diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9c6508ca5..22cffbff0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,7 +7,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        ruby: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4', 'jruby']
+        ruby: ['2.7', '3.0', '3.1', '3.2', '3.3', '3.4', 'jruby', '4.0']
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v6
diff --git a/Gemfile b/Gemfile
index 623a305b0..0c25249a6 100644
--- a/Gemfile
+++ b/Gemfile
@@ -9,7 +9,11 @@ end
 group :db do
   gem 'jdbc-sqlite3', '~> 3.7.2', platform: :jruby
   gem 'sequel'
-  gem 'sqlite3', platforms: [:mri]
+  if RUBY_VERSION < '3.1'
+    gem 'sqlite3', '~> 1.7', platforms: [:mri]
+  else
+    gem 'sqlite3', platforms: [:mri]
+  end
 end
 
 group :development do
@@ -42,9 +46,11 @@ group :development do
   end
 
   gem 'builder',          '>= 2.1.2'
+  gem 'logger',           '>= 1.6'
   gem 'minitest',         '>= 4.0'
+  gem 'minitest-mock',    '>= 1.0' if RUBY_VERSION >= '3.1'
   gem 'mocha',            '>= 2.0'
-  gem 'oga',              '>= 2.5', '< 3'
+  gem 'nokogiri',         '>= 1.13'
   gem 'rack',             '~> 3'
   gem 'rackup',           '~> 2.1'
   gem 'rack-test',        '~> 2.1'
diff --git a/padrino-cache/test/helper.rb b/padrino-cache/test/helper.rb
index 369f2411b..31770f1a9 100644
--- a/padrino-cache/test/helper.rb
+++ b/padrino-cache/test/helper.rb
@@ -2,6 +2,11 @@
 PADRINO_ROOT = __dir__ unless defined?(PADRINO_ROOT)
 
 require 'minitest/autorun'
+begin
+  require 'minitest/mock'
+rescue LoadError
+  # minitest/mock is built-in for older minitest versions
+end
 require 'minitest/pride'
 require 'rack/test'
 require 'padrino-cache'
diff --git a/padrino-core/lib/padrino-core/application/routing.rb b/padrino-core/lib/padrino-core/application/routing.rb
index a1e937669..aaa1ba3e3 100644
--- a/padrino-core/lib/padrino-core/application/routing.rb
+++ b/padrino-core/lib/padrino-core/application/routing.rb
@@ -372,7 +372,7 @@ def unlink(path, *args, &block)  route 'UNLINK',  path, *args, &block end
 
       def rebase_url(url)
         if url.start_with?('/')
-          new_url = ''
+          new_url = String.new
           new_url << conform_uri(ENV['RACK_BASE_URI']) if ENV['RACK_BASE_URI']
           new_url << conform_uri(uri_root) if defined?(uri_root)
           new_url << url
@@ -599,12 +599,14 @@ def parse_route(path, options, verb)
         # path i.e :index or :show
         if path.is_a?(Symbol)
           name = path
-          path = map&.dup || (path == :index ? '/' : path.to_s)
+          path = map&.dup || (path == :index ? '/' : path.to_s.dup)
         end
 
         # Build our controller
         controller = Array(@_controller).map(&:to_s)
 
+        path = path.dup if path.is_a?(String) && path.frozen?
+
         case path
         when String # path i.e "/index" or "/show"
           # Now we need to parse our 'with' params
diff --git a/padrino-core/lib/padrino-core/logger.rb b/padrino-core/lib/padrino-core/logger.rb
index e68bb7718..ac21d9d88 100644
--- a/padrino-core/lib/padrino-core/logger.rb
+++ b/padrino-core/lib/padrino-core/logger.rb
@@ -443,7 +443,11 @@ def flush
       return unless @buffer.size.positive?
       self.class.mutex.synchronize do
         @buffer.each do |line|
-          line.encode!(@sanitize_encoding, invalid: :replace, undef: :replace) if @sanitize_encoding
+          if @sanitize_encoding
+            line.encode!(@sanitize_encoding, invalid: :replace, undef: :replace)
+          else
+            line.encode!(@log.external_encoding || Encoding::UTF_8, invalid: :replace, undef: :replace) unless line.encoding == Encoding::UTF_8
+          end
           @log.write(line)
         end
         @buffer.clear
diff --git a/padrino-core/padrino-core.gemspec b/padrino-core/padrino-core.gemspec
index 04ce44052..1450f16f2 100755
--- a/padrino-core/padrino-core.gemspec
+++ b/padrino-core/padrino-core.gemspec
@@ -21,6 +21,7 @@ Gem::Specification.new do |s|
   s.require_paths = ['lib']
   s.rdoc_options  = ['--charset=UTF-8']
 
+  s.add_dependency('ostruct', '>= 0.6')
   s.add_dependency('padrino-support', Padrino.version)
   s.add_dependency('rackup', '~> 2.1')
   s.add_dependency('sinatra', '~> 4')
diff --git a/padrino-core/test/helper.rb b/padrino-core/test/helper.rb
index 88f477650..89d9f1dc6 100644
--- a/padrino-core/test/helper.rb
+++ b/padrino-core/test/helper.rb
@@ -2,6 +2,11 @@
 PADRINO_ROOT = __dir__ unless defined?(PADRINO_ROOT)
 
 require 'minitest/autorun'
+begin
+  require 'minitest/mock'
+rescue LoadError
+  # minitest/mock is built-in for older minitest versions
+end
 require 'minitest/pride'
 require 'i18n'
 require 'json'
diff --git a/padrino-core/test/test_logger.rb b/padrino-core/test/test_logger.rb
index 0341649a2..e163f7412 100644
--- a/padrino-core/test/test_logger.rb
+++ b/padrino-core/test/test_logger.rb
@@ -16,7 +16,7 @@
   end
 
   def setup_logger(options = {})
-    @log    = StringIO.new
+    @log    = StringIO.new(String.new(encoding: 'UTF-8'))
     @logger = Padrino::Logger.new(options.merge(stream: @log))
   end
 
@@ -102,7 +102,7 @@ def setup_logger(options = {})
       @logger.error binary_data
       @logger.error utf8_data
       @logger.flush
-      assert_match(/\.*фыв/m, @log.string.encode(Encoding.default_external))
+      assert_match(/\.*фыв/m, @log.string.encode('UTF-8'))
     end
 
     it 'should log an application' do
diff --git a/padrino/test/padrino/test-methods.rb b/padrino/test/padrino/test-methods.rb
index 84fbd60e1..0c2bc29b1 100644
--- a/padrino/test/padrino/test-methods.rb
+++ b/padrino/test/padrino/test-methods.rb
@@ -1,4 +1,4 @@
-require 'oga'
+require 'nokogiri'
 
 module Padrino
   module TestMethods
@@ -40,12 +40,12 @@ def assert_has_no_selector(html, selector, attributes)
     private
 
     def html_matched_tags(html, selector, attributes)
-      @dom ||= Oga.parse_html(html)
+      @dom ||= Nokogiri::HTML(html)
       content_requirement = attributes.delete(:content)
 
       attributes.each { |name, value| selector += %([#{name}="#{value}"]) }
       tags = @dom.css(selector.to_s.gsub(/\[([^"']*?)=([^'"]*?)\]/, '[\1="\2"]'))
-      tags = tags.select { |tag| (tag.get('content') || tag.text).index(content_requirement) } if content_requirement
+      tags = tags.select { |tag| (tag['content'] || tag.text).index(content_requirement) } if content_requirement
       tags.count
     end
   end