diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 9120ff2..618a170 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -19,10 +19,13 @@ jobs:
       matrix:
         ruby_version:
           - '3.2'
+          - '4.0'
         include:
           - ruby_version: '3.2'
             puppet_version: '~> 8.0'
-        runs-on: 
+          - ruby_version: '4.0'
+            puppet_version: '~> 9.0'
+        runs-on:
           - ubuntu-latest
           - windows-latest
     name: "spec (ruby ${{ matrix.ruby_version }} | puppet ${{ matrix.puppet_version }} - ${{ matrix.runs-on }})"
diff --git a/.github/workflows/nightly.yml b/.github/workflows/nightly.yml
index ad3b5af..8d8c4eb 100644
--- a/.github/workflows/nightly.yml
+++ b/.github/workflows/nightly.yml
@@ -12,10 +12,13 @@ jobs:
       matrix:
         ruby_version:
           - '3.2'
+          - '4.0'
         include:
           - ruby_version: '3.2'
             puppet_version: '~> 8.0'
-        runs-on: 
+          - ruby_version: '4.0'
+            puppet_version: '~> 9.0'
+        runs-on:
           - ubuntu-latest
           - windows-latest
     name: "spec (ruby ${{ matrix.ruby_version }} | puppet ${{ matrix.puppet_version }} - ${{ matrix.runs-on }})"
diff --git a/Gemfile b/Gemfile
index bd40968..8d017d0 100644
--- a/Gemfile
+++ b/Gemfile
@@ -11,6 +11,14 @@ group :test do
   gem 'rubocop', '~> 1.50.0'
   gem 'rubocop-rspec', '~> 2.19'
   gem 'rubocop-performance', '~> 1.16'
+  # rubocop 1.50.2 still requires these directly; unbundled from the standard library
+  # since Ruby 3.4.0 (base64) or 4.0.0 (benchmark, ostruct).
+  gem 'base64'
+  gem 'benchmark'
+  gem 'ostruct'
+  # required directly by spec/unit/puppetfile-resolver/spec_searchers/git/gclone_spec.rb;
+  # unbundled from the standard library since Ruby 4.0.0.
+  gem 'logger'
 
   gem 'simplecov'
   gem 'simplecov-console'