Protoc Gen Go MCP

protoc-gen-go-mcp

This is a Topeka plugin for the protoc compiler that generates a model-context-protocol(MCP) server based on a protocol buffer definition. Conceptually, this allows an AI model to use existing gRPC codebases with natural language, allowing for rapid prototyping and usage of LLM capabilities for protobuf based codebases.

Prerequisites

• Go 1.20 or later
• protoc 3.20 or later
• protoc-gen-go-grpc 1.71 or later

Running the plugin

Check out the Makefile for explicit command usage. Use make generate to generate the example MCP server from the proto file.

Debugging the plugin

make start-debugger

Will start the delve debugger in headless mode on port 2345. The file under debug will be $GOPATH/bin/protoc-gen-go-mcp. You can connect to that in VSCode with the following launch.json configuration.

{
  "name": "Connect to protoc-gen-go-mcp plugin",
  "type": "go",
  "request": "attach",
  "mode": "remote",
  "remotePath": "${workspaceFolder}",
  "port": 2345,
  "host": "127.0.0.1"
}

Testing the example

Install the example mcp-vibe server

go install ./cmd/mcp-vibe

Add the mcp-vibe server to your mcp servers:

{
  "mcpServers": {
    "vibe": {
      "command": "mcp-vibe"
    }
  }
}

Philosophical Notes

The plugin uses the existing code generation for protocol buffers and gRPC servers and builds upon that base, using and reusing parts where necessary. This gives us a healthy amount of code reuse while allowing us to control what we expose to end users. We want this plugin to provide sane, out-of-the-box functionality while allowing for easy extension.

How is this achieved?

The code is broken into composable parts:

1. The protoc-gen-go-mcp plugin generates default tools based on the request parameters for any given RPC. eg:

message SetVibeRequest {
  string vibe = 1;
}

message SetVibeResponse {
  string previous_vibe = 1;
  string vibe = 2;
}

service VibeService {
  // Set Vibe
  rpc SetVibe(SetVibeRequest) returns (SetVibeResponse) {}
}

This snippet defines the SetVibe RPC, which takes a SetVibeRequest message and contains a definition of the request parameter SetVibeRequest message. The plugin generates the following tools by default:

func (s *vibeServiceMCPServer) SetVibeTool() mcp.Tool {
	tool := mcp.NewTool("...internal instantiation of the tool")
	// ... internal implementation follows
	return tool
}

This tool can be subsequently registered with the server to make the RPC available to the model.

1. The protoc-gen-go-mcp plugin generates a default handler that interacts with a generated gRPC client for interaction with this server to parse the mcp.Tool into a defined gRPC request leveraging a generated client.

func (s *vibeServiceMCPServer) SetVibeHandler(ctx context.Context, req mcp.CallToolRequest) (*mcp.CallToolResult, error) {
	//... internal instantiation of the handler
}

1. These two pieces are combined upon registration to provide the LLM with knowledge of the RPC method and how to use them:

func (s *vibeServiceMCPServer) RegisterDefaultTools() {
	//...other tools added above
	s.MCPServer.AddTool(s.SetVibeTool(), s.SetVibeHandler)
    //...other tools added below
}

Topeka

Topeka is an open source project that provides code-generators for Model-Context-Protocol (MCP).

It is designed to facilitate the usage of MCP seamlessly against existing gRPC based applications. This is done via leveraging code generation using the protoc compiler and installing the relevant Topeka plugin.

The plugins follow Semantic Versioning and any plugin prior to 1.0.0 releases ARE still subject to breaking changes. Please note, this is applied to the generated servers, not the plugins themselves, which do not provide public APIs. This project reserves the right to change how code generation is achieved, while maintaining stable MCP server APIs.

Maintainers

Stable Kernel is the primary maintainer of this project and sponsor of the plugins, though we welcome outside contributions.

Stable Kernel is a digital transformation company building solutions that power LLM enablement for growing businesses. We have a track record of helping our partners solve their biggest challenges on their digital journey, whether they need insights or implementation. Every day, millions of people rely on software that we developed, and our custom software development and technology services have been trusted by some of the most innovative Fortune 500 companies in the world.