.Context, cc *mcp.ServerSession, params *HiParams) ([]mcp.Content, error) {
	return []mcp.Content{
		mcp.TextContent{Text: "Hi " + params.Name},
	}, nil
}

func main() {
	// Create a server with a single tool.
	server := mcp.NewServer("greeter", "v1.0.0", nil)
	server.AddTools(mcp.MakeTool("greet", "say hi", SayHi))
	// Run the server over stdin/stdout, until the client diconnects
	_ = server.Run(context.Background(), mcp.NewStdIOTransport(), nil)
}

Core Concepts

The mcp package leverages Go‘s reflect package to automatically generate the JSON schema for your tools / prompts’ input parameters. As an mcp server developer, ensure your input parameter structs include the standard "json" tags (as demonstrated in the HiParams example). Refer to the jsonschema package for detailed information on schema inference.

Tools

Tools in MCP allow servers to expose executable functions that can be invoked by clients and used by LLMs to perform actions. The server can add tools using

...
server := mcp.NewServer("greeter", "v1.0.0", nil)
server.AddTools(mcp.MakeTool("greet", "say hi", SayHi))
...

Prompts

Prompts enable servers to define reusable prompt templates and workflows that clients can easily surface to users and LLMs. The server can add prompts by using

...
server := mcp.NewServer("greeter", "v0.0.1", nil)
server.AddPrompts(mcp.MakePrompt("greet", "", PromptHi))
...

Resources

Resources are a core primitive in the Model Context Protocol (MCP) that allow servers to expose data and content that can be read by clients and used as context for LLM interactions.

Resources are not supported yet.

Testing

To test your client or server using stdio transport, you can use local transport instead of creating real stdio transportation. See example.

To test your client or server using sse transport, you can use the httptest package. See example.

Code of Conduct

This project follows the Go Community Code of Conduct. If you encounter a conduct-related issue, please mail conduct@golang.org.

License

Unless otherwise noted, the Go source files are distributed under the BSD-style license found in the LICENSE file.

Upon a potential move to modelcontextprotocol, the license will be updated to the MIT License, and the license header will reflect the Go MCP SDK Authors.