MCP (Model Context Protocol) for Go

MCP-Protocol is a Go implementation of the Model Context Protocol — a standardized way for applications to communicate with AI models. It allows developers to seamlessly bridge applications and AI models using a lightweight, JSON-RPC–based protocol.

This repository contains the shared protocol definitions and schemas for MCP. It is used by github.com/viant/mcp, which provides the actual implementation of the MCP server and client framework.

Official Model Context Protocol Specification

Overview

MCP (Model Context Protocol) is designed to provide a standardized communication layer between applications and AI models. The protocol simplifies the integration of AI capabilities into applications by offering a consistent interface for resource access, prompt management, model interaction, and tool invocation.

Key features:

• JSON-RPC 2.0–based communication
• Support for multiple transport protocols (HTTP/SSE, stdio)
• Server-side features:
  • Resource management
  • Model prompting and completion
  • Tool invocation
  • Subscriptions for resource updates
  • Logging
  • Progress reporting
  • Request cancellation
• Client-side features:
  • Roots
  • Sampling

Module Structure

github.com/viant/mcp-protocol is the Go module containing the shared Model Context Protocol (MCP) contracts:

• schema: JSON-RPC request, result, and notification types generated from the MCP JSON schema.
• server: server.Operations, server.Handler interface and server.DefaultHandler default handler with no-op stubs.
• client: client.Operations, client.Client interface for MCP clients.
• logger: logging interface (Logger) for implementers to emit JSON-RPC notifications.
• oauth2: defines meta information for OAuth2 authorization and authentication flows.
• authorization: authentication definition for global and fine grain resource/tool level authorization

Quick Start

Creating MCP server

package example

import (
  "context"
  "fmt"
  "github.com/viant/jsonrpc"
  "github.com/viant/mcp-protocol/schema"
  serverproto "github.com/viant/mcp-protocol/server"
  "github.com/viant/mcp/server"
  "log"
)

func Usage_Example() {

  newHandler := serverproto.WithDefaultHandler(context.Background(), func(h *serverproto.DefaultHandler) {
    // Register a simple resource
    h.RegisterResource(schema.Resource{Name: "hello", Uri: "/hello"},
      func(ctx context.Context, request *schema.ReadResourceRequest) (*schema.ReadResourceResult, *jsonrpc.Error) {
        return &schema.ReadResourceResult{Contents: []schema.ReadResourceResultContentsElem{{Text: "Hello, world!"}}}, nil
      })

    type Addition struct {
      A int `json:"a"`
      B int `json:"b"`
    }
    // Register a simple calculator tool: adds two integers
    if err := serverproto.RegisterTool[*Addition](h, "add", "Add two integers", func(ctx context.Context, input *Addition) (*schema.CallToolResult, *jsonrpc.Error) {
      sum := input.A + input.B
      return &schema.CallToolResult{Content: []schema.CallToolResultContentElem{{Text: fmt.Sprintf("%d", sum)}}}, nil
    }); err != nil {
      panic(err)
    }
  })

  srv, err := server.New(
    server.WithNewHandler(newHandler),
    server.WithImplementation(schema.Implementation{"default", "1.0"}),
  )
  if err != nil {
    log.Fatalf("Failed to create server: %v", err)
  }

  log.Fatal(srv.HTTP(context.Background(), ":4981").ListenAndServe())
}

Creating Custom Implementer

You can create custom implementer to extend the default behavior:

package myimpl

import (
  "context"
  "github.com/viant/jsonrpc"
  "github.com/viant/jsonrpc/transport"
  "github.com/viant/mcp-protocol/client"
  "github.com/viant/mcp-protocol/logger"
  "github.com/viant/mcp-protocol/schema"
  "github.com/viant/mcp-protocol/server"
)

// MyImplementer is a sample MCP implementer embedding the default Base.
type MyImplementer struct {
  *server.DefaultHandler
}

