diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a7d0f92..12e3294 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -9,24 +9,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: [ '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4' ]
-        bundler: [ '1.17.3', '2.3.12' ]
+        ruby: [ '2.6', '2.7', '3.0', '3.1', '3.2', '3.3', '3.4', '4.0' ]
         database: [ 'mysql2', 'postgresql', 'sqlite3', 'trilogy' ]
         exclude:
-          - ruby: '2.6'
-            bundler: '2.3.12'
-          - ruby: '2.7'
-            bundler: '1.17.3'
-          - ruby: '3.0'
-            bundler: '1.17.3'
-          - ruby: '3.1'
-            bundler: '1.17.3'
-          - ruby: '3.2'
-            bundler: '1.17.3'
-          - ruby: '3.3'
-            bundler: '1.17.3'
-          - ruby: '3.4'
-            bundler: '1.17.3'
           - ruby: '2.6'
             database: 'trilogy'
           - ruby: '2.7'
@@ -53,26 +38,19 @@ jobs:
 
     steps:
       - name: Check out code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v6
       - name: Set up ruby
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
-          bundler: ${{ matrix.bundler }}
           bundler-cache: true
-
-      - name: Set up Bundler
-        run: |
-          export BUNDLER_VERSION=${{ matrix.bundler }}
-          export BUNDLE_PATH=$PWD/vendor/bundle
-
-          bundle _${{ matrix.bundler }}_ config set path $PWD/$BUNDLE_PATH
-          bundle _${{ matrix.bundler }}_ install --jobs=4 --retry=3
-          bundle _${{ matrix.bundler }}_ update
       - name: Set up Appraisal
+        env:
+          BUNDLE_PATH: ${{ github.workspace }}/vendor/bundle
         run: bundle exec appraisal update
       - name: Test
         env:
+          BUNDLE_PATH: ${{ github.workspace }}/vendor/bundle
           CI: "true"
           DB_ADAPTER: ${{ matrix.database }}
           DB_USERNAME: root
@@ -88,7 +66,7 @@ jobs:
 
     steps:
       - name: Check out code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v6
       - name: Set up ruby
         uses: ruby/setup-ruby@v1
         with:
diff --git a/Appraisals b/Appraisals
index 703292d..897fad6 100644
--- a/Appraisals
+++ b/Appraisals
@@ -29,7 +29,7 @@ if RUBY_VERSION.to_f >= 2.5 && RUBY_VERSION.to_f < 3.1
   end
 end
 
-if RUBY_VERSION.to_f >= 2.5
+if RUBY_VERSION.to_f >= 2.5 && RUBY_VERSION.to_f < 4.0
   appraise "rails-6.1" do
     gem "rails", "~> 6.1.0"
     gem "mysql2", "~> 0.5.0"
@@ -40,7 +40,7 @@ if RUBY_VERSION.to_f >= 2.5
   end
 end
 
-if RUBY_VERSION.to_f >= 2.7
+if RUBY_VERSION.to_f >= 3.0
   appraise "rails-7.0" do
     gem "rails", "~> 7.0.1"
     gem "mysql2", "~> 0.5.0"
@@ -72,7 +72,9 @@ if RUBY_VERSION.to_f >= 3.2
     gem "sqlite3", "~> 2.1"
     gem "trilogy", "~> 2.9"
   end
+end
 
+if RUBY_VERSION.to_f >= 3.3
   appraise "rails-edge" do
     gem "rails", :git => "https://github.com/rails/rails.git", :branch => "main"
     gem "mysql2", "~> 0.5.0"
diff --git a/Gemfile b/Gemfile
index 9d3cba2..de13105 100644
--- a/Gemfile
+++ b/Gemfile
@@ -13,5 +13,7 @@ end
 gem "rubocop",           "~> 0.92"
 gem "rubocop-packaging", "~> 0.5"
 
-# Required for testing Rails 6.1 on MRI 3.1+
+# Gems that are no longer part of stdlib:
 gem "net-smtp" if RUBY_VERSION.to_f > 3.0
+gem "ostruct" if RUBY_VERSION.to_f > 3.4
+gem "stringio" if RUBY_VERSION.to_f > 3.1
diff --git a/README.md b/README.md
index 7a25a2a..dddf97d 100644
--- a/README.md
+++ b/README.md
@@ -187,7 +187,7 @@ end
 
 ## Compatibility
 
-The current test matrix covers MRI 2.6 to 3.4, and Rails 5.0 to 8.1. It will possibly work on older versions and other Ruby implementations as well.
+The current test matrix covers MRI 2.6 to 4.0, and Rails 5.0 to 8.1. It will possibly work on older versions and other Ruby implementations as well.
 
 You can also use Combustion with multiple versions of Rails to test compatibility across them. [Appraisal](https://github.com/thoughtbot/appraisal) is a gem that can help with this, and a good starting reference is the [Thinking Sphinx](https://github.com/pat/thinking-sphinx) test suite, which runs against [multiple versions](https://github.com/pat/thinking-sphinx/blob/develop/Appraisals) of Rails.
 
diff --git a/combustion.gemspec b/combustion.gemspec
index 7ecbc16..04fb2e0 100644
--- a/combustion.gemspec
+++ b/combustion.gemspec
@@ -37,6 +37,6 @@ Gem::Specification.new do |s|
   s.add_development_dependency "pg"
   s.add_development_dependency "rails"
   s.add_development_dependency "rspec"
-  s.add_development_dependency "rubocop", "~> 0.81.0"
+  s.add_development_dependency "rubocop", "~> 0.92"
   s.add_development_dependency "sqlite3"
 end