diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ce2a71cddb..a89e1e122f 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,5 +1,8 @@
 name: CI
-on: [push, pull_request]
+on:
+  push:
+    branches: [master]
+  pull_request:
 permissions:
   contents: read
 
@@ -10,11 +13,13 @@ jobs:
       matrix:
         os: [ ubuntu, macos, windows ]
         ruby: [ 3.2.9, 3.3.9, 3.4.7 ]
+        mspecopt: [""]
         rubyopt: [""]
         exclude:
           - { os: windows, ruby: 3.2.9 } # 3.2.9 has some issue with file time microseconds
         include:
           - { os: windows, ruby: 3.2.8 } # 3.2.9 has some issue with file time microseconds
+          - { os: ubuntu, ruby: 3.4.7, mspecopt: "--repeat 2" }
           - { os: ubuntu, ruby: 3.4.7, rubyopt: "--enable-frozen-string-literal" }
           - { os: ubuntu, ruby: 3.4.7, rubyopt: "--parser=parse.y" }
 
@@ -37,14 +42,14 @@ jobs:
       env:
         CHECK_LEAKS: true
         RUBYOPT: "${{ matrix.rubyopt }}"
-      run: ../mspec/bin/mspec -j --timeout 30
+      run: ../mspec/bin/mspec -j --timeout 30 ${{ matrix.mspecopt }}
 
     - name: Run specs (macOS)
       if: matrix.os == 'macos'
       env:
         CHECK_LEAKS: true
       # Use a larger timeout on macOS since it seems to have less reliable performance
-      run: ../mspec/bin/mspec -j --timeout 90
+      run: ../mspec/bin/mspec -j --timeout 90 ${{ matrix.mspecopt }}
 
     - name: Run specs (Windows)
       if: matrix.os == 'windows'
@@ -54,13 +59,13 @@ jobs:
         # Actions uses UTF8, causes test failures, similar to normal OS setup
         [Console]::OutputEncoding = [System.Text.Encoding]::GetEncoding("IBM437")
         [Console]::InputEncoding  = [System.Text.Encoding]::GetEncoding("IBM437")
-        ../mspec/bin/mspec -j
+        ../mspec/bin/mspec -j ${{ matrix.mspecopt }}
 
     - name: Run C-API specs as C++
       if: matrix.os != 'windows'
       env:
         SPEC_CAPI_CXX: true
-      run: ../mspec/bin/mspec :capi
+      run: ../mspec/bin/mspec :capi ${{ matrix.mspecopt }}
 
     - name: Run require_relative checker
       if: matrix.os == 'ubuntu'
diff --git a/core/enumerable/shared/inject.rb b/core/enumerable/shared/inject.rb
index aae9e06c97..8fb7e98c2b 100644
--- a/core/enumerable/shared/inject.rb
+++ b/core/enumerable/shared/inject.rb
@@ -103,7 +103,7 @@ def name.to_str; "-"; end
 
   it "without inject arguments(legacy rubycon)" do
     # no inject argument
-    EnumerableSpecs::EachDefiner.new(2).send(@method) {|acc,x| 999 } .should == 2
+    EnumerableSpecs::EachDefiner.new(2).send(@method) {|acc,x| 999 }.should == 2
     EnumerableSpecs::EachDefiner.new(2).send(@method) {|acc,x| acc }.should == 2
     EnumerableSpecs::EachDefiner.new(2).send(@method) {|acc,x| x }.should == 2
 
diff --git a/core/kernel/autoload_spec.rb b/core/kernel/autoload_spec.rb
index 0404caec6d..5edb70541d 100644
--- a/core/kernel/autoload_spec.rb
+++ b/core/kernel/autoload_spec.rb
@@ -7,7 +7,9 @@
 
 autoload :KSAutoloadA, "autoload_a.rb"
 autoload :KSAutoloadB, fixture(__FILE__, "autoload_b.rb")
-autoload :KSAutoloadCallsRequire, "main_autoload_not_exist.rb"
+define_autoload_KSAutoloadCallsRequire = -> {
+  autoload :KSAutoloadCallsRequire, "main_autoload_not_exist.rb"
+}
 
 def check_autoload(const)
   autoload? const
@@ -43,6 +45,7 @@ def check_autoload(const)
   end
 
   it "calls main.require(path) to load the file" do
+    define_autoload_KSAutoloadCallsRequire.call
     main = TOPLEVEL_BINDING.eval("self")
     main.should_receive(:require).with("main_autoload_not_exist.rb")
     # The constant won't be defined since require is mocked to do nothing
diff --git a/core/module/const_added_spec.rb b/core/module/const_added_spec.rb
index 739be3ead8..90cd36551a 100644
--- a/core/module/const_added_spec.rb
+++ b/core/module/const_added_spec.rb
@@ -117,6 +117,7 @@ module self::B
     end
 
