diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 159066c..1cc0774 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -19,7 +19,7 @@ jobs:
     strategy:
       matrix:
         # Test only supported versions: https://endoflife.date/ruby
-        ruby-version: [ '3.0', '3.1', '3.2', '3.3', jruby, truffleruby ]
+        ruby-version: [ '3.1', '3.2', '3.3', head, jruby, jruby-head, truffleruby, truffleruby-head ]
 
     steps:
     - uses: actions/checkout@v3
diff --git a/Appraisals b/Appraisals
index 331f538..8c2c6d8 100644
--- a/Appraisals
+++ b/Appraisals
@@ -3,14 +3,12 @@
 # Supported versions of Rails:
 # https://endoflife.date/rails
 #
-appraise "activejob-6.0" do
-  gem "activejob", "~> 6.0.x"
-  gem "activesupport", "~> 6.0.x"
-end
-
 appraise "activejob-6.1" do
   gem "activejob", "~> 6.1.x"
   gem "activesupport", "~> 6.1.x"
+  gem "base64"
+  gem "bigdecimal"
+  gem "mutex_m"
 end
 
 appraise "activejob-7.0" do
diff --git a/Gemfile b/Gemfile
index 66f559a..7ff80fe 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,6 +5,3 @@ gemspec
 
 gem "gem-release"
 gem "simplecov_json_formatter"
-
-# FIXME: mutex_m must be installed to run CI with ruby head (3.4dev)
-gem "mutex_m" # , platforms: :mri_34