There's no clean way to override the default error formatter

[dblock]

If you request a format that the API didn't know about, and call error!, you get the default error formatter, which is .txt. Without monkey patching you cannot override it.

module Grape
  module ErrorFormatter
    module Base
      class << self
        def formatter_for(api_format, options = {})
          spec = formatters(options)[api_format]
          case spec
          when nil
            # override here
            Grape::ErrorFormatter::Json
          when Symbol
            method(spec)
          else
            spec
          end
        end
      end
    end
  end
end

[dblock]

Fixed in 81fa919, added default_error_formatter. Setting format will automatically set default_error_formatter since most of the time users just want the format of the errors to match the API format.