     ScratchPad.recorded.should == [:A, :B]
+    ModuleSpecs::ConstAddedSpecs.send :remove_const, :NamedModule
   end
 
   it "is called when a new class is defined under self" do
@@ -158,6 +159,7 @@ class self::B
     end
 
     ScratchPad.recorded.should == [:A, :B]
+    ModuleSpecs::ConstAddedSpecs.send :remove_const, :NamedModuleB
   end
 
   it "is called when an autoload is defined" do
diff --git a/core/module/const_source_location_spec.rb b/core/module/const_source_location_spec.rb
index 06b3b215c2..d0708bc784 100644
--- a/core/module/const_source_location_spec.rb
+++ b/core/module/const_source_location_spec.rb
@@ -255,6 +255,17 @@
       line = ConstantSpecs::CONST_LOCATION
       ConstantSpecs.const_source_location('CONST_LOCATION').should == [file, line]
     end
+  end
+
+
+  context 'autoload' do
+    before :each do
+      @loaded_features = $".dup
+    end
+
+    after :each do
+      $".replace @loaded_features
+    end
 
     ruby_bug("#20188", ""..."3.4") do
       it 'returns the real constant location as soon as it is defined' do
@@ -265,6 +276,8 @@
         ConstantSpecs.const_source_location(:ConstSource).should == autoload_location
         ConstantSpecs::ConstSource::LOCATION.should == ConstantSpecs.const_source_location(:ConstSource)
         ConstantSpecs::BEFORE_DEFINE_LOCATION.should == autoload_location
+        ConstantSpecs.send :remove_const, :ConstSource
+        ConstantSpecs.send :remove_const, :BEFORE_DEFINE_LOCATION
       end
     end
   end
diff --git a/core/module/name_spec.rb b/core/module/name_spec.rb
index fd28ee0a33..d3318e1645 100644
--- a/core/module/name_spec.rb
+++ b/core/module/name_spec.rb
@@ -190,6 +190,7 @@ module self::B
 
     ScratchPad.recorded.should.one?(/#<Module.+>::A$/)
     ScratchPad.recorded.should.one?(/#<Module.+>::A::B$/)
+    ModuleSpecs::NameSpecs.send :remove_const, :NamedModule
   end
 
   it "returns a frozen String" do
diff --git a/core/module/set_temporary_name_spec.rb b/core/module/set_temporary_name_spec.rb
index 12c1c214dd..624b02c90e 100644
--- a/core/module/set_temporary_name_spec.rb
+++ b/core/module/set_temporary_name_spec.rb
@@ -86,6 +86,7 @@ module m::N; end
 
       ModuleSpecs::SetTemporaryNameSpec::M = m
       m::N.name.should == "ModuleSpecs::SetTemporaryNameSpec::M::N"
+      ModuleSpecs::SetTemporaryNameSpec.send :remove_const, :M
     end
 
     it "can update the name when assigned to a constant" do
diff --git a/core/random/new_spec.rb b/core/random/new_spec.rb
index 90e2a9d6f2..69210cef03 100644
--- a/core/random/new_spec.rb
+++ b/core/random/new_spec.rb
@@ -11,7 +11,7 @@
   it "returns Random instances initialized with different seeds" do
     first = Random.new
     second = Random.new
-    (0..20).map { first.rand } .should_not == (0..20).map { second.rand }
+    (0..20).map { first.rand }.should_not == (0..20).map { second.rand }
   end
 
   it "accepts an Integer seed value as an argument" do
diff --git a/language/regexp/encoding_spec.rb b/language/regexp/encoding_spec.rb
index 898b6d4ff7..ceb9cf823a 100644
--- a/language/regexp/encoding_spec.rb
+++ b/language/regexp/encoding_spec.rb
@@ -39,7 +39,11 @@
   end
 
   it "warns when using /n with a match string with non-ASCII characters and an encoding other than ASCII-8BIT" do
-    -> { /./n.match("\303\251".dup.force_encoding('utf-8')) }.should complain(%r{historical binary regexp match /.../n against UTF-8 string})
+    -> {
+      eval <<~RUBY
+      /./n.match("\303\251".dup.force_encoding('utf-8'))
+      RUBY
+    }.should complain(%r{historical binary regexp match /.../n against UTF-8 string})
   end
 
   it 'uses US-ASCII as /n encoding if all chars are 7-bit' do
diff --git a/language/rescue_spec.rb b/language/rescue_spec.rb
index 79571d689f..6be3bfd023 100644
--- a/language/rescue_spec.rb
+++ b/language/rescue_spec.rb
@@ -136,10 +136,14 @@ class << Object.new
 
     it 'captures successfully at the top-level' do
       ScratchPad.record []
+      loaded_features = $".dup
+      begin
+        require_relative 'fixtures/rescue/top_level'
 
