diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 7ebb713..0ab052d 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -3,7 +3,7 @@ name: test
 on: [push, pull_request]
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.entry.os || 'ubuntu-latest' }}
     strategy:
       fail-fast: false
       matrix:
@@ -12,7 +12,7 @@ jobs:
           - { ruby: '3.4' }
           - { ruby: '3.2', json: '2' }
           - { ruby: '3.2' }
-          - { ruby: '2.7', json: '2' }
+          - { ruby: '2.7', json: '2', os: 'ubuntu-22.04' }
           - { ruby: 'ruby-head', allowed-failure: true }
           - { ruby: 'jruby-head', allowed-failure: true }
     name: test (Ruby ${{ matrix.entry.ruby }}, JSON ${{ matrix.entry.json || '3' }})
diff --git a/Gemfile b/Gemfile
index ec2777f..e7b4449 100644
--- a/Gemfile
+++ b/Gemfile
@@ -24,13 +24,19 @@ group :development, :test do
 end
 
 group :test do
-  gem 'danger', '~> 9.5'
-  gem 'danger-changelog', '~> 0.9.0'
-  gem 'danger-pr-comment', '~> 0.1.0'
-  gem 'danger-toc', '~> 0.2.0'
   gem 'minitest'
   gem 'mocha'
   gem 'rack-test'
   gem 'spinach'
   gem 'webmock'
 end
+
+# Danger runs separately on Ruby 4.0; its plugins require Ruby 3.1+.
+if Gem::Version.new(RUBY_VERSION) >= Gem::Version.new('3.1')
+  group :danger do
+    gem 'danger', '~> 9.5'
+    gem 'danger-changelog', '~> 0.10.0'
+    gem 'danger-pr-comment', '~> 0.1.0'
+    gem 'danger-toc', '~> 0.3.0'
+  end
+end