diff --git a/.github/workflows/deploy-doc.yml b/.github/workflows/deploy-doc.yml
index 7eba579a..c2d1f7f4 100644
--- a/.github/workflows/deploy-doc.yml
+++ b/.github/workflows/deploy-doc.yml
@@ -19,7 +19,7 @@ jobs:
       - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - uses: ruby/setup-ruby@19a43a6a2428d455dbd1b85344698725179c9d8c # v1.289.0
         with:
-          ruby-version: '3.4.8'
+          ruby-version: '4.0'
       - run: bundle install
       - name: Setup Pages
         uses: actions/configure-pages@983d7736d9b0ae728b81ab479565c72886d7745b # v5.0.0
diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index 246ab182..f82d6185 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -23,7 +23,7 @@ jobs:
       - uses: ruby/setup-ruby@19a43a6a2428d455dbd1b85344698725179c9d8c # v1.289.0
         with:
           bundler-cache: true
-          ruby-version: 3.4.8
+          ruby-version: '4.0'
       - name: Update version file with the release version
         run: |
           if [ "${{ github.event_name }}" == "workflow_dispatch" ]; then
diff --git a/.github/workflows/pull_request.yml b/.github/workflows/pull_request.yml
index 2c2f38c7..29a02b69 100644
--- a/.github/workflows/pull_request.yml
+++ b/.github/workflows/pull_request.yml
@@ -18,6 +18,7 @@ jobs:
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
     name: Ruby v${{ matrix.ruby }}
     steps:
     - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
diff --git a/Gemfile b/Gemfile
index b44ad4d6..7c73b4ac 100644
--- a/Gemfile
+++ b/Gemfile
@@ -4,9 +4,11 @@ source 'https://rubygems.org'
 gemspec
 
 group :development, :test do
+  gem 'irb', '~> 1.15.0'
   gem 'rack', '~> 3.1' # for yard server
   gem 'rackup', '~> 2.1' # for yard server
   gem 'rbs', '~> 3.10.0'
+  gem 'redcarpet', '~> 3.6.0'
   gem 'rubocop', '~> 1.85.0', require: false
   gem 'steep', '~> 1.10.0'
   gem 'webrick', '~> 1.9.1'
diff --git a/Gemfile.lock b/Gemfile.lock
index ae943750..c3ea11d0 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -8,14 +8,14 @@ PATH
 GEM
   remote: https://rubygems.org/
   specs:
-    activesupport (8.0.2)
+    activesupport (8.1.2)
       base64
-      benchmark (>= 0.3)
       bigdecimal
       concurrent-ruby (~> 1.0, >= 1.3.1)
       connection_pool (>= 2.2.5)
       drb
       i18n (>= 1.6, < 2)
+      json
       logger (>= 1.4.2)
       minitest (>= 5.1)
       securerandom (>= 0.3)
@@ -25,7 +25,6 @@ GEM
       public_suffix (>= 2.0.2, < 7.0)
     ast (2.4.3)
     base64 (0.2.0)
-    benchmark (0.4.0)
     bigdecimal (3.3.1)
     concurrent-ruby (1.3.5)
     connection_pool (2.5.2)
@@ -33,14 +32,20 @@ GEM
       bigdecimal
       rexml
     csv (3.3.4)
+    date (3.5.1)
     diff-lcs (1.6.2)
     drb (2.2.1)
-    ffi (1.17.2-arm64-darwin)
-    ffi (1.17.2-x86_64-linux-gnu)
+    erb (6.0.1)
+    ffi (1.17.2)
     fileutils (1.7.3)
     hashdiff (1.2.1)
     i18n (1.14.7)
       concurrent-ruby (~> 1.0)
+    io-console (0.8.2)
+    irb (1.15.3)
+      pp (>= 0.6.0)
+      rdoc (>= 4.0.0)
+      reline (>= 0.4.2)
     json (2.18.1)
     json-schema (4.3.1)
       addressable (>= 2.8)
@@ -52,14 +57,21 @@ GEM
     logger (1.7.0)
     mcp (0.8.0)
       json-schema (>= 4.1)
-    minitest (5.25.5)
+    minitest (6.0.1)
+      prism (~> 1.5)
     multipart-post (2.4.1)
     mutex_m (0.3.0)
     parallel (1.27.0)
     parser (3.3.10.2)
       ast (~> 2.4.1)
       racc
+    pp (0.6.3)
+      prettyprint
+    prettyprint (0.2.0)
     prism (1.9.0)
+    psych (5.3.1)
+      date
+      stringio
     public_suffix (6.0.2)
     racc (1.8.1)
     rack (3.2.5)
@@ -73,7 +85,14 @@ GEM
     rbs (3.10.3)
       logger
       tsort
+    rdoc (7.2.0)
+      erb
+      psych (>= 4.0.0)
+      tsort
+    redcarpet (3.6.1)
     regexp_parser (2.11.3)
+    reline (0.6.3)
+      io-console (~> 0.5)
     rexml (3.4.4)
     rspec (3.13.2)
       rspec-core (~> 3.13.0)
@@ -122,6 +141,7 @@ GEM
       strscan (>= 1.0.0)
       terminal-table (>= 2, < 5)
       uri (>= 0.12.0)
+    stringio (3.2.0)
     strscan (3.1.3)
     terminal-table (4.0.0)
       unicode-display_width (>= 1.1.1, < 4)
@@ -142,14 +162,17 @@ GEM
 PLATFORMS
   arm64-darwin-21
   arm64-darwin-22
+  arm64-darwin-24
   x86_64-linux
 
