diff --git a/Gemfile b/Gemfile
index b487f9d2c..2decba57b 100644
--- a/Gemfile
+++ b/Gemfile
@@ -45,8 +45,9 @@ group :development do
   gem "minitest", ">= 4.0"
   gem "mocha",    ">= 2.0"
   gem "oga",       ">= 2.5", "< 3"
-  gem "rack",      ">= 1.3.0", "< 3"
-  gem "rack-test", ">= 0.6.3"
+  gem "rack",      "~> 3"
+  gem "rackup",    "~> 2.1"
+  gem "rack-test", "~> 2.1"
   gem "rake",      ">= 10.5.0"
   gem 'rb-readline', '~> 0.4.2'
   gem 'rubocop', '~> 1.6', :platforms => [:mri]
diff --git a/padrino-core/lib/padrino-core/application/application_setup.rb b/padrino-core/lib/padrino-core/application/application_setup.rb
index fdf70b4ae..4018f4c81 100644
--- a/padrino-core/lib/padrino-core/application/application_setup.rb
+++ b/padrino-core/lib/padrino-core/application/application_setup.rb
@@ -134,7 +134,7 @@ def default_errors
         configure :production do
           error ::Exception do
             logger.exception env['sinatra.error']
-            halt(500, { 'Content-Type' => 'text/html' }, ['<h1>Internal Server Error</h1>'])
+            halt(500, { 'content-type' => 'text/html' }, ['<h1>Internal Server Error</h1>'])
           end unless errors.has_key?(::Exception)
         end
       end
diff --git a/padrino-core/lib/padrino-core/router.rb b/padrino-core/lib/padrino-core/router.rb
index a7e09168c..8f0f1fb9d 100644
--- a/padrino-core/lib/padrino-core/router.rb
+++ b/padrino-core/lib/padrino-core/router.rb
@@ -91,7 +91,7 @@ def call(env)
         env['SCRIPT_NAME'] = script_name
         env['PATH_INFO'] = path_info
         Padrino::Logger::Rack.new(nil,'/').send(:log, env, 404, {}, began_at) if logger.debug?
-        [404, {"Content-Type" => "text/plain", "X-Cascade" => "pass"}, ["Not Found: #{path_info}"]]
+        [404, {"content-type" => "text/plain", "x-cascade" => "pass"}, ["Not Found: #{path_info}"]]
       end
     end
   end
diff --git a/padrino-core/lib/padrino-core/server.rb b/padrino-core/lib/padrino-core/server.rb
index 8245b40ba..dd4fbc061 100644
--- a/padrino-core/lib/padrino-core/server.rb
+++ b/padrino-core/lib/padrino-core/server.rb
@@ -30,7 +30,7 @@ def self.detect_application(options)
   ##
   # This module builds a Padrino server to run the project based on available handlers.
   #
-  class Server < Rack::Server
+  class Server < Rackup::Server
     DEFAULT_ADDRESS = { :Host => '127.0.0.1', :Port => 3000 }
 
     # Server Handlers
diff --git a/padrino-core/padrino-core.gemspec b/padrino-core/padrino-core.gemspec
index 500c518c7..010eae412 100755
--- a/padrino-core/padrino-core.gemspec
+++ b/padrino-core/padrino-core.gemspec
@@ -21,6 +21,6 @@ Gem::Specification.new do |s|
   s.rdoc_options  = ["--charset=UTF-8"]
 
   s.add_dependency("padrino-support", Padrino.version)
-  s.add_dependency("sinatra", ">= 2.2.4")
+  s.add_dependency("sinatra", "~> 4")
   s.add_dependency("thor", "~> 1.0")
 end
diff --git a/padrino-gen/lib/padrino-gen/generators/templates/project_bin.tt b/padrino-gen/lib/padrino-gen/generators/templates/project_bin.tt
index ca282524b..57d33b6b6 100644
--- a/padrino-gen/lib/padrino-gen/generators/templates/project_bin.tt
+++ b/padrino-gen/lib/padrino-gen/generators/templates/project_bin.tt
@@ -9,6 +9,6 @@ require 'padrino-core/cli/launcher'
 ARGV.unshift('start') if ARGV.first.nil? || ARGV.first.start_with?('-')
 Padrino::Cli::Launcher.start ARGV
 
-# Start the app with Rack::Server
-#require "rack"
-#Rack::Server.start
+# Start the app with Rackup::Server
+#require "rackup"
+#Rackup::Server.start