How do you debug a Sinatra app like a Rails app?

You could try adding a before filter that prints out the parameters

before do
  puts '[Params]'
  p params
end

My opinion is that for debug you should use configure :development do because some debugging flags are turned on in this scenario. So, under your configure do block you can enable the flags:

enable :logging, :dump_errors, :raise_errors

and for your logging facility:

log = File.new("sinatra.log", "a")
STDOUT.reopen(log)
STDERR.reopen(log)

From the Sinatra handbook:

• dump_errors option controls whether the backtrace is dumped to rack.errors when an exception is raised from a route. The option is enabled by default for top-level apps.

• raise_errors - allow exceptions to propagate outside of the app (...) The :raise_errors option is disabled by default for classic style apps and enabled by default for Sinatra::Base subclasses.

I also use the

puts "something" + myvar.inspect

method in the middle of my controllers.