diff --git a/Library/Homebrew/extend/pathname.rb b/Library/Homebrew/extend/pathname.rb
index e329ca97282a9..175f834c879d7 100644
--- a/Library/Homebrew/extend/pathname.rb
+++ b/Library/Homebrew/extend/pathname.rb
@@ -3,6 +3,7 @@
 
 require "system_command"
 require "extend/pathname/disk_usage_extension"
+require "extend/pathname/eager_initialize_extension"
 require "extend/pathname/observer_pathname_extension"
 require "extend/pathname/write_mkpath_extension"
 require "utils/output"
@@ -29,6 +30,7 @@ class Pathname
   include SystemCommand::Mixin
   include DiskUsageExtension
   include Utils::Output::Mixin
+  prepend EagerInitializeExtension
 
   sig { void }
   def self.activate_extensions!
diff --git a/Library/Homebrew/extend/pathname/eager_initialize_extension.rb b/Library/Homebrew/extend/pathname/eager_initialize_extension.rb
new file mode 100644
index 0000000000000..14c689ebcf61e
--- /dev/null
+++ b/Library/Homebrew/extend/pathname/eager_initialize_extension.rb
@@ -0,0 +1,24 @@
+# typed: strict
+# frozen_string_literal: true
+
+# Eagerly initialises {Pathname}'s lazy memoised ivars so every instance
+# shares one object shape, avoiding Ruby's shape-variation warning.
+#
+# Any new `@x ||= ...` ivar added to {Pathname} or its mixed-in extensions
+# must also be added to `#initialize` below to keep the shape stable.
+module EagerInitializeExtension
+  extend T::Helpers
+
+  requires_ancestor { Pathname }
+
+  sig { params(args: T.untyped).void }
+  def initialize(*args)
+    @magic_number = T.let(nil, T.nilable(String))
+    @file_type = T.let(nil, T.nilable(String))
+    @zipinfo = T.let(nil, T.nilable(T::Array[String]))
+    @which_install_info = T.let(nil, T.nilable(String))
+    @disk_usage = T.let(nil, T.nilable(Integer))
+    @file_count = T.let(nil, T.nilable(Integer))
+    super
+  end
+end
diff --git a/Library/Homebrew/test/pathname_spec.rb b/Library/Homebrew/test/pathname_spec.rb
index 4f498505dd3de..71bbd9903314c 100644
--- a/Library/Homebrew/test/pathname_spec.rb
+++ b/Library/Homebrew/test/pathname_spec.rb
@@ -12,6 +12,16 @@
   let(:file) { src/"foo" }
   let(:dir) { src/"bar" }
 
+  describe EagerInitializeExtension do
+    it "defines the lazy memoised ivars on every new Pathname" do
+      pathname = Pathname.new(file.to_s)
+      [:@magic_number, :@file_type, :@zipinfo, :@which_install_info, :@disk_usage, :@file_count].each do |ivar|
+        expect(pathname.instance_variable_defined?(ivar)).to be(true), "expected #{ivar} to be defined"
+        expect(pathname.instance_variable_get(ivar)).to be_nil
+      end
+    end
+  end
+
   describe DiskUsageExtension do
     before do
       mkdir_p dir/"a-directory"