diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0070ff6..628afc8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,10 +7,10 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby: ["3.2", "3.3", "3.4"]
+        ruby: ["3.2", "3.3", "3.4", "4.0"]
     steps:
       - name: Checkout
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
       - name: Setup Ruby
         uses: ruby/setup-ruby@v1
         with:
diff --git a/Gemfile b/Gemfile
index 6756fe7..2c2c617 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,6 +5,10 @@ source "https://rubygems.org"
 # development dependencies will be added by default to the :development group.
 gemspec
 
+# json 3.0 made JSON.parse options keyword-only, which breaks
+# ActiveSupport::JSON.decode (as of Rails 8.1.3.1); remove once Rails supports json 3
+gem "json", "< 3"
+
 # Declare any dependencies that are still in development here instead of in
 # your gemspec. These might include edge Rails or gems from your path or
 # Git. Remember to move these dependencies to your gemspec before releasing
diff --git a/ep_postmaster.gemspec b/ep_postmaster.gemspec
index fb4e7e0..06a37d9 100644
--- a/ep_postmaster.gemspec
+++ b/ep_postmaster.gemspec
@@ -16,7 +16,7 @@ Gem::Specification.new do |s|
   s.files = Dir["{app,config,db,lib}/**/*", "MIT-LICENSE", "Rakefile", "README.rdoc"]
   s.test_files = Dir["test/**/*"]
 
-  s.add_dependency "rails", ">= 4.2.1", "< 8.1"
+  s.add_dependency "rails", ">= 4.2.1", "< 8.2"
 
   s.add_development_dependency "sqlite3"
   s.add_development_dependency "rr"