diff --git a/lib/hanami/lambda/env.rb b/lib/hanami/lambda/env.rb
new file mode 100644
index 0000000..6e18105
--- /dev/null
+++ b/lib/hanami/lambda/env.rb
@@ -0,0 +1,38 @@
+# frozen_string_literal: true
+
+require "rack"
+
+module Hanami
+  module Lambda
+    class Env
+      def initialize(event:, headers:, context:)
+        @event = event
+        @headers = standardize_headers(headers)
+        @context = context
+      end
+
+      def to_h
+        {
+          ::Rack::REQUEST_METHOD => @event["httpMethod"],
+          ::Rack::PATH_INFO => @event["path"] || "",
+          ::Rack::RACK_VERSION => ::Rack.release,
+          ::Rack::RACK_INPUT => StringIO.new(@event["body"] || ""),
+          ::Hanami::Lambda::LAMBDA_EVENT => @event,
+          ::Hanami::Lambda::LAMBDA_CONTEXT => @context
+        }.merge(@headers)
+      end
+
+      private
+
+      def standardize_headers(headers)
+        headers.transform_keys do |key|
+          if ::Rack::Headers::KNOWN_HEADERS[key.downcase.tr("_", "-")]
+            key.upcase.tr("-", "_")
+          else
+            "HTTP_#{key.upcase.tr('-', '_')}"
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/lib/hanami/lambda/rack.rb b/lib/hanami/lambda/rack.rb
index 9fe9ec7..7fc8f3b 100644
--- a/lib/hanami/lambda/rack.rb
+++ b/lib/hanami/lambda/rack.rb
@@ -41,19 +41,7 @@ def call(event:, context:)
       #
       # @since 0.1.0
       def build_env(event, headers, context)
-        {
-          ::Rack::REQUEST_METHOD => event["httpMethod"],
-          ::Rack::PATH_INFO => event["path"] || "",
-          ::Rack::RACK_VERSION => ::Rack.release,
-          ::Rack::RACK_INPUT => StringIO.new(event["body"] || ""),
-          ::Hanami::Lambda::LAMBDA_EVENT => event,
-          ::Hanami::Lambda::LAMBDA_CONTEXT => context
-        }.tap do |env|
-          content_type = headers.delete("Content-Type") ||
-                         headers.delete("content-type") ||
-                         headers.delete("CONTENT_TYPE")
-          env["CONTENT_TYPE"] = content_type if content_type
-        end.merge(headers.transform_keys { |k| "HTTP_#{k.upcase.tr('-', '_')}" })
+        Env.new(event: event, headers: headers, context: context).to_h
       end
     end
   end
diff --git a/sig/hanami/lambda/env.rbs b/sig/hanami/lambda/env.rbs
new file mode 100644
index 0000000..ca0adfb
--- /dev/null
+++ b/sig/hanami/lambda/env.rbs
@@ -0,0 +1,23 @@
+module Hanami
+  module Lambda
+    class Env
+      @event: Hash[String, untyped]
+
+      @headers: Hash[String, String]
+
+      @context: Object
+
+      def initialize: (event: Hash[String, untyped], headers: Hash[String, String], context: Object) -> void
+
+      # Build a hash of the Rack environment
+      #
+      # @return [Hash] the hash of Rack environment
+      #
+      def to_h: () -> ::Hash[String, untyped]
+
+      private
+
+      def standardize_headers: (Hash[String, String]) -> Hash[String, String]
+    end
+  end
+end
diff --git a/sig/shims/rack.rbs b/sig/shims/rack.rbs
new file mode 100644
index 0000000..d9b6b3d
--- /dev/null
+++ b/sig/shims/rack.rbs
@@ -0,0 +1,5 @@
+module Rack
+  class Headers
+    KNOWN_HEADERS: Hash[String, String]
+  end
+end
diff --git a/spec/hanami/lambda/env_spec.rb b/spec/hanami/lambda/env_spec.rb
new file mode 100644
index 0000000..2348889
--- /dev/null
+++ b/spec/hanami/lambda/env_spec.rb
@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+RSpec.describe Hanami::Lambda::Env do
+  subject(:env) { described_class.new(event: event, headers: headers, context: context) }
+
+  let(:event) do
+    {
+      "httpMethod" => "GET",
+      "path" => "/",
+      "body" => ""
+    }
+  end
+
+  let(:headers) do
+    {
+      "Content-Type" => "text/plain",
+      "X-Custom-Header" => "Custom Value"
+    }
+  end
+
+  let(:context) { double(:context) }
+
+  describe "#to_h" do
+    subject(:hash) { env.to_h }
+
+    it { is_expected.to include(::Hanami::Lambda::LAMBDA_EVENT) }
+    it { is_expected.to include(::Hanami::Lambda::LAMBDA_CONTEXT) }
+    it { is_expected.to include("CONTENT_TYPE" => "text/plain") }
+    it { is_expected.to include("HTTP_X_CUSTOM_HEADER" => "Custom Value") }
+  end
+end
diff --git a/spec/hanami/lambda/rack_spec.rb b/spec/hanami/lambda/rack_spec.rb
index dec496a..05bffaf 100644
--- a/spec/hanami/lambda/rack_spec.rb
+++ b/spec/hanami/lambda/rack_spec.rb
@@ -33,13 +33,4 @@
     it { is_expected.to include(body: "Hello from Rack") }
     it { is_expected.to include(headers: {}) }
   end
-
-  describe "#env" do
-    subject(:env) { rack.build_env(event, headers, context) }
-
-    it { is_expected.to include(::Hanami::Lambda::LAMBDA_EVENT) }
-    it { is_expected.to include(::Hanami::Lambda::LAMBDA_CONTEXT) }
-    it { is_expected.to include("CONTENT_TYPE" => "text/plain") }
-    it { is_expected.to include("HTTP_X_CUSTOM_HEADER" => "Custom Value") }
-  end
 end