diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index 270e31e..0dc56e6 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -8,33 +8,18 @@ jobs:
       fail-fast: false
       matrix:
         ruby-version:
-          - '3.0'
-          - '3.1'
           - '3.2'
           - '3.3'
           - '3.4'
+          - '4.0'
         gemfile:
-          - gemfiles/Gemfile.rails61
-          - gemfiles/Gemfile.rails70
           - gemfiles/Gemfile.rails71
           - gemfiles/Gemfile.rails72
           - gemfiles/Gemfile.rails80
+          - gemfiles/Gemfile.rails81
         exclude:
-          # rails 6.1 requires ruby < 3.4
-          - ruby-version: '3.4'
-            gemfile: 'gemfiles/Gemfile.rails61'
-          # rails 7.0 requires ruby >= 2.7, < 3.4
-          # https://www.fastruby.io/blog/ruby/rails/versions/compatibility-table.html
-          - ruby-version: '3.4'
-            gemfile: 'gemfiles/Gemfile.rails70'
-          # rails 7.2 requires ruby >= 3.1
-          # https://www.fastruby.io/blog/ruby/rails/versions/compatibility-table.html
-          - ruby-version: '3.0'
-            gemfile: 'gemfiles/Gemfile.rails72'
           # rails 8.0 requires ruby >= 3.2
           # https://www.fastruby.io/blog/ruby/rails/versions/compatibility-table.html
-          - ruby-version: '3.0'
-            gemfile: 'gemfiles/Gemfile.rails80'
           - ruby-version: '3.1'
             gemfile: 'gemfiles/Gemfile.rails80'
 
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 87dffbc..8c03fa6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,6 @@
 ## [Unreleased]
-* no unreleased changes
+### Fixed
+* Support Rails 8.1, Ruby 4.0
 
 ## 2.4.3 / 2025-11-07
 ### Fixed
diff --git a/gemfiles/Gemfile.rails61 b/gemfiles/Gemfile.rails61
deleted file mode 100644
index 8fdb206..0000000
--- a/gemfiles/Gemfile.rails61
+++ /dev/null
@@ -1,12 +0,0 @@
-source 'https://rubygems.org'
-
-gemspec path: '..'
-
-gem 'rails', '~> 6.1.0'
-
-# Rails 6.1 does not support sqlite3 2.x; it specifies gem "sqlite3", "~> 1.4"
-# in lib/active_record/connection_adapters/sqlite3_adapter.rb
-gem 'sqlite3', '~> 1.7'
-
-# Latest concurrent-ruby breaks Rails < 7.1. See https://github.com/rails/rails/issues/54260
-gem 'concurrent-ruby', '1.3.4'
diff --git a/gemfiles/Gemfile.rails70 b/gemfiles/Gemfile.rails70
deleted file mode 100644
index 085807e..0000000
--- a/gemfiles/Gemfile.rails70
+++ /dev/null
@@ -1,12 +0,0 @@
-source 'https://rubygems.org'
-
-gemspec path: '..'
-
-gem 'rails', '~> 7.0.0'
-
-# Rails 7.0 does not support sqlite3 2.x; it specifies gem "sqlite3", "~> 1.4"
-# in lib/active_record/connection_adapters/sqlite3_adapter.rb
-gem 'sqlite3', '~> 1.7'
-
-# Latest concurrent-ruby breaks Rails < 7.1. See https://github.com/rails/rails/issues/54260
-gem 'concurrent-ruby', '1.3.4'
diff --git a/gemfiles/Gemfile.rails71 b/gemfiles/Gemfile.rails71
index 50624d7..173bb3f 100644
--- a/gemfiles/Gemfile.rails71
+++ b/gemfiles/Gemfile.rails71
@@ -3,6 +3,3 @@ source 'https://rubygems.org'
 gemspec path: '..'
 
 gem 'rails', '~> 7.1.0'
-
-# sqlite3 2.x is not supported on ruby 3.0
-gem 'sqlite3', '~> 1.7'
diff --git a/gemfiles/Gemfile.rails81 b/gemfiles/Gemfile.rails81
new file mode 100644
index 0000000..4b958c0
--- /dev/null
+++ b/gemfiles/Gemfile.rails81
@@ -0,0 +1,5 @@
+source 'https://rubygems.org'
+
+gemspec path: '..'
+
+gem 'rails', '~> 8.1.0'
diff --git a/ndr_error.gemspec b/ndr_error.gemspec
index 3eb4556..7a53367 100644
--- a/ndr_error.gemspec
+++ b/ndr_error.gemspec
@@ -19,14 +19,9 @@ Gem::Specification.new do |s|
   s.files = Dir['{app,config,db,lib}/**/*', 'CHANGELOG.md', 'CODE_OF_CONDUCT.md',
                 'MIT-LICENSE', 'Rakefile', 'README.md'] - ['.travis.yml']
 
-  s.required_ruby_version = '>= 3.0'
+  s.required_ruby_version = '>= 3.2'
 
-  s.add_dependency 'rails', '>= 6.1', '< 8.1'
-
-  # Support rails 6.1 with Ruby 3.1
-  s.add_dependency 'net-imap'
-  s.add_dependency 'net-pop'
-  s.add_dependency 'net-smtp'
+  s.add_dependency 'rails', '>= 7.1', '< 8.2'
 
   s.add_dependency 'will_paginate'
 
@@ -35,23 +30,8 @@ Gem::Specification.new do |s|
   s.add_development_dependency 'pry'
   s.add_development_dependency 'puma'
 
-  # Rails 6.1 and 7.0 do not support sqlite3 2.x; they specify gem "sqlite3", "~> 1.4"
-  # in lib/active_record/connection_adapters/sqlite3_adapter.rb
-  # cf. gemfiles/Gemfile.rails70
   s.add_development_dependency 'sqlite3'
 
-  # Workaround build issue on GitHub Actions with ruby <= 3.1 when installing sass-embedded
-  # gem version 1.81.0: NoMethodError: undefined method `parse' for #<Psych::Parser...>
-  # https://bugs.ruby-lang.org/issues/19371
-  s.add_development_dependency 'psych', '< 5'
-
-  # Workaround build issue on GitHub Actions with ruby <= 3.1 when installing sass-embedded
-  # Versions 1.77.0 and above require ruby >= 3.2 for packaged x86_64-linux binaries
-  # but say they're compatible with ruby 3.1.
-  # 1.76.0 has a bug that was fixed in 1.77.1, which we can't use yet.
-  # https://github.com/sass/dart-sass/issues/2239
-  s.add_development_dependency 'sass-embedded', '~> 1.75.0' if RUBY_VERSION.start_with?('3.1') # rubocop:disable Gemspec/RubyVersionGlobalsUsage
-
   s.add_development_dependency 'mocha'
   s.add_development_dependency 'test-unit', '~> 3.0'