Neil Gupta

Building an iCal Subscription

601 words • 3 minutes to read

I recently wanted to sync events between Guilded and my personal calendar. The simplest solution is to generate an iCalendar file that is served from a personal endpoint for each user. When they subscribe, their calendar app basically acts like an RSS reader and periodically checks the endpoint for the latest list of events.

There are plenty of libraries in your chosen language to generate the .ics file, and the spec is simple enough that you could even generate it by hand. Here’s a sample with a single event:

SUMMARY:My Fun Event
LOCATION:Monkey Island

In a nutshell, you can add as many VEVENT blocks as you need to show more events. The only catch is that this file needs to show all events that you want in the user’s calendar. Unlike an RSS reader, most calendar apps won’t keep a cache of old events around. That means over time, this file can grow very large. I recommend limiting it to a sensible range (i.e., events from 1 month ago to 1 year from now), depending on your use case.

There are also some little things you need to know to actually make this calendar work with the variety of calendar services that I couldn’t find documented anywhere. After a lot of investigating (and trial and error), here’s some tips and tricks to help you build your ICS endpoint faster.

Content Headers

Make sure you’re serving your calendar file with the right headers. In particular, they should look like

Content-Type: text/calendar; charset=utf-8
Content-Disposition: attachment; filename="calendar.ics"

This is pretty straightforward, but easy to miss.

Accept Header

Your server needs to respond to Accept: text/calendar request headers from the client. It looks like iOS is the only one that actually sends the header, but if the request gets rejected, it shows an unhelpful “Unable to verify account information” error. Since a rejected Accept header returns a 406 Not Acceptable response, it’s easy to miss if you don’t normally log 4xx errors.

URL Length

This one was frustrating to discover, but Google Calendar has a max url length of 256 characters. This isn’t documented anywhere and trying to use a longer URL doesn’t show any error, the UI just silently fails. In my case, the user token was going over that limit, so I had to change my hashing algorithm to create a shorter token.

Query Params

As far as I can tell, query params do work with most calendar apps, but I’d still recommend avoiding them if possible. For example, I changed the calendar path from to to ensure that we don’t hit issues with some obscure calendar app. It also helps cut down the url length as a bonus.


It turns out the Apple Calendar on macOS is the most forgiving and works exactly like your browser. It’ll happily sync any valid ICS file without any of the setup above. That means you definitely want to test your endpoint with other apps directly, especially iOS, Google Calendar, and Outlook. I recommend using something like ngrok to quickly test your local endpoint. As a bonus, it includes a detailed dashboard at http://localhost:4040 for viewing request headers and replaying requests to make debugging much easier. This is how I resolved the 406 error above!


Written on September 3, 2020 in Chicago.