Yandex Calendar

An MCP server that gives an AI assistant access to Yandex Calendar over CalDAV. Read the schedule, create, move and delete events, invite people and reply to invitations.

Русская версия

Note on language: tool descriptions and responses are in Russian, because the server was written for a Russian-speaking assistant. The code, configuration and this document are in English. Everything else works regardless of language.

What it does

Reading

• Events for any period: today by default, or an explicit range up to 92 days.
• All calendars at once, with every event labelled by the calendar it came from — or a single calendar selected by name.
• Recurring events expanded into concrete dates. If expansion fails, the event is returned with a warning rather than presented as accurate.
• Attendees listed with how each of them replied — accepted, declined, tentative or no answer yet.
• All-day events recognised as such, not shown as midnight appointments.
• Every answer states the time zone and UTC offset it used.

Writing

• Create an event: title, start, end or duration, location, description; timed or all-day spanning several days.
• Create a recurring event — daily, weekly, monthly or yearly — bounded by a number of occurrences or by an end date.
• Update an event: move it, rename it, change location or description. Only the fields you pass are touched; a moved event keeps its original duration unless you say otherwise.
• Edit or delete a single occurrence of a recurring series, or the whole series. A changed day becomes a RECURRENCE-ID override, a removed day an EXDATE — the standard mechanisms, so the result looks right in the Yandex app and in any other client.
• Invite people: attendees can be added when creating an event, and added or removed later. Yandex mails the invitations and cancellations itself.
• Reply to someone else's invitation — accept, tentative or decline — for a single occurrence or the whole series.
• Delete an event, with the safeguards described below.

Every write is checked afterwards. After each change the event is read back from the server and compared against what was asked for. If the server accepted the request and did not apply it — which happens, see below — the tool reports failure instead of success. A calendar tool that lies about having deleted something is worse than one that cannot delete at all.

Time zones, handled deliberately

• Every timestamp is converted to the configured zone with zoneinfo. Time strings are never sliced by character position.
• Written events carry an explicit DTSTART;TZID=… plus a full VTIMEZONE block, so any calendar client reads them the same way.
• The machine running the server is usually on UTC; that never leaks into the answers, and the zone actually used is named in every response, so a mistake is visible instead of silent.

Built for an assistant to drive

• Responses are plain readable text with the event uid included, so an edit or delete can follow a listing directly.
• create_event requires an explicit calendar: people keep several calendars for different purposes, and an event silently filed into the wrong one is an event that is lost. The assistant is expected to ask rather than guess.
• Errors come back as sentences explaining what to do next, not stack traces.

Requirements

• Python 3.10 or newer (3.13 tested)
• A Yandex app password, not the account password: https://id.yandex.ru/security/app-passwords → application type "Календарь (CalDAV)"

pip install -r requirements.txt

tzdata is in the requirements on purpose: slim Linux images often ship without a time zone database, and ZoneInfo("Europe/Moscow") then fails.

Configuration

All settings come from environment variables.

Variable	Required	Default	Meaning
YANDEX_USERNAME	yes	—	[email protected]
YANDEX_PASSWORD	yes	—	app password
YANDEX_CALDAV_URL	no	https://caldav.yandex.ru	CalDAV endpoint
YANDEX_TIMEZONE	no	Europe/Moscow	IANA zone for all input and output
YANDEX_TRASH_DIR	no	<project>/trash	where copies of changed and deleted events are kept
YANDEX_ORGANIZER	no	value of YANDEX_USERNAME	address that appears as the meeting organizer in invitations

The server speaks MCP over stdio. Example client configuration is in examples/.

