diff --git a/lib/irb/cmd/debug.rb b/lib/irb/cmd/debug.rb
index 01bbf49d5..9eca96421 100644
--- a/lib/irb/cmd/debug.rb
+++ b/lib/irb/cmd/debug.rb
@@ -35,6 +35,11 @@ def execute(pre_cmds: nil, do_cmds: nil)
             return
           end
 
+          if IRB.respond_to?(:JobManager)
+            warn "Can't start the debugger when IRB is running in a multi-IRB session."
+            return
+          end
+
           unless IRB::Debug.setup(irb_context.irb)
             puts <<~MSG
               You need to install the debug gem before using this command.
diff --git a/lib/irb/cmd/subirb.rb b/lib/irb/cmd/subirb.rb
index 5f3e02c98..777ebf150 100644
--- a/lib/irb/cmd/subirb.rb
+++ b/lib/irb/cmd/subirb.rb
@@ -11,8 +11,7 @@ module IRB
 
   module ExtendCommand
     class MultiIRBCommand < Nop
-      def initialize(conf)
-        super
+      def execute(*args)
         extend_irb_context
       end
 
@@ -29,6 +28,12 @@ class IrbCommand < MultiIRBCommand
       description "Start a child IRB."
 
       def execute(*obj)
+        if irb_context.with_debugger
+          warn "Multi-IRB commands are not available when the debugger is enabled."
+          return
+        end
+
+        super
         IRB.irb(nil, *obj)
       end
     end
@@ -38,6 +43,12 @@ class Jobs < MultiIRBCommand
       description "List of current sessions."
 
       def execute
+        if irb_context.with_debugger
+          warn "Multi-IRB commands are not available when the debugger is enabled."
+          return
+        end
+
+        super
         IRB.JobManager
       end
     end
@@ -47,6 +58,12 @@ class Foreground < MultiIRBCommand
       description "Switches to the session of the given number."
 
       def execute(key = nil)
+        if irb_context.with_debugger
+          warn "Multi-IRB commands are not available when the debugger is enabled."
+          return
+        end
+
+        super
         raise CommandArgumentError.new("Please specify the id of target IRB job (listed in the `jobs` command).") unless key
         IRB.JobManager.switch(key)
       end
@@ -57,6 +74,12 @@ class Kill < MultiIRBCommand
       description "Kills the session with the given number."
 
       def execute(*keys)
+        if irb_context.with_debugger
+          warn "Multi-IRB commands are not available when the debugger is enabled."
+          return
+        end
+
+        super
         IRB.JobManager.kill(*keys)
       end
     end
diff --git a/test/irb/test_debug_cmd.rb b/test/irb/test_debug_cmd.rb
index c8b92bec1..cbc1f864b 100644
--- a/test/irb/test_debug_cmd.rb
+++ b/test/irb/test_debug_cmd.rb
@@ -365,6 +365,38 @@ def bar
       assert_include(output, "show_source    Show the source code of a given method or constant.")
     end
 
+    def test_debugger_cant_be_activated_while_multi_irb_is_active
+      write_ruby <<~'ruby'
+        binding.irb
+        a = 1
+      ruby
+
+      output = run_ruby_file do
+        type "jobs"
+        type "next"
+        type "exit"
+      end
+
+      assert_match(/irb\(main\):001:0> jobs/, output)
+      assert_include(output, "Can't start the debugger when IRB is running in a multi-IRB session.")
+    end
+
+    def test_multi_irb_commands_are_not_available_after_activating_the_debugger
+      write_ruby <<~'ruby'
+        binding.irb
+        a = 1
+      ruby
+
+      output = run_ruby_file do
+        type "next"
+        type "jobs"
+        type "continue"
+      end
+
+      assert_match(/irb\(main\):001:0> next/, output)
+      assert_include(output, "Multi-IRB commands are not available when the debugger is enabled.")
+    end
+
     private
 
     TIMEOUT_SEC = 3