Not quite clear.

Given I have no tiers, I am covered just with cache-duration and can completely ignore message-expiry-duration?

I have a single service that POSTs messages to ntfy which sends them to subscribers. Any subscriber will have to connect within cache-duration or else ntfy deletes messages older than that from the database. Is that accurate? The intention is to keep the sqlite db lean and small.

That would work great! So anything sent under "extras" would be forwarded verbatim to the mobile app.

Is it too much trouble to implement?

My backend POSTs notification JSON to self-hosted ntfy server:

{
 "topic": "1234", 
 "message": "foo",
 "title": "bar",
 "customField": "baz"
}

The ntfy server should broadcast all fields, including customField to the websockets/subscribers of this topic.

Am I missing something?

I also see log-file option. Is it possible to log both to a file (for later inspection) and to stderr?

Nevermind. I found the problem. It was not NTFY’s fault. Some of the notifications that were POSTed to NTFY, didn’t follow the correct syntax (was sending “attach” in the JSON as string[] instead of a single string).

Would be nice though if NTFY could provide a debug log to catch such cases.