caldav-blade-mcp

A precision CalDAV MCP server that gives AI agents structured access to calendar data. Built for the Model Context Protocol with token efficiency as a first-class design goal.

Why another calendar MCP?

Most calendar integrations dump entire iCalendar blobs into the context window. A single week view can burn 3,000+ tokens before the model even starts reasoning. caldav-blade-mcp takes a different approach:

• Compact output — pipe-delimited, one line per event, null fields omitted. A day's events in ~200 tokens.
• Batch operations — cal_events_batch fetches N calendars in one tool call. One call instead of eleven for a family digest.
• Purpose-built views — cal_today, cal_week, cal_freebusy give you exactly what you need with zero configuration.
• Write-gated — read by default, writes require explicit opt-in. cal_delete has an additional confirm=true safety gate.

Quick start

# Install
uv pip install -e .

# Configure (single provider)
export CALDAV_URL="https://caldav.fastmail.com/dav/calendars/user/you@fastmail.com"
export CALDAV_USERNAME="you@fastmail.com"
export CALDAV_PASSWORD="app-specific-password"

# Run
caldav-blade-mcp

13 tools, 3 categories

Read (9 tools)

Tool	Purpose	Token cost
cal_info	Health check — providers, connection, write gate	~50
cal_calendars	List all calendars (name, UID, provider)	~20/cal
cal_events	Events from one calendar in date range	~30/event
cal_events_batch	Events from N calendars in one call	~30/event
cal_event	Full detail by UID (attendees, recurrence)	~100
cal_search	Search by text, attendee, location	~30/event
cal_today	Today's events across all calendars	~30/event
cal_week	This week's events across all calendars	~30/event
cal_freebusy	Busy periods only — cheapest availability check	~15/period

Write (4 tools, gated)

Tool	Purpose
cal_create	Create event with all fields (description, location, recurrence, attendees, alarm)
cal_update	Partial update — only changed fields, auto-increments SEQUENCE
cal_delete	Delete by UID — requires confirm=true
cal_move	Move event between calendars

Output format

── Work ──
08:30-09:30 | Standup | uid=abc123
14:00-15:00 | Design review @ Level 3 | loc=Level 3 | uid=def456

── Personal ──
All day | School holidays | uid=ghi789
18:30-19:30 | Swim squad | uid=jkl012

Multi-provider support

Run against multiple CalDAV servers simultaneously — useful when calendars span Fastmail, iCloud, Google, or self-hosted (Radicale, Baïkal, Nextcloud).

export CALDAV_PROVIDERS="fastmail,icloud"
export CALDAV_FASTMAIL_URL="https://caldav.fastmail.com/dav/calendars/user/..."
export CALDAV_FASTMAIL_USERNAME="you@fastmail.com"
export CALDAV_FASTMAIL_PASSWORD="fm-app-password"
export CALDAV_ICLOUD_URL="https://caldav.icloud.com/"
export CALDAV_ICLOUD_USERNAME="you@icloud.com"
export CALDAV_ICLOUD_PASSWORD="app-specific-password"

Single-provider mode (the CALDAV_URL / CALDAV_USERNAME / CALDAV_PASSWORD env vars) remains fully supported for simple setups.

Security model

Layer	Mechanism
Write gate	CALDAV_WRITE_ENABLED=true required for any mutation
Delete safety	cal_delete additionally requires confirm=true
Credential scrubbing	Passwords and server URLs stripped from all error output
Bearer auth	Optional CALDAV_MCP_API_TOKEN for HTTP transport
No caching	Credentials read from env at startup, never persisted

Claude Code integration

Add to your MCP config (e.g. claude.nix or settings.json):

{
  "mcpServers": {
    "calendar-piers": {
      "type": "stdio",
      "command": "uv",
      "args": ["--directory", "~/src/caldav-blade-mcp", "run", "caldav-blade-mcp"],
      "env": {
        "CALDAV_URL": "...",
        "CALDAV_USERNAME": "...",
        "CALDAV_PASSWORD": "...",
        "CALDAV_WRITE_ENABLED": "false"
      }
    }
  }
}

A bundled SKILL.md teaches Claude the optimal tool-calling patterns — batch over individual, freebusy over full event scans, convenience views for common queries.

Development

make install-dev    # Install with dev + test dependencies
make test           # Unit tests (mocked caldav, no server needed)
make check          # Lint + format + type-check
make run            # Start MCP server (stdio)

Architecture

src/caldav_blade_mcp/
├── server.py       — FastMCP 2.0 server, 13 @mcp.tool decorators
├── client.py       — CalDAVClient with multi-provider, typed exceptions
├── formatters.py   — Token-efficient output (pipe-delimited, null omission)
├── models.py       — Provider config, write-gate, constants
└── auth.py         — Bearer token middleware for HTTP transport

Built with FastMCP 2.0, caldav, and python-dateutil.

License

MIT