 DEPENDENCIES
+  irb (~> 1.15.0)
   line-bot-api!
   rack (~> 3.1)
   rackup (~> 2.1)
   rake (~> 13.0)
   rbs (~> 3.10.0)
+  redcarpet (~> 3.6.0)
   rspec (~> 3.13.0)
   rubocop (~> 1.85.0)
   steep (~> 1.10.0)
diff --git a/Rakefile b/Rakefile
index 6f7861d7..91ab596d 100644
--- a/Rakefile
+++ b/Rakefile
@@ -5,6 +5,13 @@ RSpec::Core::RakeTask.new(:spec)
 
 task default: :spec
 
+# To avoid double "bundle exec" calls, e.g. bundle exec ... in bundle exec rake ...
+def bundle_exec(command)
+  Bundler.with_unbundled_env do
+    sh "bundle exec #{command}"
+  end
+end
+
 # We don't want to push tags to source control when releasing the gem
 Rake::Task["release"].clear
 
@@ -24,17 +31,19 @@ task :test_normal, [:skip_tag] do |_, args|
   rspec_opts = []
   rspec_opts << "--tag '~#{skip_tag}'" if skip_tag
 
-  sh "bundle exec rspec --exclude-pattern 'spec/line/bot/line_bot_api_gem_spec.rb,spec/line/bot/line_bot_gem_spec.rb' #{rspec_opts.join(' ')}"
+  bundle_exec(
+    "rspec --exclude-pattern 'spec/line/bot/line_bot_api_gem_spec.rb,spec/line/bot/line_bot_gem_spec.rb' #{rspec_opts.join(' ')}"
+  )
 end
 
 desc "Test line-bot-api gem spec"
 task :test_line_bot_api do
-  sh "bundle exec rspec --pattern 'spec/line/bot/line_bot_api_gem_spec.rb'"
+  bundle_exec("rspec --pattern 'spec/line/bot/line_bot_api_gem_spec.rb'")
 end
 
 desc "Test line/bot gem(?) spec"
 task :test_line_bot do
-  sh "bundle exec rspec --pattern 'spec/line/bot/line_bot_gem_spec.rb'"
+  bundle_exec("rspec --pattern 'spec/line/bot/line_bot_gem_spec.rb'")
 end
 
 desc "Run all tests in separate processes"
@@ -46,18 +55,18 @@ end
 
 desc "Validate comment for YARD"
 task :validate_yard_comment do
-  sh "bundle exec yard stats ./lib/line/bot/v2 --fail-on-warning"
+  bundle_exec("yard stats ./lib/line/bot/v2 --fail-on-warning")
 end
 
 desc "RBS type check"
 task :rbs do
-  sh "bundle exec rbs collection install"
-  sh "bundle exec rbs -I sig validate"
+  bundle_exec("rbs collection install")
+  bundle_exec("rbs -I sig validate")
 end
 
 desc "RBS type check (with steep)"
 task :rbs_steep do
-  sh "bundle exec steep check"
+  bundle_exec("steep check")
 end
 
 desc "RBS type check (with test)"
@@ -86,17 +95,17 @@ end
 
 desc "Run rubocop"
 task :rubocop do
-  sh "bundle exec rubocop"
+  bundle_exec("rubocop")
 end
 
 desc "Fix rubocop errors"
 task :rubocop_fix do
-  sh "bundle exec rubocop -A"
+  bundle_exec("rubocop -A")
 end
 
