diff --git a/Library/Homebrew/debrew/irb.rb b/Library/Homebrew/debrew/irb.rb
index a18f892225..6f2ed6c8ed 100644
--- a/Library/Homebrew/debrew/irb.rb
+++ b/Library/Homebrew/debrew/irb.rb
@@ -1,13 +1,15 @@
-# typed: true # rubocop:todo Sorbet/StrictSigil
+# typed: strict
 # frozen_string_literal: true
 
 require "irb"
 
 module IRB
+  sig { params(binding: Binding).void }
   def self.start_within(binding)
     old_stdout_sync = $stdout.sync
     $stdout.sync = true
 
+    @setup_done ||= T.let(false, T.nilable(T::Boolean))
     unless @setup_done
       setup(nil, argv: [])
       @setup_done = true