Return a JSON response to a malformed JSON request which throws an MultiJson::DecodeError exception in ActionDispatch before a controller is instanciated. Without this the ShowExceptions rack middleware will render an HTML error page to your JSON respons

application.rb

# config/application.rb
# You must either set the following in this file or set consider_all_requests_local to false in config/environments/development.rb

config.middleware.delete ActionDispatch::DebugExceptions

development.rb

# config/environments/development.rb
# You must either set the following in this file or disable ActionDispatch::DebugExceptions in config/application.rb

config.consider_all_requests_local = false

show_exceptions.rb

# config/initializers/show_exceptions.rb
# Tell ShowExceptions rack middleware to render a JSON response to a MultiJson::DecodeError exception instead of an HTML page.

class ActionDispatch::ShowExceptions
  def call(env)
    status, headers, body = @app.call(env)

    if headers['X-Cascade'] == 'pass'
      raise ActionController::RoutingError, "No route matches #{env['PATH_INFO'].inspect}"
    end

    [status, headers, body]
  rescue Exception => exception
    raise exception if env['action_dispatch.show_exceptions'] == false
    if exception.class == MultiJson::DecodeError
      [422, {'Content-Type' => 'application/json'}, ['{"error":"Invalid JSON"}']]
    else
      render_exception(env, exception)
    end
  end
end