diff --git a/Rakefile b/Rakefile
index 820656c32b87..82b22ccf3a82 100644
--- a/Rakefile
+++ b/Rakefile
@@ -9,7 +9,7 @@ require "rake/testtask"
 require_relative "spec/support/rubygems_ext"
 
 desc "Setup Rubygems dev environment"
-task setup: [:"dev:deps", "vendor:compact_index"] do
+task setup: [:"dev:deps"] do
   Dir.glob("tool/bundler/*_gems.rb").each do |file|
     name = File.basename(file, ".rb")
     next if name == "dev_gems"
@@ -17,36 +17,6 @@ task setup: [:"dev:deps", "vendor:compact_index"] do
   end
 end
 
-namespace :vendor do
-  # Pinned to rubygems/rubygems.org#6504 so `rake setup` stays reproducible.
-  # Override with REF=<sha> to test against a newer compact_index.
-  desc "Sync vendored compact_index from rubygems/rubygems.org"
-  task :compact_index do
-    require "open-uri"
-    require "fileutils"
-
-    ref = ENV["REF"] || "7c68a7b39761c61a66f9299f85b889ec39afc02c"
-    repo = "rubygems/rubygems.org"
-    paths = %w[
-      lib/compact_index.rb
-      lib/compact_index/dependency.rb
-      lib/compact_index/gem.rb
-      lib/compact_index/gem_version.rb
-      lib/compact_index/versions_file.rb
-    ]
-
-    target_root = Spec::Path.tmp_root.join("compact_index")
-    paths.each do |path|
-      url = "https://raw.githubusercontent.com/#{repo}/#{ref}/#{path}"
-      puts "Fetching #{url}"
-      content = URI.parse(url).open(&:read)
-      target = File.join(target_root, path)
-      FileUtils.mkdir_p(File.dirname(target))
-      File.write(target, content)
-    end
-  end
-end
-
 desc "Update Rubygems dev environment"
 task :update do
   Spec::Rubygems.dev_bundle "update", "--all"
diff --git a/spec/support/rubygems_ext.rb b/spec/support/rubygems_ext.rb
index cf639a660a04..9ff0138f2ccd 100644
--- a/spec/support/rubygems_ext.rb
+++ b/spec/support/rubygems_ext.rb
@@ -73,6 +73,38 @@ def install_test_deps
 
       require_relative "helpers"
       Helpers.install_dev_bundler
+
+      install_vendored_compact_index
+    end
+
+    # Vendor `rubygems/rubygems.org#lib/compact_index/` under `tmp/compact_index/`
+    # so the artifice can serve compact-index responses without a runtime gem
+    # dependency. Pinned to a reviewed commit; override with COMPACT_INDEX_REF
+    # to refresh against another ref (the existing vendor copy is discarded).
+    def install_vendored_compact_index
+      target_root = Path.tmp_root.join("compact_index")
+      require "fileutils"
+
+      if ENV["COMPACT_INDEX_REF"]
+        FileUtils.rm_rf(target_root)
+      elsif File.exist?(target_root.join("lib/compact_index.rb"))
+        return
+      end
+
+      require "open-uri"
+      ref = ENV["COMPACT_INDEX_REF"] || "7c68a7b39761c61a66f9299f85b889ec39afc02c"
+      %w[
+        lib/compact_index.rb
+        lib/compact_index/dependency.rb
+        lib/compact_index/gem.rb
+        lib/compact_index/gem_version.rb
+        lib/compact_index/versions_file.rb
+      ].each do |path|
+        url = "https://raw.githubusercontent.com/rubygems/rubygems.org/#{ref}/#{path}"
+        target = target_root.join(path)
+        FileUtils.mkdir_p(File.dirname(target))
+        File.write(target, URI.parse(url).open(&:read))
+      end
     end
 
     def check_source_control_changes(success_message:, error_message:)