diff --git a/.gitignore b/.gitignore
index 54fa3ab9d..c136b0d03 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,7 @@
 *.xcworkspace/
 .DS_Store
 .android-test-*
+.bundle/*
 .ccache/*
 !.ccache/ccache.conf
 .cxx/
diff --git a/packages/app/.rubocop.yml b/.rubocop.yml
similarity index 93%
rename from packages/app/.rubocop.yml
rename to .rubocop.yml
index 9c2403a3c..f35cabf2d 100644
--- a/packages/app/.rubocop.yml
+++ b/.rubocop.yml
@@ -62,8 +62,8 @@ Style/HashTransformValues:
 
 Style/OneClassPerFile:
   Exclude:
-    - ios/test_app.rb
-    - test/test_test_app.rb
+    - packages/app/ios/test_app.rb
+    - packages/app/test/test_test_app.rb
 
 Style/TrailingCommaInArrayLiteral:
   EnforcedStyleForMultiline: comma
diff --git a/packages/app/package.json b/packages/app/package.json
index 9ee534f9b..1cfc18f4f 100644
--- a/packages/app/package.json
+++ b/packages/app/package.json
@@ -83,7 +83,7 @@
     "test": "node scripts/internal/test.mts",
     "test:js": "node --no-warnings --test $(git ls-files '*.test.mts')",
     "test:matrix": "node scripts/testing/test-matrix.mts",
-    "test:rb": "bundle exec ruby -Ilib:test -e \"Dir.glob('./test/test_*.rb').each { |file| require(file) }\""
+    "test:rb": "bundle exec ruby -Ilib:test -rgithub_annotations -e \"Dir.glob('./test/test_*.rb').each { |file| require(file) }\""
   },
   "dependencies": {
     "@rnx-kit/react-native-host": "^0.5.21",
diff --git a/packages/app/test/github_annotations.rb b/packages/app/test/github_annotations.rb
new file mode 100644
index 000000000..5f722076a
--- /dev/null
+++ b/packages/app/test/github_annotations.rb
@@ -0,0 +1,55 @@
+require('minitest')
+
+# Reports test failures as GitHub Actions annotations. Unlike problem matchers,
+# workflow commands support multiline messages (newlines are encoded as `%0A`).
+#
+# See https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-commands
+module GitHubAnnotations
+  class Reporter < Minitest::AbstractReporter
+    def initialize(io)
+      super()
+      @io = io
+      @annotations = []
+    end
+
+    def record(result)
+      return if result.passed? || result.skipped?
+
+      failure = result.failure
+      file, line = parse_location(failure.location)
+      title = "#{result.class_name}##{result.name}"
+      @annotations << "::error file=#{escape_property(file)},line=#{line}," \
+                      "title=#{escape_property(title)}::#{escape_data(failure.message.rstrip)}"
+    end
+
+    # Workflow commands must start on a new line, so we cannot print them
+    # while progress is being reported.
+    def report
+      @io.puts(@annotations) unless @annotations.empty?
+    end
+
+    private
+
+    def escape_data(value)
+      value.to_s.gsub('%', '%25').gsub("\r", '%0D').gsub("\n", '%0A')
+    end
+
+    def escape_property(value)
+      escape_data(value).gsub(':', '%3A').gsub(',', '%2C')
+    end
+
+    # The runner makes absolute paths under `GITHUB_WORKSPACE` relative to it.
+    def parse_location(location)
+      file, line = location.match(/\A(.*):(\d+)\z/)&.captures || [location, 1]
+      [File.expand_path(file), line]
+    end
+  end
+
+  def self.minitest_plugin_init(options)
+    return unless ENV['GITHUB_ACTIONS'] == 'true'
+
+    Minitest.reporter << Reporter.new(options[:io])
+  end
+end
+
+Minitest.register_plugin(GitHubAnnotations)