diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index b4b36ffe..31c8c87c 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -22,6 +22,7 @@ jobs:
           - '3.3'
           - '3.4'
           - '4.0'
+          - 'ruby-head'
     steps:
     - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6
     - name: Set up Ruby
diff --git a/lib/rage/router/dsl.rb b/lib/rage/router/dsl.rb
index e040f9db..acdd8c4a 100644
--- a/lib/rage/router/dsl.rb
+++ b/lib/rage/router/dsl.rb
@@ -420,11 +420,11 @@ def mount(app, at:, via: :all)
       at = at.delete_suffix("/") if at.end_with?("/")
 
       http_methods = if via == :all || via.nil?
-        @default_match_methods.map { |method| method.to_s.upcase! }
+        @default_match_methods.map { |method| method.to_s.upcase }
       else
-        Array(via).map! do |method|
+        Array(via).map do |method|
           raise ArgumentError, "Invalid HTTP method: #{method}" unless @default_match_methods.include?(method)
-          method.to_s.upcase!
+          method.to_s.upcase
         end
       end
 
@@ -501,8 +501,7 @@ def __resource_scope(resource, path, mod, &block)
     end
 
     def to_singular(str)
-      @active_support_loaded ||= str.respond_to?(:singularize) || :false
-      return str.singularize if @active_support_loaded != :false
+      return str.singularize if str.respond_to?(:singularize)
 
       @endings ||= {
         "ves" => "fe",
@@ -519,8 +518,7 @@ def to_singular(str)
     end
 
     def to_plural(str)
-      @active_support_loaded ||= str.respond_to?(:pluralize) || :false
-      return str.pluralize if @active_support_loaded != :false
+      return str.pluralize if str.respond_to?(:pluralize)
 
       str.end_with?("s") ? str : "#{str}s"
     end
diff --git a/spec/openapi/parsers/ext/blueprinter_spec.rb b/spec/openapi/parsers/ext/blueprinter_spec.rb
index ec21c8a6..2f6cdb81 100644
--- a/spec/openapi/parsers/ext/blueprinter_spec.rb
+++ b/spec/openapi/parsers/ext/blueprinter_spec.rb
@@ -883,8 +883,12 @@
       end
 
       before do
-        allow_any_instance_of(String).to receive(:respond_to?).and_call_original
-        allow_any_instance_of(String).to receive(:respond_to?).with(:singularize).and_return(false)
+        String.alias_method(:__singularize__, :singularize)
+        String.undef_method(:singularize)
+      end
+
+      after do
+        String.alias_method(:singularize, :__singularize__)
       end
 
       it "falls back to array, since cardinality cannot be determined without singularize" do
@@ -2573,8 +2577,12 @@
       end
 
       before do
-        allow_any_instance_of(String).to receive(:respond_to?).and_call_original
-        allow_any_instance_of(String).to receive(:respond_to?).with(:singularize).and_return(false)
+        String.alias_method(:__singularize__, :singularize)
+        String.undef_method(:singularize)
+      end
+
+      after do
+        String.alias_method(:singularize, :__singularize__)
       end
 
       it "falls back to array, since cardinality cannot be determined without singularize" do
diff --git a/spec/router/dsl_spec.rb b/spec/router/dsl_spec.rb
index 69e04467..a9040e90 100644
--- a/spec/router/dsl_spec.rb
+++ b/spec/router/dsl_spec.rb
@@ -764,12 +764,13 @@ def call(env)
     end
 
     it "uses activesupport" do
-      allow_any_instance_of(String).to receive(:singularize).and_return("image")
+      _resource = +"photos"
+      allow(_resource).to receive(:singularize).and_return("image")
 
       expect(router).to receive(:on).with("POST", "/photos/:image_id/mark", "photos#mark", instance_of(Hash))
 
       dsl.draw do
-        resources :photos, only: [] do
+        resources _resource, only: [] do
           post :mark
         end
       end