Kalendis – API-First Scheduling Backend for AI Agents

TL;DR

TL;DR: Kalendis is an API-first scheduling backend that handles timezone math, DST edge cases, and conflict-safe bookings so you can keep your own UI.

Source and Accuracy Notes

⚠️ This section is MANDATORY. All links must be verified from actual source, not guessed.

• Project page: kalendis.dev
• HN launch thread: news.ycombinator.com/item?id=45956154
• License: Not publicly confirmed from open-source repo (API service, not OSS)
• Source last checked: 2026-07-28 (HN launch post + developer description)

What Is Kalendis?

Kalendis is an API-first scheduling backend for developers and AI agents. Rather than embedding a hosted scheduling UI, you keep full control of the user experience and delegate the complex parts: recurrence rules, timezone and DST handling, and conflict-safe booking endpoints. The HN launch description puts it directly:

Kalendis is an API-first scheduling backend. You keep your UI; we handle the gnarly parts (recurrence, time zones, DST, conflict-safe bookings).

The service ships as a REST API with ISO-8601 timestamps and IANA timezones, paired with an MCP tool that generates typed clients and route handlers for Next.js, Express, Fastify, and Nest.

Features

MCP tool generator. Drop in a typed client or route handler and call the API directly from an IDE or AI agent tooling. Supports Next.js, Express, Fastify, and Nest.

Availability engine. Define recurring availability rules plus one-off exceptions and blackout periods. The API returns a clean, queryable shape you can feed directly into a calendar UI.

Conflict-safe bookings. Endpoints for creating, updating, and canceling slots — with built-in conflict detection so double-bookings do not happen.

Timezone and DST handling. Uses IANA timezone identifiers so DST transitions are handled automatically on every request.

Focused scope. The API covers four resources: users, availability, exceptions, and bookings. No monolithic feature suite to integrate.

Setup Workflow

Step 1: Get an API Key

Sign up at kalendis.dev to receive an API key. The REST API base URL and authentication details are provided in your dashboard.

Step 2: Explore the Endpoints

The API covers three areas:

• /users — manage users and their timezone preferences
• /availability — define recurring rules and exceptions
• /bookings — create, update, and cancel bookings with conflict detection

All requests use ISO-8601 timestamps and IANA timezone identifiers.

Step 3: Generate an MCP Typed Client

The MCP tool generates scaffolded clients for your stack. Drop in a typed client or route handler and call the API directly — the MCP server exposes the full scheduling API as callable tools for Next.js, Express, Fastify, or Nest.

Step 4: Integrate Into Your UI

With the typed client in place, call availability and booking endpoints from your existing calendar UI. The API returns structured data — render it however your product requires.

Practical Evaluation Checklist

• [ ] API key obtained and first request sent
• [ ] Availability rules defined with recurring patterns
• [ ] DST boundary tested (e.g., spring forward / fall back)
• [ ] Booking conflict scenario triggered and handled
• [ ] MCP typed client generated for your framework
• [ ] Agent tool call tested end-to-end

FAQ

Q: How does Kalendis handle timezones? A: Uses IANA timezone identifiers with ISO-8601 timestamps. DST transitions are handled server-side — availability queries return correct offsets for any date.

Q: Can AI agents use Kalendis? A: Yes. The MCP tool exposes the full API as typed tools that an agent can call directly from an IDE or agent runtime without writing glue code.

Q: What frameworks does the MCP tool support? A: Next.js, Express, Fastify, and Nest. The generator produces typed clients and route handlers ready to drop into your project.

Q: Is this a hosted service or self-hosted? A: Hosted API service. You connect over REST — no self-hosting required.

Q: How does it compare to Cal.com as a scheduling backend? A: Cal.com bundles a hosted UI with its scheduling engine. Kalendis is purely API-first — you bring your own UI and delegate only the scheduling logic.

Q: Does it support recurring events? A: Yes. The availability engine handles recurring rules with one-off exceptions and blackout periods composably.

Conclusion

Kalendis solves the scheduling problem at the backend layer so you retain full UI control. The MCP tool generator is the standout feature for developer and agent workflows — typed clients scaffold automatically for Next.js, Express, Fastify, and Nest. If you have ever rebuilt DST handling or conflict detection from scratch, Kalendis is worth evaluating as a drop-in replacement.