^{Originally posted by baumanno January 17, 2026}
Hello! Thanks for maintaining this nifty piece of software.

TL;DR: ktor sets the Accept-Charset header, which causes the Exchange Web Services instance I'm interfacing with to refuse to respond. Any chance of not setting that header?

For several months now, I've been getting a notification that a subscribed calendar was producing 403 Forbidden errors. Today, I noticed the calendar hadn't updated in quite a while, and dug deeper.
The org where my calendar is hosted uses Microsoft Exchange Web Services, unfortunately I don't know which version.

This is the stack trace from ICSx5 upon newly subscribing to the calendar in question:

HTTP 403 Forbidden

java.io.IOException: HTTP 403 Forbidden
	at at.bitfire.icsdroid.CalendarFetcher.fetchNetwork$icsx5_92_2_4_3_standardRelease(SourceFile:181)
	at at.bitfire.icsdroid.CalendarFetcher$fetchNetwork$1.invokeSuspend(Unknown Source:11)
	at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(SourceFile:34)
	at kotlinx.coroutines.DispatchedTask.run(SourceFile:100)
	at kotlinx.coroutines.internal.LimitedDispatcher$Worker.run(SourceFile:124)
	at kotlinx.coroutines.scheduling.TaskImpl.run(SourceFile:89)
	at kotlinx.coroutines.scheduling.CoroutineScheduler.runSafely(SourceFile:586)
	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.executeTask(SourceFile:820)
	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.runWorker(SourceFile:717)
	at kotlinx.coroutines.scheduling.CoroutineScheduler$Worker.run(SourceFile:704)

Using PCAPdroid, I captured the HTTPS session for the feed validation during subscription. This resulted in the following headers:

GET /owa/calendar/<REDACTED>/reachcalendar.ics HTTP/1.1
Accept: text/calendar, */*;q=0.9
User-Agent: ICSx5/2.4.0 (ical4j/3.2.19 okhttp/5.1.0 Android/11)
Accept-Charset: UTF-8
Host: mail.<REDACTED>
Connection: Keep-Alive
Accept-Encoding: gzip

After playing around with various user-agent strings I found in other discussions, I tried running plain curl within termux, and it worked; so the UA wasn't the issue. I could also access the .ics from various browsers in private mode. What didn't work was setting the above headers in a curl command, so I successively removed headers until it worked, narrowing it down to the Accept-Charset: UTF-8 header. This apparently bothers the Exchange server so much it refuses to cooperate.

It seems that ktor sets the header Accept-Charset: UTF-8 by default, if not overridden otherwise.

Is there any chance of not setting the Accept-Charset header at all? According to the note in RFC 9110 §12.5.2 it is deprecated.

Thanks for considering this!