diff --git a/.gitignore b/.gitignore
index a2258b23..87d7767b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,7 @@ gemfiles/*.gemfile.lock
 node_modules
 pkg
 spec/examples.txt
+spec/internal
 tmp
 zeus.server.log
 zeus.server-start.log
diff --git a/Appraisals b/Appraisals
index c6e8c6de..5cb2556e 100644
--- a/Appraisals
+++ b/Appraisals
@@ -3,6 +3,7 @@ rails_dependencies =
     gem "activerecord-jdbcsqlite3-adapter", platform: :jruby
     gem "jdbc-sqlite3", platform: :jruby
     gem "net-ftp"
+    gem "combustion"
   end
 
 appraisals = {
@@ -10,24 +11,21 @@ appraisals = {
     proc do
       instance_eval(&rails_dependencies)
 
-      gem "activerecord", "~> 6.0.0"
-      gem "railties", "~> 6.0.0"
+      gem "rails", "~> 6.0.0"
       gem "sqlite3", "~> 1.4.0", platform: %i[ruby mswin mingw]
     end,
   rails_6_1:
     proc do
       instance_eval(&rails_dependencies)
 
-      gem "activerecord", "~> 6.1.0"
-      gem "railties", "~> 6.1.0"
+      gem "rails", "~> 6.1.0"
       gem "sqlite3", "~> 1.4.0", platform: %i[ruby mswin mingw]
     end,
   rails_7_0:
     proc do
       instance_eval(&rails_dependencies)
 
