Tool comparison

Kalender Sync vs. inovex CalendarSync – CLI or managed service?

CalendarSync by inovex is a likeable open-source project from Germany: a stateless command-line tool that syncs events without routing data through third-party servers. For developers it's a valid choice – we say that without reservation. The honest question is whether you want to operate a CLI with a YAML config, or use a service that just runs. Here's the fair comparison.

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Background

What can inovex CalendarSync do?

CalendarSync is an open-source CLI tool (MIT licence, Go) by inovex GmbH, a German IT project company. It syncs events one-way from a source to a sink – supporting Google Calendar and Outlook (plus the time-tracking tool ZEP as a source). Everything is configured via a YAML file: time window, OAuth credentials, transformers (keep or replace titles, carry over attendees, location, description, reminders) and filters (declined events, all-day events, regex patterns). The clever part: the tool is stateless and runs locally – credentials are stored encrypted on your machine, and event data never passes through a third-party server. Version 0.11.0 shipped in January 2026; the repo has around 230 GitHub stars.

Strengths

Where inovex CalendarSync is strong

Stateless and local – maximum data minimisation

CalendarSync runs on your own machine and stores no event data on anyone else's servers; OAuth tokens live locally, encrypted. If you don't want to give any cloud service access to your calendar, this is the most consistent approach out there – and we genuinely respect it.

Powerful transformers and filters

Title prefixes, regex filters, selectively carrying over attendees, location or meeting links – for developers who want precise control over their sync, the YAML configuration is a sharp tool.

German IT company, MIT licence, free

Behind the project stands inovex, an established German IT company; the code is MIT-licensed and the tool completely free. For IT professionals who want to wire a script into their toolchain, an absolutely valid choice.

Where Kalender Sync wins

2 minutes in the browser instead of CLI, YAML and OAuth apps

CalendarSync explicitly targets developers: install a binary, write YAML, create your own OAuth credentials with Google and Microsoft – and build a cron job for continuous operation, because the CLI only syncs while it's running. Kalender Sync is set up in the browser in 2 minutes and then runs permanently as a service – monitored by us, not by you.

Genuine two-way sync instead of a one-way street

CalendarSync syncs one-way from source to sink. Kalender Sync is bidirectional: changes flow in both directions, no matter in which calendar you create or move an event.

9 provider types instead of Google + Outlook

CalendarSync supports Google and Outlook. Kalender Sync connects 9 provider types: Google, Microsoft Outlook/M365, Apple iCloud, GMX, WEB.DE, mailbox.org, Infomaniak kSuite, CalDAV and iCal feeds – and is the only tool in this comparison with the German consumer providers GMX and WEB.DE.

Live-sharing into other calendars

CalendarSync reconciles your own accounts. Kalender Sync can additionally embed your availability into another person's calendar by invitation – they accept once, and from then on everything stays in sync automatically. That sharing flow is a core feature for us, not a scripting exercise.

Three visibility levels as a choice instead of regex config

With CalendarSync, what gets transferred is governed by transformers in a YAML file. With Kalender Sync it's a simple choice per sync: "busy only", "title + time" or "full details" – including custom display names for blocked slots. The same control, without a config file.

A service with support and a DPA

CalendarSync is a tool, not a service: no support commitment, no contractual partner, no DPA – entirely fair for a free CLI, but a gap for company use. Kalender Sync is run by PPJ Venture Labs UG in Munich, GDPR- and BDSG-compliant, hosted at Hetzner in Nuremberg, with a DPA for business customers and German-language support.

A permanently free plan – without the tinkering

One synchronisation is permanently free with Kalender Sync – no credit card, no trial expiry. Beyond that: Basic from €4/month, Business at €10/month, 14 days of all Business features free. The price difference to a free CLI is real – so is the setup and maintenance time you save.

Decision guide

When should you pick which?

inovex CalendarSync is the right choice if:

You're a developer and deliberately prefer a CLI with a YAML config.
Your event data must never pass through a third-party server.
Google and Outlook are all the providers you need, and one-way sync is enough.
You want to manage cron jobs, OAuth setup and updates yourself – and see that as a feature.

Kalender Sync is the right choice if:

You want to be up and running in 2 minutes – no CLI, YAML or your own OAuth app.
You need genuine two-way sync, not just source-to-sink.
You want to share your availability live into other people's calendars.
You need a German contractual partner with a DPA, GDPR/BDSG compliance and support.
You want the broadest provider coverage in this comparison: 9 provider types – Google, Microsoft Outlook/M365, Apple iCloud, GMX, WEB.DE, mailbox.org, Infomaniak kSuite, CalDAV and iCal feeds. Kalender Sync is the only tool in this comparison with the German providers GMX & WEB.DE.

Ready for calendar sync without YAML and cron jobs?

Connect your calendars in a few clicks – GDPR-compliant, hosted in Germany, with two-way sync and genuine live-sharing.

Try free for 14 days

Currently supported providers

Google Microsoft Apple iCloud KSuite iCal-Feeds

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