diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 25fff31..cab09e3 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -8,12 +8,17 @@ on:
 jobs:
   build:
     runs-on: ubuntu-latest
+    env:
+      FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true
     name: Ruby ${{ matrix.ruby }}
     strategy:
+      fail-fast: false
       matrix:
         ruby:
           - "3.2"
           - "3.3"
+          - "3.4"
+          - "4.0"
           - "ruby-head"
 
     steps:
diff --git a/.gitignore b/.gitignore
index 867fc18..b82e7a1 100644
--- a/.gitignore
+++ b/.gitignore
@@ -16,3 +16,10 @@ Gemfile.lock
 
 # MacOS
 .DS_Store
+
+# Dummy app artifacts
+spec/dummy/log/
+spec/dummy/tmp/
+
+# Claude
+.claude/settings.local.json
diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
index ef4145c..e2516b0 100644
--- a/.rubocop_todo.yml
+++ b/.rubocop_todo.yml
@@ -1,12 +1,17 @@
 # This configuration was generated by
 # `rubocop --auto-gen-config`
-# on 2025-11-17 23:28:22 UTC using RuboCop version 1.81.7.
+# on 2026-01-25 10:40:25 UTC using RuboCop version 1.81.7.
 # The point is for the user to remove these configuration records
 # one by one as the offenses are removed from the code base.
 # Note that changes in the inspected code, or installation of new
 # versions of RuboCop, may require this file to be generated again.
 
-# Offense count: 2
+# Offense count: 1
+# Configuration parameters: CountComments, CountAsOne, AllowedMethods, AllowedPatterns.
+Metrics/MethodLength:
+  Max: 12
+
+# Offense count: 6
 # Configuration parameters: CountAsOne.
 RSpec/ExampleLength:
   Max: 14
diff --git a/AGENTS.md b/AGENTS.md
new file mode 100644
index 0000000..ea351c4
--- /dev/null
+++ b/AGENTS.md
@@ -0,0 +1,5 @@
+# Agents
+
+## Style
+
+- Always use Ruby keyword argument shorthand (e.g., `super(**kwargs, json:)` not `super(**kwargs, json: json)`).
diff --git a/Gemfile b/Gemfile
index 5fbbfc1..ab93cd5 100644
--- a/Gemfile
+++ b/Gemfile
@@ -7,7 +7,10 @@ gemspec
 
 gem "rake", "~> 13.0"
 
+gem "rails", ">= 7.2"
+
 gem "rspec", "~> 3.0"
+gem "rspec-rails", "~> 7.0"
 
 gem "rubocop", "~> 1.21"
 gem "rubocop-rspec", require: false
@@ -18,5 +21,7 @@ gem "undercover"
 
 gem "pry"
 
+gem "sqlite3"
+
 gem "gem-release", require: false
 gem "solargraph", require: false
diff --git a/lib/transmutation/serialization/rendering.rb b/lib/transmutation/serialization/rendering.rb
index 1cfaeab..2f99712 100644
--- a/lib/transmutation/serialization/rendering.rb
+++ b/lib/transmutation/serialization/rendering.rb
@@ -4,11 +4,26 @@ module Transmutation
   module Serialization
     module Rendering
       # Serializes the value of the `json` parameter before calling the existing render method.
-      def render(json: nil, serialize: true, namespace: nil, serializer: nil, max_depth: 1, **args)
-        return super(**args) unless json
-        return super(**args, json:) unless serialize
+      #
+      # Handles both Rails-style hash arguments (e.g., from send_data) and keyword arguments.
+      def render(options = nil, **kwargs)
+        # Handle Rails-style positional hash argument (e.g., from send_data calling render(body: data))
+        if options.is_a?(Hash)
+          return super(options) unless options.key?(:json)
 
