cal-scheduler

If you are an agent installing this MCP, read AGENTS.md first.

cal-scheduler

A thin, timezone-correct MCP server for scheduling against any CalDAV calendar. It gives an LLM agent a small, strict set of calendar tools and does the deterministic, fiddly parts of iCalendar correctly so the model doesn't have to.

It is not an NLP layer: the agent phrases the request, the server validates it, rejects bad input loudly, and persists clean zoned .ics.

Why it exists

Most calendar tooling an LLM reaches for gets three things subtly wrong. cal-scheduler fixes them by construction:

• Stores zoned, not bare UTC. Events are written with TZID/VTIMEZONE, so a weekly 9am stays 9am across a daylight-saving boundary instead of drifting an hour. Storing bare UTC is the classic cause of that drift.
• Validates recurrence. It rejects an RRULE whose anchor contradicts it (e.g. a series starting on the 30th but set to repeat on the 1st), and normalises UNTIL to UTC under a zoned DTSTART as RFC 5545 requires.
• Does real single-occurrence edits. Exclude one instance (EXDATE) or move one instance (RECURRENCE-ID) without disturbing the rest of the series — the operations naive wrappers tend to lack.

How it works

A small uv Python package that composes mature libraries rather than implementing a calendar engine:

Module	Role
config.py	environment config (CALDAV_*, CAL_DEFAULT_TZ)
timezones.py	parse datetimes; naive → assume default-zone wall time, offset → normalise into the zone; report what was assumed
ical.py	build/parse VEVENTs (icalendar), expand ranges (recurring-ical-events), recurrence validation, EXDATE/RECURRENCE-ID ops
store.py	CalDAV transport (caldav) — list/create/delete calendars, get/put/delete events, read-modify-write with etag
server.py	the FastMCP stdio server and the tool surface

It runs as a stdio MCP server that an MCP host (Claude, an agent harness, etc.) spawns as a subprocess.

Install

Requires Python ≥ 3.11. The package is on PyPI as cal-scheduler-mcp:

uv tool install cal-scheduler-mcp

(or pip install cal-scheduler-mcp, uvx --from cal-scheduler-mcp cal-scheduler, etc.)

For editable dev work, install from a clone:

# run straight from the repo with uv (no global install)
uv run --directory /path/to/cal-scheduler-mcp cal-scheduler

# or install the console script into a tool environment
gh repo clone limey/cal-scheduler-mcp
uv tool install --editable /path/to/cal-scheduler-mcp
# or, with SSH GitHub access, in one step:
uv tool install git+ssh://git@github.com/limey/cal-scheduler-mcp

Don't uv add cal-scheduler-mcp for the MCP — uv add writes into whatever project you're sitting in, not into the tool environment. For an MCP server (spawned as a subprocess), uv tool install is the right shape.

Configure

All configuration is via environment variables:

Variable	Required	Default	Meaning
CALDAV_BASE_URL	✅	—	CalDAV server URL, e.g. http://127.0.0.1:5232
CALDAV_USERNAME		—	CalDAV account user
CALDAV_PASSWORD		—	CalDAV account password
CAL_DEFAULT_TZ		Pacific/Auckland	IANA zone every event is stored in

Many MCP hosts strip inherited environment from stdio servers, so set these in the host's per-server env block rather than relying on the ambient shell.

Example MCP host config

The MCP runs as a stdio subprocess that the host spawns. Many hosts strip inherited PATH from that subprocess, so wire the uv run --directory form rather than relying on the cal-scheduler shim being on the spawn host's PATH:

{
  "mcpServers": {
    "cal-scheduler": {
      "command": "uv",
      "args": ["run", "--directory", "/abs/path/to/cal-scheduler-mcp", "cal-scheduler"],
      "env": {
        "CALDAV_BASE_URL": "http://127.0.0.1:5232",
        "CALDAV_USERNAME": "me",
        "CALDAV_PASSWORD": "secret",
        "CAL_DEFAULT_TZ": "Pacific/Auckland"
      }
    }
  }
}

/abs/path/to/cal-scheduler-mcp is the absolute path to a local clone of this repo (see Install above).

If you installed from PyPI with uv tool install cal-scheduler-mcp, use the installed shim directly:

{
  "mcpServers": {
    "cal-scheduler": {
      "command": "/path/to/cal-scheduler",
      "env": {
        "CALDAV_BASE_URL": "http://127.0.0.1:5232",
        "CALDAV_USERNAME": "me",
        "CALDAV_PASSWORD": "secret",
        "CAL_DEFAULT_TZ": "Pacific/Auckland"
      }
    }
  }
}

Find the shim path with which cal-scheduler or uv tool list --show-paths.

Pair it with any CalDAV server. A simple self-hosted option is Radicale (plain http://, no TLS needed for local use).

Tool surface

Events — list_events(start, end, [calendar]), create_event(summary, start, [end, calendar, description, location, rrule]), update_event(uid, …), delete_event(uid, [calendar]), exclude_occurrence(uid, occurrence, [calendar]), move_occurrence(uid, occurrence, new_start, [new_end, calendar]), mark_done(uid, [calendar]) — tag an event done and record the timestamp.

Calendars — list_calendars, create_calendar(name), delete_calendar(name).

Helper — resolve_datetime(value) — preview how a datetime will be interpreted, without writing anything.

The timezone rule (the whole point)

Every event is stored zoned to CAL_DEFAULT_TZ.

• A naive datetime (2026-06-30T21:00) is assumed to be wall time in that zone, and the tool response says so ("assumed Pacific/Auckland wall time").
• An offset-qualified datetime (…+12:00) is honoured as an instant and re-expressed in the zone — same wall clock when the offset matches, a correct conversion otherwise.
• A date-only value (2026-06-30) is an all-day event.

Develop

uv sync                 # install deps + dev tools
uv run ruff check       # lint
uv run pytest           # unit tests (no server required)

The unit tests cover the pure layers (timezone resolution, recurrence validation, EXDATE/RECURRENCE-ID construction) and need no running CalDAV server. To exercise the full stack end to end, point CALDAV_BASE_URL at a throwaway CalDAV account.

License

MIT © 2026 Robert Clark