// ListResources implements the resources/list method.
func (i *MyImplementer) ListResources(
        ctx context.Context,
        req *schema.ListResourcesRequest,
) (*schema.ListResourcesResult, *jsonrpc.Error) {
  // Custom implementation
  return &schema.ListResourcesResult{}, nil
}

// Implements indicates which methods this implementer supports.
func (i *MyImplementer) Implements(method string) bool {
  switch method {
  case schema.MethodResourcesList, schema.MethodResourcesRead:
    return true
  }
  return i.DefaultHandler.Implements(method)
}

func (i *MyImplementer) ReadResource(ctx context.Context, request *schema.ReadResourceRequest) (*schema.ReadResourceResult, *jsonrpc.Error) {
  // Custom implementation
  return &schema.ReadResourceResult{}, nil
}

// NewMyImplementer returns a factory for MyImplementer.
func NewMyImplementer() server.NewHandler {
  return func(
          ctx context.Context,
          notifier transport.Notifier,
          log logger.Logger,
          aClient client.Operations,
) server.Handler {
    base := server.NewDefaultHandler(notifier, log, aClient)
    return &MyImplementer{DefaultHandler: base}
  }
}

Example: Comprehensive Custom Implementer

Use the example/custom package for a more advanced implementer with polling, notifications, and resource watching:

package main

import (
    "context"
    "embed"
    "log"

    "github.com/viant/afs/storage"
    "github.com/viant/mcp/example/custom"
    "github.com/viant/mcp/server"
    "github.com/viant/mcp-protocol/schema"
)

//go:embed data/*
var embedFS embed.FS

func main() {
    config := &custom.Config{
        BaseURL: "embed://data",
        Options: []storage.Option{embedFS},
    }
    newImplementer := custom.New(config)
    srv, err := server.New(
        server.WithNewImplementer(newImplementer),
        server.WithImplementation(schema.Implementation{"custom", "1.0"})
    )
    if err != nil {
        log.Fatalf("Failed to create server: %v", err)
    }
    log.Fatal(srv.HTTP(context.Background(), ":4981").ListenAndServe())
}

Key Components

JSON-RPC Methods

The protocol defines standard methods for communication:

// JSON-RPC method names for MCP protocol
const (
    MethodInitialize                  = "initialize"
    MethodPing                        = "ping"
    MethodResourcesList               = "resources/list"
    MethodResourcesTemplatesList      = "resources/templates/list"
    MethodResourcesRead               = "resources/read"
    MethodSubscribe                   = "resources/subscribe"
    MethodUnsubscribe                 = "resources/unsubscribe"
    MethodPromptsList                 = "prompts/list"
    MethodPromptsGet                  = "prompts/get"
    MethodToolsList                   = "tools/list"
    MethodToolsCall                   = "tools/call"
    MethodComplete                    = "complete"
    MethodLoggingSetLevel             = "logging/setLevel"
    MethodNotificationInitialized     = "notification/initialized"
    MethodNotificationMessage         = "notification/message"
    MethodNotificationCancel          = "notification/cancel"
    MethodNotificationResourceUpdated = "notifications/resources/updated"
    MethodRootsList                   = "roots/list"
    MethodSamplingCreateMessage       = "sampling/createMessage"
)

Authorization

The protocol supports OAuth2 (official MCP spect) and fine-grained authorization (experimental):

// Config holds OAuth2/OIDC configuration for fine-grained control.
type Config struct {
    // Global resource protection metadata
    Global *Authorization `json:"global,omitempty"`
    // ExcludeURI skips middleware on matching paths
    ExcludeURI string `json:"excludeURI,omitempty"`
    // Per-tool authorization metadata
    Tools map[string]*Authorization `json:"tools,omitempty"`
    // Per-tenant authorization metadata
    Tenants map[string]*Authorization `json:"tenants,omitempty"`
}

Contributing

Contributions welcome—please fork and submit a Pull Request.

Credits

Author: Adrian Witas

This project is maintained by Viant.

License

Apache License 2.0. See the LICENSE file in the project root.