Stateless Google Calendar MCP Proxy

This is a heavily streamlined, stateless implementation of the Google Calendar Model Context Protocol (MCP) server, designed exclusively for headless integration with orchestrators like n8n.

By aggressively adhering to the "ponytail rule" (lazy senior dev: extreme simplicity, YAGNI, no speculative complexity), all stateful components—such as SQLite databases, embedded web interfaces, complex local multi-tenancy mappings, and internal token refreshing background jobs—have been stripped out.

Instead, this server acts as a thin, highly reliable proxy between the MCP standard and the Google Calendar API, leveraging an external Baserow database as the single source of truth for Google OAuth credentials.

Architecture

• Stateless by Design: This application does not persist any local state.

• HTTP/SSE Transport: This server implements the standard MCP SSEServerTransport. It listens on a specified port (default 3000) for Server-Sent Events (SSE) connections at /sse and POST message payloads at /messages.

• Application Layer Authentication: Multi-tenancy is handled at the tool schema level. Every tool exposed to the LLM requires a dentist_id parameter.

• Baserow Auth Engine: Before a tool accesses Google Calendar, the handler intercepts the dentist_id argument, securely pulls the tenant's encrypted Google OAuth credentials directly from Baserow via the REST API, decrypts them in-memory, instantiates a headless Google OAuth2Client, and performs the requested operation.

• Global App Credentials: The Google Cloud application credentials (client_id and client_secret) are shared globally and stored in environment variables, keeping the Baserow table lean.

Related MCP server: @calendar-mcp/server

Setup Guide

1. Prerequisites

You need a Baserow Database configured with a table (e.g., Table 759) containing the following fields for per-tenant Google OAuth credentials:

• GCal_access_token (Base64 encrypted)

• GCal_refresh_token (Base64 encrypted)

• expiry_date

• GCal_Auth_Status (Optional, object containing a value, e.g., 'Revoked')

2. Configuration

Create a .env file in the root of the project with your connection and encryption details:

# Baserow Setup
BASEROW_URL=https://api.baserow.io
BASEROW_DATABASE_TOKEN=your_secure_database_token
BASEROW_TABLE_ID=your_table_id_here

# Encryption
MASTER_ENCRYPTION_KEY=your_32_byte_aes_256_key_here

# Google Cloud Global App Credentials
GOOGLE_CLIENT_ID=your_global_google_app_client_id
GOOGLE_CLIENT_SECRET=your_global_google_app_client_secret

# Server Port
PORT=3000

3. Installation & Execution

Option A: Local Node.js

# Install dependencies
npm ci

# Build the project
npm run build

# Start the server
npm start

Option B: Docker / Portainer Deployment

• Do not hardcode secrets into the docker-compose.yml.

• Use the Portainer "Environment variables" UI to store sensitive keys securely.

• Provide the following template for the stack configuration, demonstrating the ${VAR_NAME} mapping technique:

version: '3.8'

services:
  gcal-mcp:
    build: https://github.com/adibaLLC/GCal-mcp.git
    container_name: gcal-mcp
    restart: unless-stopped
    expose:
      - "3000"
    environment:
      # Hardcoded non-sensitive settings:
      - PORT=3000
      - BASEROW_URL=https://api.baserow.io
      - BASEROW_TABLE_ID=your_table_id
      # Secrets securely mapped from Portainer UI fields:
      - BASEROW_DATABASE_TOKEN=${BASEROW_API_TOKEN}
      - MASTER_ENCRYPTION_KEY=${MASTER_ENCRYPTION_KEY}
      - GOOGLE_CLIENT_ID=${GOOGLE_CLIENT_ID}
      - GOOGLE_CLIENT_SECRET=${GOOGLE_CLIENT_SECRET}
    networks:
      proxy:
        aliases:
          - gcal-mcp

networks:
  proxy:
    external: true

4. Integration with n8n

To use this server in your n8n workflow, configure your MCP node as follows:

1. Connection URL: Connect to the Server-Sent Events (SSE) endpoint: http://localhost:3000/sse

2. Tool Execution: When the LLM calls a calendar tool (like create-event), it will automatically prompt for the dentist_id argument based on the tool schema. N8n simply passes the prompt and the LLM fulfills it. No custom headers (X-Baserow-Row-Id) are required at the HTTP layer.

Tools Available

This MCP server exposes the following Google Calendar tools. Note that every tool requires a dentist_id parameter.

• list-calendars: Fetch calendars the authenticated user has access to.

• list-events: Fetch events from a calendar.

• search-events: Free-text search across a calendar.

• get-event: Retrieve a specific event by ID.

• create-event: Insert a new event.

• create-events: Bulk insert multiple events.

• update-event: Patch an existing event.

• delete-event: Remove an event.

• get-freebusy: Check free/busy availability.

• find-available-slots: Fetch exact available time slots for a given dentist and date, enforcing clinic working hours and treatment durations. Automatically scans forward up to 30 days to find the next 2 available slots.

• get-current-time: Helpful for LLMs to understand the relative local time.

• respond-to-event: Accept, decline, or mark tentative for invitations.

Developer Note (Ponytail Architecture)

If you are extending this server:

1. Do not add state.

2. Do not add caching layers for tokens; let Baserow handle the data persistence. If performance becomes an issue, implement caching at the n8n layer.

3. Keep handlers simple. Extract data from this.getClient(), use googleapis, format the response, and return.

4. Schema sanitization: additionalProperties is automatically stripped from tool schemas in server.ts to maintain compatibility with Google Gemini's function calling API.