Deltat

@open-deltat/mcp

Give an agent a calendar it can actually book on, without ever double-booking.

npx @open-deltat/mcp

Most scheduling APIs give an agent one verb: *book*. That is fine for a form submission and wrong for an agent, because an agent takes seconds to decide. It reads that 14:30 is free, says "how about 14:30?", waits for a human to answer, and by then someone else has taken it. Checking and booking are two separate moments, and everything in between is a race.

This server has no one-step booking verb. Booking is:

find_slots  →  hold_slot  →  commit_hold

The hold is the point. It reserves the slot for a few minutes, so between offering a time and confirming it, that time is yours and nobody else can take it. Holds expire on their own, so an abandoned conversation cleans itself up and holding costs nothing.

Underneath is deltat, a database whose one invariant is that two bookings can never occupy the same span. The refusal comes from the storage engine, not from application code that checked first and hoped.

Tools

That last row is deliberate. Every other calendar API has a one-step booking verb, so models reach for one here and find nothing. Rather than let that become a hallucinated tool call or an abandoned conversation, the verb is registered and refuses, explaining the two real steps in the response. It never books anything.

Times cross the boundary as RFC 3339 strings with a mandatory offset, never bare epoch milliseconds. A zoneless timestamp is rejected rather than silently interpreted in whatever zone the process runs in. Errors come back with a typed prefix to branch on: CONFLICT (pick another slot), EXPIRED (re-hold), INVALID, NOT_FOUND.

A refusal tells you when

When a slot is taken or the time falls outside opening hours, the refusal carries the times that would have worked, so an agent can counter-offer in the same turn instead of starting over:

{

"error": "CONFLICT", "booked": false, "held": false, "reserved": false, "retry_same_time": true, "alternatives": [ { "start": "2026-06-01T12:00:00.000Z", "end": "2026-06-01T13:00:00.000Z", "start_local": "1 Jun 2026, 2:00 pm" } ], "next": "These times were free a moment ago but are NOT reserved. Offer one, then call hold_slot on it before you tell anyone it is theirs." }

On a live phone call that removes a whole model turn, which is otherwise silence.

reserved is always false, and it is stated rather than implied because a model that skims to alternatives must not conclude something was set aside. An alternative is a time that was free; taking one still means calling hold_slot.

A refusal with nothing to offer keeps the plain CODE: message shape, so a kernel with counter-offers disabled (DELTAT_COUNTER_OFFER=0) or one older than the feature behaves exactly as before.

Book several calendars together

A camera body, its lens and the crew. A surgeon and an operating room. An appointment and the drive to it. Pass every calendar to the same three tools:

find_slots({ calendar_ids: [body, lens, crew], from, to })    times when ALL are free

hold_slot({ calendar_ids: [body, lens, crew], start, end }) holds every one, or none commit_hold({ hold_ids, label }) books every one, or none

One calendar is the same call with a one-item list. The commit is a single record in deltat, so neither a competing booker nor a crash can leave the body booked and the lens free.

When a kit is refused, the alternatives are times when every calendar is free for the same length, never the time just refused, in the same shape as above.

Configure it

You need a deltat server, 0.4.0 or later. Self-host it with Docker:

docker run -p 5433:5433 -e DELTAT_PASSWORD=<your-password> ghcr.io/open-deltat/deltat

Then add the MCP server. In Claude Code:

claude mcp add deltat \

--env DELTAT_HOST=localhost \ --env DELTAT_PORT=5433 \ --env DELTAT_DATABASE=public \ --env DELTAT_PASSWORD= \ -- npx -y @open-deltat/mcp

In Claude Desktop, edit claude_desktop_config.json and restart:

{

"mcpServers": { "deltat": { "command": "npx", "args": ["-y", "@open-deltat/mcp"], "env": { "DELTAT_HOST": "localhost", "DELTAT_PORT": "5433", "DELTAT_DATABASE": "public", "DELTAT_PASSWORD": " " } } } }

DELTAT_PASSWORD is required and has no default; the server refuses to start without it rather than trying a guessable one. Everything else defaults to a local deltat on localhost:5433, database public. Each database name is an isolated tenant.

Then just ask: *"Create a calendar called Haircuts open weekday afternoons, then book me the first free 30-minute slot tomorrow."*

Authentication, honestly

This stdio server has no login. The deltat password in your config is the credential, and the server runs with your user's permissions. That is the single-operator self-host model: you own the deltat, you own the agent, and it needs no OAuth.

A hosted multi-tenant server is a different problem. There an agent authenticates with OAuth 2.1: the server answers an unauthenticated call with 401 plus protected-resource metadata (RFC 9728), and a capable client runs authorization-code + PKCE itself, registering via a Client ID Metadata Document with one-time human consent. The OidcAdapter in this package is the resource-server half of that, verifying a bearer JWT offline against the issuer's JWKS with no per-request network call.

That hosted transport is not in this package yet. The stdio server above is what runs today.

Embed it

import { createDeltatMcpServer } from "@open-deltat/mcp";

import { DeltaT } from "@open-deltat/client";

const server = createDeltatMcpServer(new DeltaT({ /* ... */ })); await server.connect(yourTransport);

Prove it end to end

With a deltat running:

bun scripts/smoke.ts

drives the whole loop through an in-memory MCP client and asserts the round trip (create → availability → find → hold → commit → list → cancel).

License

MIT