How to serialize Exception

The JSON module doesn't extend Exception by default. You have to require "json/add/exception". I'm not sure if this is documented anywhere:

require "json/add/exception"

begin
  nil.foo
rescue => exception
  ex = exception
end

puts ex.to_json
# => {"json_class":"NoMethodError","m":"undefined method `foo' for nil:NilClass","b":["prog.rb:5:in `<main>'"]}

Check out ext/json/lib/json/add in the Ruby source to see which classes work this way. If you do require "json/add/core" it will load JSON extensions for Date, DateTime, Exception, OpenStruct, Range, Regexp, Struct, Symbol, Time, and others.