diff --git a/lib/singed/rack_middleware.rb b/lib/singed/rack_middleware.rb
index bce1e2f..f267974 100644
--- a/lib/singed/rack_middleware.rb
+++ b/lib/singed/rack_middleware.rb
@@ -9,15 +9,11 @@ def initialize(app)
     end
 
     def call(env)
-      status, headers, body = if capture_flamegraph?(env)
-        flamegraph do
-          @app.call(env)
-        end
+      if capture_flamegraph?(env)
+        flamegraph { @app.call(env) }
       else
         @app.call(env)
       end
-
-      [status, headers, body]
     end
 
     def capture_flamegraph?(env)
diff --git a/spec/singed/middleware_spec.rb b/spec/singed/middleware_spec.rb
new file mode 100644
index 0000000..0e54183
--- /dev/null
+++ b/spec/singed/middleware_spec.rb
@@ -0,0 +1,84 @@
+# frozen_string_literal: true
+
+describe Singed::RackMiddleware do
+  subject do
+    instance.call(env)
+  end
+
+  let(:app_response) { [200, {"content-type" => "text/plain"}, ["OK"]] }
+  let(:app) { ->(*) { app_response } }
+  let(:instance) { described_class.new(app) }
+  let(:env) { Rack::MockRequest.env_for("/", headers) }
+  let(:headers) { {} }
+
+  it "returns a proper rack response" do
+    linted_app = Rack::Lint.new(instance)
+    expect { linted_app.call(env) }.not_to raise_error
+  end
+
+  it "passes through the app response unchanged" do
+    expect(subject).to eq(app_response)
+  end
+
+  context "when enabled" do
+    before do
+      allow_any_instance_of(Singed::Flamegraph).to receive(:open)
+      allow(instance).to receive(:capture_flamegraph?).and_return(true)
+    end
+
+    it "captures a flamegraph" do
+      expect(instance).to receive(:flamegraph).and_call_original
+      subject
+    end
+
+    it "returns a proper rack response" do
+      linted_app = Rack::Lint.new(instance)
+      expect { linted_app.call(env) }.not_to raise_error
+    end
+
+    it "passes through the app response unchanged" do
+      expect(subject).to eq(app_response)
+    end
+  end
+
+  describe "#capture_flamegraph?" do
+    subject { instance.capture_flamegraph?(env) }
+
+    it { is_expected.to be false }
+
+    context "when HTTP_X_SINGED is true" do
+      let(:headers) { {"HTTP_X_SINGED" => "true"} }
+
+      it { is_expected.to be true }
+    end
+
+    context "when SINGED_MIDDLEWARE_ALWAYS_CAPTURE env var is set" do
+      around do |example|
+        original = ENV["SINGED_MIDDLEWARE_ALWAYS_CAPTURE"]
+        described_class.remove_instance_variable(:@always_capture) if described_class.instance_variable_defined?(:@always_capture)
+        example.run
+      ensure
+        if original.nil?
+          ENV.delete("SINGED_MIDDLEWARE_ALWAYS_CAPTURE")
+        else
+          ENV["SINGED_MIDDLEWARE_ALWAYS_CAPTURE"] = original
+        end
+        described_class.remove_instance_variable(:@always_capture) if described_class.instance_variable_defined?(:@always_capture)
+      end
+
+      %w[true 1 yes].each do |truthy_value|
+        context "when SINGED_MIDDLEWARE_ALWAYS_CAPTURE=#{truthy_value}" do
+          before { ENV["SINGED_MIDDLEWARE_ALWAYS_CAPTURE"] = truthy_value }
+
+          it { is_expected.to be true }
+        end
+      end
+
+      context "when SINGED_MIDDLEWARE_ALWAYS_CAPTURE=false" do
+        before { ENV["SINGED_MIDDLEWARE_ALWAYS_CAPTURE"] = "false" }
+
+        it { is_expected.to be false }
+      end
+    end
+  end
+end