diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e545741..d89df2b 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,7 +14,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['3.2', '3.3', '3.4']
+        ruby: ['3.3', '3.4', '4.0']
         gemfile:
           - gemfiles/activeadmin_3.5.gemfile
           - gemfiles/activeadmin_4.0.gemfile
diff --git a/activeadmin-oidc.gemspec b/activeadmin-oidc.gemspec
index 80e9904..55cc693 100644
--- a/activeadmin-oidc.gemspec
+++ b/activeadmin-oidc.gemspec
@@ -16,7 +16,7 @@ Gem::Specification.new do |spec|
   spec.license  = "MIT"
   spec.homepage = "https://github.com/activeadmin-plugins/activeadmin-oidc"
 
-  spec.required_ruby_version = ">= 3.2.0"
+  spec.required_ruby_version = ">= 3.3.0"
 
   spec.files = Dir[
     "lib/**/*",
diff --git a/gemfiles/activeadmin_3.5.gemfile b/gemfiles/activeadmin_3.5.gemfile
index 2fdf8c4..f4cc083 100644
--- a/gemfiles/activeadmin_3.5.gemfile
+++ b/gemfiles/activeadmin_3.5.gemfile
@@ -6,7 +6,7 @@ gemspec path: ".."
 
 # ActiveAdmin 3.5 line: classic Sprockets + Sassc asset pipeline.
 gem "activeadmin", "~> 3.5.0"
-gem "rails",       "~> 7.2.0"
+gem "rails",       "~> 8.0.0"
 
 # openid_connect 1.x line (httpclient-based, no faraday dep). Pinning the
 # 0.6.x series here keeps CI exercising the pre-faraday strategy path.
diff --git a/gemfiles/activeadmin_4.0.gemfile b/gemfiles/activeadmin_4.0.gemfile
index 538c275..f649b68 100644
--- a/gemfiles/activeadmin_4.0.gemfile
+++ b/gemfiles/activeadmin_4.0.gemfile
@@ -8,6 +8,9 @@ gemspec path: ".."
 # the Sprockets/Sassc pipeline with Propshaft + importmap + cssbundling +
 # Tailwind, so the asset gems differ entirely from the 3.5 gemfile.
 gem "activeadmin", ">= 4.0.0.beta", "< 5"
+# Blocked on Rails 8.1: railties 8.1 calls `delegate_missing_to` in
+# Rails::Initializable::Collection without requiring the ActiveSupport
+# core ext, so this gem's `require "rails/railtie"` raises NoMethodError.
 gem "rails",       "~> 8.0.0"
 
 # openid_connect 2.x line (faraday 2.x).