Google Calendar MCP Server (Go)

A Model Context Protocol (MCP) server implementation in Go that provides Google Calendar integration with HTTP transport.

Features

• Event Management: Create, read, update, and delete calendar events
• Calendar Management: Manage multiple calendars
• Free/Busy Queries: Check availability across calendars
• All-day Events: Support for both timed and all-day events
• Attendee Management: Add and manage event attendees
• Time Zone Support: Handle events across different time zones
• OAuth2 Authentication: Secure Google OAuth2 flow
• JSON-RPC 2.0: Standard MCP protocol implementation

Tools Available

Event Operations

• create_event - Create new calendar events with attendees and reminders
• get_event - Retrieve event details by ID
• update_event - Modify existing events
• delete_event - Remove events from calendar
• list_events - Search and filter calendar events

Calendar Management

• list_calendars - List all accessible calendars
• get_calendar - Get details for a specific calendar
• create_calendar - Create new calendars
• delete_calendar - Remove calendars

Availability

• get_freebusy - Query free/busy information across calendars

Installation

1. Clone the repository:

git clone https://github.com/phildougherty/mcp-google-calendar-go.git
cd mcp-google-calendar-go

2. Install dependencies:

go mod tidy

3. Set up Google OAuth2 credentials:
   • Go to Google Cloud Console
   • Create a new project or select existing
   • Enable Google Calendar API
   • Create OAuth2 credentials (Desktop application)
   • Download the credentials as gcp-oauth.keys.json

Configuration

Place your gcp-oauth.keys.json file in:

• Current directory, or
• ~/.calendar-mcp/gcp-oauth.keys.json

Environment variables:

• CALENDAR_OAUTH_PATH - Custom path to OAuth keys file
• CALENDAR_CREDENTIALS_PATH - Custom path to stored credentials

Usage

1. Authentication

First, authenticate with Google:

go run main.go -auth

This will open a browser window for OAuth2 authentication.

2. Start the Server

go run main.go -port 8080

Optional flags:

• -port - Server port (default: 8080)
• -debug - Enable debug logging

3. Test the Server

Check server health:

curl http://localhost:8080/health

4. Using MCP Tools

The server implements JSON-RPC 2.0 over HTTP. Example requests:

Initialize connection:

curl -X POST http://localhost:8080/ \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
      "protocolVersion": "2024-11-05",
      "capabilities": {},
      "clientInfo": {"name": "test-client", "version": "1.0.0"}
    }
  }'

List available tools:

curl -X POST http://localhost:8080/ \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 2,
    "method": "tools/list"
  }'

Create a calendar event:

curl -X POST http://localhost:8080/ \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 3,
    "method": "tools/call",
    "params": {
      "name": "create_event",
      "arguments": {
        "summary": "Team Meeting",
        "description": "Weekly team sync",
        "location": "Conference Room A",
        "startTime": "2024-01-15T10:00:00Z",
        "endTime": "2024-01-15T11:00:00Z",
        "timeZone": "America/New_York",
        "attendees": ["colleague@example.com"]
      }
    }
  }'

List upcoming events:

curl -X POST http://localhost:8080/ \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 4,
    "method": "tools/call",
    "params": {
      "name": "list_events",
      "arguments": {
        "timeMin": "2024-01-01T00:00:00Z",
        "timeMax": "2024-12-31T23:59:59Z",
        "maxResults": 10,
        "orderBy": "startTime"
      }
    }
  }'

Development

Building

go build -o calendar-mcp-server main.go

Running Tests

go test ./...

Project Structure

├── main.go                 # Application entry point
├── internal/
│   ├── config/            # Configuration management
│   │   └── config.go
│   ├── calendar/          # Google Calendar API client
│   │   ├── client.go      # OAuth2 and service setup
│   │   └── operations.go  # Calendar operations
│   ├── mcp/               # MCP protocol implementation
│   │   ├── server.go      # HTTP server and JSON-RPC
│   │   ├── tools.go       # Tool registry and handlers
│   │   └── types.go       # Data structures and schemas
│   └── types/             # Shared type definitions
│       └── types.go
├── go.mod
└── README.md

API Reference

Authentication Flow

1. Start server with -auth flag
2. Browser opens for Google OAuth2
3. Grant permissions to Calendar API
4. Credentials saved locally for future use

Tool Schemas

All tools follow JSON Schema specifications. See internal/mcp/types.go for complete schema definitions.

Event Time Formats

• Timed Events: Use RFC3339 format (e.g., 2024-01-15T10:00:00Z)
• All-day Events: Use date format (e.g., 2024-01-15)
• Time Zones: Use IANA time zone names (e.g., America/New_York)

Error Handling

• JSON-RPC 2.0 error responses for protocol errors
• Tool execution errors returned in response with isError: true
• Detailed error messages provided for debugging

Security

• OAuth2 tokens stored securely in user home directory
• HTTPS recommended for production deployments
• CORS headers configured for cross-origin requests

Example Integration

This server can be integrated with MCP clients like Claude Desktop. Add to your MCP configuration:

{
  "mcpServers": {
    "google-calendar": {
      "command": "/path/to/calendar-mcp-server",
      "args": ["-port", "8080"]
    }
  }
}

Contributing

1. Fork the repository
2. Create a feature branch
3. Make your changes
4. Add tests if applicable
5. Submit a pull request

License

This project is licensed under the MIT License - see the LICENSE file for details.

Support

For issues and questions:

• Create an issue on GitHub
• Check the Google Calendar API documentation
• Review MCP protocol specifications