-      require_relative 'fixtures/rescue/top_level'
-
-      ScratchPad.recorded.should == ["message"]
+        ScratchPad.recorded.should == ["message"]
+      ensure
+        $".replace loaded_features
+      end
     end
   end
 
diff --git a/library/net-http/HTTPServerException_spec.rb b/library/net-http/HTTPServerException_spec.rb
index 5e0a833fee..020d3cce85 100644
--- a/library/net-http/HTTPServerException_spec.rb
+++ b/library/net-http/HTTPServerException_spec.rb
@@ -3,10 +3,10 @@
 
 describe "Net::HTTPServerException" do
   it "is a subclass of Net::ProtoServerError and is warned as deprecated" do
-    -> { Net::HTTPServerException.should < Net::ProtoServerError }.should complain(/warning: constant Net::HTTPServerException is deprecated/)
+    -> { eval("Net::HTTPServerException").should < Net::ProtoServerError }.should complain(/warning: constant Net::HTTPServerException is deprecated/)
   end
 
   it "includes the Net::HTTPExceptions module and is warned as deprecated" do
-    -> { Net::HTTPServerException.should < Net::HTTPExceptions }.should complain(/warning: constant Net::HTTPServerException is deprecated/)
+    -> { eval("Net::HTTPServerException").should < Net::HTTPExceptions }.should complain(/warning: constant Net::HTTPServerException is deprecated/)
   end
 end
diff --git a/optional/capi/encoding_spec.rb b/optional/capi/encoding_spec.rb
index 0c3c98a5c0..c14983c7ea 100644
--- a/optional/capi/encoding_spec.rb
+++ b/optional/capi/encoding_spec.rb
@@ -724,14 +724,16 @@
   end
 
   describe "rb_define_dummy_encoding" do
+    run = 0
+
     it "defines the dummy encoding" do
-      @s.rb_define_dummy_encoding("FOO")
-      enc = Encoding.find("FOO")
+      @s.rb_define_dummy_encoding("FOO#{run += 1}")
+      enc = Encoding.find("FOO#{run}")
       enc.should.dummy?
     end
 
     it "returns the index of the dummy encoding" do
-      index = @s.rb_define_dummy_encoding("BAR")
+      index = @s.rb_define_dummy_encoding("BAR#{run += 1}")
       index.should == Encoding.list.size - 1
     end
 
diff --git a/optional/capi/globals_spec.rb b/optional/capi/globals_spec.rb
index 48677620bc..4657293e15 100644
--- a/optional/capi/globals_spec.rb
+++ b/optional/capi/globals_spec.rb
@@ -41,14 +41,19 @@
     @f.sb_get_global_value.should == "XYZ"
   end
 
+  run = 0
+
   it "rb_define_readonly_variable should define a new readonly global variable" do
+    name = "ro_gvar#{run += 1}"
+    eval <<~RUBY
     # Check the gvar doesn't exist and ensure rb_gv_get doesn't implicitly declare the gvar,
     # otherwise the rb_define_readonly_variable call will conflict.
-    suppress_warning { @f.sb_gv_get("ro_gvar") } .should == nil
+    suppress_warning { @f.sb_gv_get("#{name}") }.should == nil
 
-    @f.rb_define_readonly_variable("ro_gvar", 15)
-    $ro_gvar.should == 15
-    -> { $ro_gvar = 10 }.should raise_error(NameError)
+    @f.rb_define_readonly_variable("#{name}", 15)
+    $#{name}.should == 15
+    -> { $#{name} = 10 }.should raise_error(NameError)
+    RUBY
   end
 
   it "rb_define_hooked_variable should define a C hooked global variable" do
diff --git a/optional/capi/module_spec.rb b/optional/capi/module_spec.rb
index b7684e566b..af39ec0192 100644
--- a/optional/capi/module_spec.rb
+++ b/optional/capi/module_spec.rb
@@ -38,7 +38,7 @@
         CApiModuleSpecs::C.const_set(:_INVALID, 1)
       }.should raise_error(NameError, /wrong constant name/)
 
-      @m.rb_const_set(CApiModuleSpecs::C, :_INVALID, 2)
+      suppress_warning { @m.rb_const_set(CApiModuleSpecs::C, :_INVALID, 2) }
       @m.rb_const_get(CApiModuleSpecs::C, :_INVALID).should == 2
 
       # Ruby-level should still not allow access