The `format: false` removes the optional segment from the end of the route but doesn't affect any path parameters so you need the regexp constraint to allow matching on periods.

routes.rb

NoFormatTest::Application.routes.draw do
  root 'root#index'
  scope '/foo/:bar', format: false, bar: /[^\/\?]+/ do
    get '/', to: 'root#index'
    get '/baz', to: 'root#index'
  end
end