Stylistically, this might be better as a constant. E.g., API_URL.

And I took care of this as well.

You provided default values for the token and the user which means if the user does not empty those fields your validation will always pass, i would change the default options for token and user to an empty string.

Personally i am not a big fan of determinating the state of an agent which is receiving events based on the time frame it received the last event. I would rather base it on the (un)successful delivery of the events it received.

Yeah I can see that. I'll make this change later today. Really I'd like to take into account the response from pushover as to whether or not the notification was successful, a failure that can be retried or a failure that will never be successful and shouldn't be retried.

The only other change I'd suggest is to use .presence on these, so that empty fields are treated as missing.

post_params['title'] = event.payload['title'].presence || event.payload['subject'].presence || options['title']

and also below, anywhere you think that makes sense. E.g,

url = (event.payload['url'].presence || options['url'] || '').to_s

url_title = (event.payload['url_title'].presence || options['url_title']).to_s

post_params['priority'] = (event.payload['priority'].presence || options['priority']).to_i

etc.

sound, retry, expire, ...

Anything where empty string should be treated as nil.