-        super(**args, json: serialize(json, namespace:, serializer:, max_depth:))
+          kwargs = options.merge(kwargs)
+        end
+
+        json = kwargs.delete(:json)
+        should_serialize = kwargs.key?(:serialize) ? kwargs.delete(:serialize) : true
+        namespace = kwargs.delete(:namespace)
+        serializer = kwargs.delete(:serializer)
+        max_depth = kwargs.delete(:max_depth) || Transmutation.max_depth
+
+        return super(**kwargs) unless json
+        return super(**kwargs, json:) unless should_serialize
+
+        super(**kwargs, json: serialize(json, namespace:, serializer:, max_depth:))
       end
     end
   end
diff --git a/spec/system/dummy/controllers/api/application_controller.rb b/spec/dummy/app/controllers/api/application_controller.rb
similarity index 62%
rename from spec/system/dummy/controllers/api/application_controller.rb
rename to spec/dummy/app/controllers/api/application_controller.rb
index f633cff..0c2735a 100644
--- a/spec/system/dummy/controllers/api/application_controller.rb
+++ b/spec/dummy/app/controllers/api/application_controller.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module Api
-  class ApplicationController < BaseController
+  class ApplicationController < ::ApplicationController
     include Transmutation::Serialization
   end
 end
diff --git a/spec/system/dummy/controllers/api/v1/health_controller.rb b/spec/dummy/app/controllers/api/v1/health_controller.rb
similarity index 60%
rename from spec/system/dummy/controllers/api/v1/health_controller.rb
rename to spec/dummy/app/controllers/api/v1/health_controller.rb
index 8767c66..f792c9c 100644
--- a/spec/system/dummy/controllers/api/v1/health_controller.rb
+++ b/spec/dummy/app/controllers/api/v1/health_controller.rb
@@ -4,11 +4,11 @@ module Api
   module V1
     class HealthController < Api::ApplicationController
       def index
-        render(json: { ok: true })
+        render json: { ok: true }
       end
 
       def download
-        send_data("binary data content", filename: "report.txt", type: "text/plain")
+        send_data "binary data content", filename: "report.txt", type: "text/plain"
       end
     end
   end
diff --git a/spec/system/dummy/controllers/api/v1/posts_controller.rb b/spec/dummy/app/controllers/api/v1/posts_controller.rb
similarity index 83%
rename from spec/system/dummy/controllers/api/v1/posts_controller.rb
rename to spec/dummy/app/controllers/api/v1/posts_controller.rb
index e7c6bd8..e658831 100644
--- a/spec/system/dummy/controllers/api/v1/posts_controller.rb
+++ b/spec/dummy/app/controllers/api/v1/posts_controller.rb
@@ -5,13 +5,11 @@ module V1
     class PostsController < Api::ApplicationController
       def index
         posts = Post.all
-
         render json: posts
       end
 
-      def show(id)
-        post = Post.find(id)
-
+      def show
+        post = Post.find(params[:id])
         render json: post, namespace: "Detailed"
       end
     end
diff --git a/spec/system/dummy/controllers/api/v1/products_controller.rb b/spec/dummy/app/controllers/api/v1/products_controller.rb
similarity index 61%
rename from spec/system/dummy/controllers/api/v1/products_controller.rb
rename to spec/dummy/app/controllers/api/v1/products_controller.rb
index 4887e5d..0ded5d1 100644
--- a/spec/system/dummy/controllers/api/v1/products_controller.rb
+++ b/spec/dummy/app/controllers/api/v1/products_controller.rb
@@ -3,10 +3,9 @@
 module Api
   module V1
     class ProductsController < Api::ApplicationController
-      def show(id)
-        post = Product.find(id)
-
-        render json: post
+      def show
+        product = Product.find(params[:id])
+        render json: product
       end
     end
   end
diff --git a/spec/system/dummy/controllers/api/v1/users_controller.rb b/spec/dummy/app/controllers/api/v1/users_controller.rb
similarity index 84%
rename from spec/system/dummy/controllers/api/v1/users_controller.rb
rename to spec/dummy/app/controllers/api/v1/users_controller.rb
index bf7a47e..eff3a96 100644
--- a/spec/system/dummy/controllers/api/v1/users_controller.rb
+++ b/spec/dummy/app/controllers/api/v1/users_controller.rb
@@ -5,13 +5,11 @@ module V1
     class UsersController < Api::ApplicationController
       def index
         users = User.all
-
         render json: users
       end
 
