api_steps.rb

When /^I send a GET request for "([^"]*)"$/ do |path|
    get path
end

Then /^the response code should be "([^"]*)"$/ do |code|
    last_response.status.should == code.to_i
end

my_dumb_api.feature

# features/my_dumb_api.feature
Feature: My Dumb API
  In order to retrieve an API response
  As a web API developer
  I want an API to respond to my requests

  Scenario: API is available
    When I send a GET request for "http://example.com/api/"
    Then the response code should be "200"

routes.rb

# routes.rb
RailsApp::Application.routes.draw do
  mount proc { |env|
    [200, {}, ["OK"]]
  } => "/api"

  # your other Rails routes
end