Heroku: Force Error Backtrace To Display In Development Mode

Question

I am running a sinatra application on Heroku, and I'm seeing the generic error page:

Stuff I've Tried So Far

1. heroku logs returns a generic "H10 - Application Crashed" error, which doesn't tell me what the actual cause is.

2. I've used heroku config vars to set RACK_ENV=development and verified that it is set that way using heroku config. I thought this would force a backtrace to be displayed on the screen but it does not.

Questions

1. How can I force a backtrace to be displayed in dev mode?
2. What else can I do to track down the source of this error?

Answer 1

I got a response from Heroku support about this.

Everything I did was correct, the problem is the application boot errors won't display to the screen, and this was a boot error. The source of the error was further back in the backtrace in heroku logs, so that was in fact the right place to look in this case, I just missed it.

Setting RACK_ENV=development is the correct way to display application runtime errors to the screen, according to support.