diff --git a/Rakefile b/Rakefile
index 56237d2..57c50c3 100644
--- a/Rakefile
+++ b/Rakefile
@@ -5,7 +5,7 @@ require "rake/testtask"
 
 Rake::TestTask.new(:test) do |t|
   t.libs << "test"
-  t.test_files = FileList["test/test_*.rb"]
+  t.test_files = FileList["test/spec_*.rb"]
 end
 
 task default: :test
diff --git a/lib/nack.rb b/lib/nack.rb
index 598cf7b..ecb346f 100644
--- a/lib/nack.rb
+++ b/lib/nack.rb
@@ -1,3 +1,4 @@
 # frozen_string_literal: true
 
 require_relative "nack/version"
+require_relative "nack/utils"
diff --git a/lib/nack/utils.rb b/lib/nack/utils.rb
new file mode 100644
index 0000000..695ba3f
--- /dev/null
+++ b/lib/nack/utils.rb
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+module Nack
+  module Utils
+    module_function
+
+    def no_entity_body?(status)
+      (100..199).cover?(status) || status == 204 || status == 304
+    end
+
+    def get_header(pairs, name)
+      pairs.assoc(name)&.last
+    end
+
+    def set_header(pairs, name, value)
+      pairs.reject! { |key, _| key == name }
+      pairs << [name, value]
+      pairs
+    end
+
+    def delete_header(pairs, name)
+      pairs.reject! { |key, _| key == name }
+      pairs
+    end
+
+    def body_size(body)
+      case body
+      when nil then 0
+      when IO::Buffer then body.size
+      else body.to_s.bytesize
+      end
+    end
+  end
+end
diff --git a/nack.gemspec b/nack.gemspec
index 3bd56d3..64e6b4a 100644
--- a/nack.gemspec
+++ b/nack.gemspec
@@ -28,4 +28,5 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency "bundler"
   spec.add_development_dependency "rake"
   spec.add_development_dependency "minitest"
+  spec.add_development_dependency "minitest-global_expectations"
 end
diff --git a/test/helper.rb b/test/helper.rb
index 2e49dfb..ae7390c 100644
--- a/test/helper.rb
+++ b/test/helper.rb
@@ -3,4 +3,4 @@
 $LOAD_PATH.unshift(File.expand_path("../lib", __dir__))
 
 require "nack"
-require "minitest/autorun"
+require "minitest/global_expectations/autorun"
diff --git a/test/spec_utils.rb b/test/spec_utils.rb
new file mode 100644
index 0000000..1e6a4b9
--- /dev/null
+++ b/test/spec_utils.rb
@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+require_relative "helper"
+
+describe Nack::Utils do
+  it "knows which statuses forbid an entity body" do
+    Nack::Utils.no_entity_body?(100).must_equal true
+    Nack::Utils.no_entity_body?(199).must_equal true
+    Nack::Utils.no_entity_body?(204).must_equal true
+    Nack::Utils.no_entity_body?(304).must_equal true
+    Nack::Utils.no_entity_body?(200).must_equal false
+    Nack::Utils.no_entity_body?(404).must_equal false
+  end
+
+  it "returns the first value for a header name" do
+    pairs = [["content-type", "text/html"], ["x-a", "1"], ["x-a", "2"]]
+    Nack::Utils.get_header(pairs, "content-type").must_equal "text/html"
+    Nack::Utils.get_header(pairs, "x-a").must_equal "1"
+    Nack::Utils.get_header(pairs, "missing").must_be_nil
+  end
+
+  it "replaces all occurrences when setting a header" do
+    pairs = [["b", "2"], ["a", "1"], ["b", "3"]]
+    Nack::Utils.set_header(pairs, "b", "9")
+    pairs.must_equal [["a", "1"], ["b", "9"]]
+  end
+
+  it "appends when setting a header not yet present" do
+    pairs = [["a", "1"]]
+    Nack::Utils.set_header(pairs, "b", "2")
+    pairs.must_equal [["a", "1"], ["b", "2"]]
+  end
+
+  it "removes all occurrences when deleting a header" do
+    pairs = [["a", "1"], ["a", "2"], ["b", "3"]]
+    Nack::Utils.delete_header(pairs, "a")
+    pairs.must_equal [["b", "3"]]
+  end
+
+  it "measures body size in bytes" do
+    Nack::Utils.body_size(nil).must_equal 0
+    Nack::Utils.body_size("hello").must_equal 5
+    Nack::Utils.body_size("héllo").must_equal 6
+    Nack::Utils.body_size(IO::Buffer.for("abc")).must_equal 3
+  end
+end