gistfile1.rb

enable :sessions

get "/foo" do
  session[:something] = "hello"
  "set"
end

get "/bar" do
  session[:something]
end

# and the test:

class TestSessions < Test::Unit::TestCase
  include Sinatra::Test

  def test_session
    get "/foo"
    assert_equal "set", body
    get "/bar"
    assert_equal "hello", body # fails - body is ""
  end
end