 desc "Check buildable and its contents"
 task :build_test do
-  sh "bundle exec rake build"
+  bundle_exec("rake build")
   sh "tar -xvf pkg/line-bot-api-*.gem"
 
   puts "<<Show the contents of the gem>>"
diff --git a/rbs_collection.lock.yaml b/rbs_collection.lock.yaml
index 9ed3b8cc..27db5b87 100644
--- a/rbs_collection.lock.yaml
+++ b/rbs_collection.lock.yaml
@@ -25,6 +25,10 @@ gems:
     revision: 0ef449166f72c767f58f920cc36aca818fbf082f
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
+- name: dbm
+  version: '0'
+  source:
+    type: stdlib
 - name: diff-lcs
   version: '1.5'
   source:
@@ -37,6 +41,10 @@ gems:
   version: '0'
   source:
     type: stdlib
+- name: erb
+  version: '0'
+  source:
+    type: stdlib
 - name: hashdiff
   version: '1.1'
   source:
@@ -45,6 +53,10 @@ gems:
     revision: 0ef449166f72c767f58f920cc36aca818fbf082f
     remote: https://github.com/ruby/gem_rbs_collection.git
     repo_dir: gems
+- name: io-console
+  version: '0'
+  source:
+    type: stdlib
 - name: json
   version: '0'
   source:
@@ -61,6 +73,26 @@ gems:
   version: '0'
   source:
     type: stdlib
+- name: pp
+  version: '0'
+  source:
+    type: stdlib
+- name: prettyprint
+  version: '0'
+  source:
+    type: stdlib
+- name: pstore
+  version: '0'
+  source:
+    type: stdlib
+- name: psych
+  version: '0'
+  source:
+    type: stdlib
+- name: rdoc
+  version: '0'
+  source:
+    type: stdlib
 - name: socket
   version: '0'
   source:
@@ -69,6 +101,10 @@ gems:
   version: '0'
   source:
     type: stdlib
+- name: tsort
+  version: '0'
+  source:
+    type: stdlib
 - name: uri
   version: '0'
   source:
diff --git a/spec/workflows/ruby_version_consistency_spec.rb b/spec/workflows/ruby_version_consistency_spec.rb
new file mode 100644
index 00000000..5c799438
--- /dev/null
+++ b/spec/workflows/ruby_version_consistency_spec.rb
@@ -0,0 +1,54 @@
+require 'yaml'
+
+RSpec.describe "GitHub Actions workflow Ruby version consistency" do
+  let(:workflows_dir) { File.expand_path('../../.github/workflows', __dir__) }
+
+  let(:latest_matrix_version) do
+    pr_workflow = YAML.safe_load(
+      File.read(File.join(workflows_dir, 'pull_request.yml')),
+      permitted_classes: [Symbol]
+    )
+    versions = pr_workflow.dig('jobs', 'build', 'strategy', 'matrix', 'ruby')
+    expect(versions).not_to be_nil, "No ruby matrix found in pull_request.yml"
+    versions.map(&:to_s).max_by { |v| Gem::Version.new(v) }
+  end
+
+  let(:hardcoded_ruby_versions) do
+    versions = {}
+
+    Dir[File.join(workflows_dir, '*.yml')].each do |file|
+      workflow = YAML.safe_load(File.read(file), permitted_classes: [Symbol])
+      basename = File.basename(file)
+
+      next unless workflow['jobs']
+
+      workflow['jobs'].each do |job_name, job|
+        steps = job['steps'] || []
+        steps.each do |step|
+          next unless step.is_a?(Hash) && step.dig('with', 'ruby-version')
+
+          ruby_version = step['with']['ruby-version'].to_s
+          next if ruby_version.include?('${{')
+
+          versions["#{basename} -> #{job_name}"] = ruby_version
+        end
+      end
+    end
+
+    versions
+  end
+
+  it "uses the same Ruby version as the latest in pull_request.yml matrix" do
+    expect(hardcoded_ruby_versions).not_to(
+      be_empty, "No hardcoded ruby-version found in workflows"
+    )
+
+    mismatched = hardcoded_ruby_versions.reject { |_, v| v == latest_matrix_version }
+    expect(mismatched).to(
+      be_empty,
+      "Expected all hardcoded ruby-version to be '#{latest_matrix_version}' " \
+      "(latest in pull_request.yml matrix), but found:\n" \
+      "#{mismatched.map { |k, v| "  #{k}: #{v}" }.join("\n")}"
+    )
+  end
+end