-      gem "activerecord", "~> 7.0.0"
-      gem "railties", "~> 7.0.0"
+      gem "rails", "~> 7.0.0"
       gem "sqlite3", "~> 1.4.0", platform: %i[ruby mswin mingw]
     end,
   no_rails: proc {},
diff --git a/gemfiles/rails_6_0_rspec_gte_3_10.gemfile b/gemfiles/rails_6_0_rspec_gte_3_10.gemfile
index 77e9c2c2..2651bb8f 100644
--- a/gemfiles/rails_6_0_rspec_gte_3_10.gemfile
+++ b/gemfiles/rails_6_0_rspec_gte_3_10.gemfile
@@ -17,8 +17,8 @@ gem "warnings_logger"
 gem "activerecord-jdbcsqlite3-adapter", platform: :jruby
 gem "jdbc-sqlite3", platform: :jruby
 gem "net-ftp"
-gem "activerecord", "~> 6.0.0"
-gem "railties", "~> 6.0.0"
+gem "combustion"
+gem "rails", "~> 6.0.0"
 gem "sqlite3", "~> 1.4.0", platform: [:ruby, :mswin, :mingw]
 gem "rspec", ">= 3.10", "< 4"
 gem "rspec-rails"
diff --git a/gemfiles/rails_6_0_rspec_lt_3_10.gemfile b/gemfiles/rails_6_0_rspec_lt_3_10.gemfile
index 3a9fd1e2..43a69561 100644
--- a/gemfiles/rails_6_0_rspec_lt_3_10.gemfile
+++ b/gemfiles/rails_6_0_rspec_lt_3_10.gemfile
@@ -17,8 +17,8 @@ gem "warnings_logger"
 gem "activerecord-jdbcsqlite3-adapter", platform: :jruby
 gem "jdbc-sqlite3", platform: :jruby
 gem "net-ftp"
-gem "activerecord", "~> 6.0.0"
-gem "railties", "~> 6.0.0"
+gem "combustion"
+gem "rails", "~> 6.0.0"
 gem "sqlite3", "~> 1.4.0", platform: [:ruby, :mswin, :mingw]
 gem "rspec", "~> 3.9.0"
 gem "rspec-rails"
diff --git a/gemfiles/rails_6_1_rspec_gte_3_10.gemfile b/gemfiles/rails_6_1_rspec_gte_3_10.gemfile
index 44a7eb41..20d7982a 100644
--- a/gemfiles/rails_6_1_rspec_gte_3_10.gemfile
+++ b/gemfiles/rails_6_1_rspec_gte_3_10.gemfile
@@ -17,8 +17,8 @@ gem "warnings_logger"
 gem "activerecord-jdbcsqlite3-adapter", platform: :jruby
 gem "jdbc-sqlite3", platform: :jruby
 gem "net-ftp"
-gem "activerecord", "~> 6.1.0"
-gem "railties", "~> 6.1.0"
+gem "combustion"
+gem "rails", "~> 6.1.0"
 gem "sqlite3", "~> 1.4.0", platform: [:ruby, :mswin, :mingw]
 gem "rspec", ">= 3.10", "< 4"
 gem "rspec-rails"
diff --git a/gemfiles/rails_6_1_rspec_lt_3_10.gemfile b/gemfiles/rails_6_1_rspec_lt_3_10.gemfile
index 6a6472df..3d4a0e98 100644
--- a/gemfiles/rails_6_1_rspec_lt_3_10.gemfile
+++ b/gemfiles/rails_6_1_rspec_lt_3_10.gemfile
@@ -17,8 +17,8 @@ gem "warnings_logger"
 gem "activerecord-jdbcsqlite3-adapter", platform: :jruby
 gem "jdbc-sqlite3", platform: :jruby
 gem "net-ftp"
-gem "activerecord", "~> 6.1.0"
-gem "railties", "~> 6.1.0"
+gem "combustion"
+gem "rails", "~> 6.1.0"
 gem "sqlite3", "~> 1.4.0", platform: [:ruby, :mswin, :mingw]
 gem "rspec", "~> 3.9.0"
 gem "rspec-rails"
diff --git a/gemfiles/rails_7_0_rspec_gte_3_10.gemfile b/gemfiles/rails_7_0_rspec_gte_3_10.gemfile
index 70e656a9..93abe4dd 100644
--- a/gemfiles/rails_7_0_rspec_gte_3_10.gemfile
+++ b/gemfiles/rails_7_0_rspec_gte_3_10.gemfile
@@ -17,8 +17,8 @@ gem "warnings_logger"
 gem "activerecord-jdbcsqlite3-adapter", platform: :jruby
 gem "jdbc-sqlite3", platform: :jruby
 gem "net-ftp"
-gem "activerecord", "~> 7.0.0"
-gem "railties", "~> 7.0.0"
+gem "combustion"
+gem "rails", "~> 7.0.0"
 gem "sqlite3", "~> 1.4.0", platform: [:ruby, :mswin, :mingw]
 gem "rspec", ">= 3.10", "< 4"
 gem "rspec-rails"
diff --git a/gemfiles/rails_7_0_rspec_lt_3_10.gemfile b/gemfiles/rails_7_0_rspec_lt_3_10.gemfile
index 07f2a8f9..d77e94d3 100644
--- a/gemfiles/rails_7_0_rspec_lt_3_10.gemfile
+++ b/gemfiles/rails_7_0_rspec_lt_3_10.gemfile
@@ -17,8 +17,8 @@ gem "warnings_logger"
 gem "activerecord-jdbcsqlite3-adapter", platform: :jruby
 gem "jdbc-sqlite3", platform: :jruby
 gem "net-ftp"
-gem "activerecord", "~> 7.0.0"
-gem "railties", "~> 7.0.0"
+gem "combustion"
+gem "rails", "~> 7.0.0"
 gem "sqlite3", "~> 1.4.0", platform: [:ruby, :mswin, :mingw]
 gem "rspec", "~> 3.9.0"
 gem "rspec-rails"
diff --git a/lib/super_diff/rails.rb b/lib/super_diff/rails.rb
index 300a23e7..fb7b98a4 100644
--- a/lib/super_diff/rails.rb
+++ b/lib/super_diff/rails.rb
@@ -1 +1,2 @@
-require "super_diff/active_record"
+require "super_diff/active_support"
+require "super_diff/active_record" if defined?(ActiveRecord)
diff --git a/spec/integration/rails/engines_spec.rb b/spec/integration/rails/engines_spec.rb
new file mode 100644
index 00000000..1d0e5755
--- /dev/null
+++ b/spec/integration/rails/engines_spec.rb
@@ -0,0 +1,20 @@
+require "spec_helper"
+
+RSpec.describe "Integration with Rails engines", type: :integration do
+  context "when ActiveRecord is not loaded via Combustion" do
+    it "does not fail to load" do
+      as_both_colored_and_uncolored do |color_enabled|
+        program =
+          make_rspec_rails_engine_program(
+            "expect(true).to be(true)",
+            combustion_initialize: [:action_controller],
+            color_enabled: color_enabled
+          )
+
+        expect(program).not_to produce_output_when_run(
+          "uninitialized constant ActiveRecord"
+        ).in_color(color_enabled)
+      end
+    end
+  end
+end
diff --git a/spec/support/integration/helpers.rb b/spec/support/integration/helpers.rb
index 3561a740..e30ac813 100644
--- a/spec/support/integration/helpers.rb
+++ b/spec/support/integration/helpers.rb
@@ -32,6 +32,18 @@ def make_rspec_rails_test_program(test, color_enabled:)
       TestPrograms::RSpecRails.new(test, color_enabled: color_enabled)
     end
 
+    def make_rspec_rails_engine_program(
+      test,
+      color_enabled:,
+      combustion_initialize:
+    )
+      TestPrograms::RspecRailsEngine.new(
+        test,
+        color_enabled: color_enabled,
+        combustion_initialize: combustion_initialize
+      )
+    end
+
     def build_expected_output(
       color_enabled:,
       snippet:,
diff --git a/spec/support/integration/matchers/produce_output_when_run_matcher.rb b/spec/support/integration/matchers/produce_output_when_run_matcher.rb
index 18500602..66859e40 100644
--- a/spec/support/integration/matchers/produce_output_when_run_matcher.rb
+++ b/spec/support/integration/matchers/produce_output_when_run_matcher.rb
@@ -55,6 +55,12 @@ def failure_message
         end
       end
 