Tools

Tool	What it does
list_events	events for a period; defaults to today; expands recurrences
list_calendars	calendar names and URLs
create_event	create an event, one-off or recurring, with attendees; calendar is required
update_event	move, rename, change location or description, add or remove attendees; one day of a series or the whole series
delete_event	delete your own event; one day of a series or the whole series
respond_event	reply to someone else’s invitation: accept, tentative or decline; one occurrence or the whole series

Deleting is irreversible — what protects you

1. Title check. delete_event takes both the uid and the event title. They are compared; on mismatch nothing is deleted and the response names both titles and the calendar. A stale or confused uid is the realistic way to destroy the wrong event, and this is what catches it.
2. Trash. Before every delete and every update, the full original event is written to YANDEX_TRASH_DIR as a timestamped .ics file. If the copy cannot be written, the operation does not run at all. Recovery is importing that file back.
3. Recurring series. Any edit or delete on a series has to say what it applies to: occurrence for one day, apply_to_series for the whole series. With neither, the tool refuses and explains the choice. This is what keeps "cancel Tuesday" from erasing a year of meetings.

Someone else's meetings: reply, don't edit

An event created by another person belongs to its organizer. You are only an attendee, and this is not a matter of politeness — it is enforced:

• Yandex silently discards an attendee's edit. Excluding one day of another person's series, or changing its title or time, comes back 200 OK and changes nothing. Verified on live data: the exclusion list was identical before and after. update_event and delete_event therefore refuse this outright and explain what to do instead, rather than reporting a success that did not happen.
• The way out is respond_event. accept, tentative or decline, for one occurrence or the whole series — the same choice the Yandex app offers. The organizer is notified.
• A declined occurrence disappears from your calendar rather than staying marked "declined". That is how Yandex implements it: the day is dropped from your copy of the series while the other days stay untouched. The tool recognises this outcome as success and says so in plain words; it does not demand to see a "declined" flag that the server never stores.

Inviting people sends real email

Yandex advertises calendar-auto-schedule, so adding an attendee makes the server send an invitation, and removing one sends a cancellation. Neither can be recalled. This was verified end to end, not merely from the server's advertised capabilities: an invitation added through create_event arrived in the recipient's mailbox.

What follows from that:

• Addresses are taken literally and validated. The server never derives an address from a name — an assistant that half-remembers a contact would otherwise email a stranger.
• update_event adds and removes attendees instead of replacing the list, so answers already given by the others are preserved.
• Every response lists exactly who is on the event and how they replied, so a wrong address is visible immediately.

An assistant driving this server should confirm the address list with its user before calling. Sending an invitation is not an undoable action.

Testing

python tests/write_cycle.py "Calendar name"

Self-cleaning rounds against a real calendar through the actual protocol. A one-off event: create → attempt to delete with a wrong title (must be refused) → move and rename → confirm it appears in the listing → delete → confirm it is gone. A three-occurrence series: edit one day, delete one day, delete the whole series, checking each time that exactly one day changed. If a delete fails, the leftover event's uid is printed instead of being silently left behind.

The attendee round only runs when an address is passed explicitly, because it sends real email:

python tests/write_cycle.py "Calendar name" [email protected]

tests/probe.jsonl is a set of raw protocol messages for checking the handshake and read path:

python src/server.py < tests/probe.jsonl

Known limitations

• All-day events can be created, but their time cannot be moved.
• Recurrence rules are created in their common forms (every day / week / month / year, with a count or an end date). More elaborate rules — "every second Tuesday", weekday sets — are read and expanded correctly but cannot be created through the tool.
• Reminders and alarms are not handled.
• A meeting whose organizer is outside Yandex (one that originated in Google, say) takes a reply only halfway. Yandex refuses to change your own copy, so the server falls back to the scheduling outbox: the reply is handed over for delivery to the organizer and the server answers 2.0;Success. The meeting still shows in your own calendar afterwards and has to be removed in the Yandex app. The tool says exactly that instead of claiming a clean success. Verified on a live event.
• Delivery to the organizer is confirmed only as far as the server's own acknowledgement goes; whether the mail arrived cannot be seen from here.
• Attendees are always invited as required participants; optional attendees and per-attendee roles are not exposed.
• Verified against Yandex Calendar only. Other CalDAV servers are likely to work but are untested.

License

MIT — see LICENSE.