+      def failure_message_when_negated
+        "Expected output of test not to produce output, but it did.\n\n" +
+          "Actual output:\n\n" +
+          SuperDiff::Test::OutputHelpers.bookended(actual_output)
+      end
+
       private
 
       attr_reader :expected_output, :program, :output_processor
diff --git a/spec/support/integration/test_programs/base.rb b/spec/support/integration/test_programs/base.rb
index de27f5dc..aab55097 100644
--- a/spec/support/integration/test_programs/base.rb
+++ b/spec/support/integration/test_programs/base.rb
@@ -112,7 +112,6 @@ def program
                 color_enabled: #{color_enabled?.inspect},
                 configuration: #{configuration.inspect}
               )
-              test_plan.boot
               #{test_plan_prelude}
               test_plan.#{test_plan_command}
 
diff --git a/spec/support/integration/test_programs/plain.rb b/spec/support/integration/test_programs/plain.rb
index c9157678..7465203b 100644
--- a/spec/support/integration/test_programs/plain.rb
+++ b/spec/support/integration/test_programs/plain.rb
@@ -4,6 +4,12 @@ module TestPrograms
       class Plain < Base
         protected
 
+        def test_plan_prelude
+          <<~PRELUDE.strip
+            test_plan.boot
+          PRELUDE
+        end
+
         def test_plan_command
           "run_plain_test"
         end
diff --git a/spec/support/integration/test_programs/rspec_active_record.rb b/spec/support/integration/test_programs/rspec_active_record.rb
index b5691b61..e303f387 100644
--- a/spec/support/integration/test_programs/rspec_active_record.rb
+++ b/spec/support/integration/test_programs/rspec_active_record.rb
@@ -5,7 +5,10 @@ class RSpecActiveRecord < Base
         protected
 
         def test_plan_prelude
-          "test_plan.boot_active_record"
+          <<~PRELUDE.strip
+            test_plan.boot
+            test_plan.boot_active_record
+          PRELUDE
         end
 
         def test_plan_command
diff --git a/spec/support/integration/test_programs/rspec_active_support.rb b/spec/support/integration/test_programs/rspec_active_support.rb
index 31ab8d93..bce2ec9d 100644
--- a/spec/support/integration/test_programs/rspec_active_support.rb
+++ b/spec/support/integration/test_programs/rspec_active_support.rb
@@ -5,7 +5,10 @@ class RSpecActiveSupport < Base
         protected
 
         def test_plan_prelude
-          "test_plan.boot_active_support"
+          <<~PRELUDE.strip
+            test_plan.boot
+            test_plan.boot_active_support
+          PRELUDE
         end
 
         def test_plan_command
diff --git a/spec/support/integration/test_programs/rspec_rails.rb b/spec/support/integration/test_programs/rspec_rails.rb
index fe405e10..4ce1152b 100644
--- a/spec/support/integration/test_programs/rspec_rails.rb
+++ b/spec/support/integration/test_programs/rspec_rails.rb
@@ -5,7 +5,10 @@ class RSpecRails < Base
         protected
 
         def test_plan_prelude
-          "test_plan.boot_active_record"
+          <<~PRELUDE.strip
+            test_plan.boot
+            test_plan.boot_active_record
+          PRELUDE
         end
 
         def test_plan_command
diff --git a/spec/support/integration/test_programs/rspec_rails_engine.rb b/spec/support/integration/test_programs/rspec_rails_engine.rb
new file mode 100644
index 00000000..439f3cf9
--- /dev/null
+++ b/spec/support/integration/test_programs/rspec_rails_engine.rb
@@ -0,0 +1,30 @@
+module SuperDiff
+  module IntegrationTests
+    module TestPrograms
+      class RspecRailsEngine < Base
+        def initialize(*args, combustion_initialize:, **options)
+          super(*args, **options)
+          @combustion_initialize = combustion_initialize
+        end
+
+        protected
+
+        def test_plan_prelude
+          <<~PRELUDE.strip
+            test_plan.boot_rails_engine(
+              combustion_initialize: #{combustion_initialize.inspect}
+            )
+          PRELUDE
+        end
+
+        def test_plan_command
+          "run_rspec_rails_test"
+        end
+
+        private
+
+        attr_reader :combustion_initialize
+      end
+    end
+  end
+end
diff --git a/support/test_plan.rb b/support/test_plan.rb
index 8f4073b4..f76f98e1 100644
--- a/support/test_plan.rb
+++ b/support/test_plan.rb
@@ -110,6 +110,11 @@ def boot_rails
     boot_active_record
   end
 
+  def boot_rails_engine(combustion_initialize: [])
+    require "combustion"
+    Combustion.initialize!(*combustion_initialize)
+  end
+
   def run_plain_test
     run_test_with_libraries("super_diff/rspec")
   end
diff --git a/zeus.json b/zeus.json
index 6e696e7f..41644621 100644
--- a/zeus.json
+++ b/zeus.json
@@ -12,6 +12,7 @@
       "boot_rails": {
         "run_rspec_rails_test": []
       }
-    }
+    },
+    "boot_rails_engine": []
   }
 }