-      def show(id)
-        user = User.find(id)
-
+      def show
+        user = User.find(params[:id])
         render json: user, serializer: "Detailed::UserSerializer"
       end
     end
diff --git a/spec/dummy/app/controllers/application_controller.rb b/spec/dummy/app/controllers/application_controller.rb
new file mode 100644
index 0000000..13c271f
--- /dev/null
+++ b/spec/dummy/app/controllers/application_controller.rb
@@ -0,0 +1,4 @@
+# frozen_string_literal: true
+
+class ApplicationController < ActionController::API
+end
diff --git a/spec/dummy/app/models/comment.rb b/spec/dummy/app/models/comment.rb
new file mode 100644
index 0000000..00b9bd6
--- /dev/null
+++ b/spec/dummy/app/models/comment.rb
@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+class Comment < ActiveRecord::Base
+  belongs_to :user, optional: true
+end
diff --git a/spec/system/dummy/lib/money.rb b/spec/dummy/app/models/money.rb
similarity index 100%
rename from spec/system/dummy/lib/money.rb
rename to spec/dummy/app/models/money.rb
diff --git a/spec/dummy/app/models/post.rb b/spec/dummy/app/models/post.rb
new file mode 100644
index 0000000..f9cb961
--- /dev/null
+++ b/spec/dummy/app/models/post.rb
@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+class Post < ActiveRecord::Base
+  belongs_to :user, optional: true
+end
diff --git a/spec/dummy/app/models/product.rb b/spec/dummy/app/models/product.rb
new file mode 100644
index 0000000..a38fd3e
--- /dev/null
+++ b/spec/dummy/app/models/product.rb
@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class Product < ActiveRecord::Base
+  def price
+    Money.new(price_subunit, price_currency)
+  end
+end
diff --git a/spec/dummy/app/models/user.rb b/spec/dummy/app/models/user.rb
new file mode 100644
index 0000000..1907662
--- /dev/null
+++ b/spec/dummy/app/models/user.rb
@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class User < ActiveRecord::Base
+  has_many :posts
+  has_many :comments
+end
diff --git a/spec/system/dummy/serializers/api/v1/comment_serializer.rb b/spec/dummy/app/serializers/api/v1/comment_serializer.rb
similarity index 100%
rename from spec/system/dummy/serializers/api/v1/comment_serializer.rb
rename to spec/dummy/app/serializers/api/v1/comment_serializer.rb
diff --git a/spec/system/dummy/serializers/api/v1/detailed/post_serializer.rb b/spec/dummy/app/serializers/api/v1/detailed/post_serializer.rb
similarity index 100%
rename from spec/system/dummy/serializers/api/v1/detailed/post_serializer.rb
rename to spec/dummy/app/serializers/api/v1/detailed/post_serializer.rb
diff --git a/spec/system/dummy/serializers/api/v1/detailed/user_serializer.rb b/spec/dummy/app/serializers/api/v1/detailed/user_serializer.rb
similarity index 100%
rename from spec/system/dummy/serializers/api/v1/detailed/user_serializer.rb
rename to spec/dummy/app/serializers/api/v1/detailed/user_serializer.rb
diff --git a/spec/system/dummy/serializers/api/v1/post_serializer.rb b/spec/dummy/app/serializers/api/v1/post_serializer.rb
similarity index 100%
rename from spec/system/dummy/serializers/api/v1/post_serializer.rb
rename to spec/dummy/app/serializers/api/v1/post_serializer.rb
diff --git a/spec/system/dummy/serializers/api/v1/user_serializer.rb b/spec/dummy/app/serializers/api/v1/user_serializer.rb
similarity index 100%
rename from spec/system/dummy/serializers/api/v1/user_serializer.rb
rename to spec/dummy/app/serializers/api/v1/user_serializer.rb
diff --git a/spec/system/dummy/serializers/money_serializer.rb b/spec/dummy/app/serializers/money_serializer.rb
similarity index 100%
rename from spec/system/dummy/serializers/money_serializer.rb
rename to spec/dummy/app/serializers/money_serializer.rb
diff --git a/spec/system/dummy/serializers/product_serializer.rb b/spec/dummy/app/serializers/product_serializer.rb
similarity index 100%
rename from spec/system/dummy/serializers/product_serializer.rb
rename to spec/dummy/app/serializers/product_serializer.rb
diff --git a/spec/dummy/config/application.rb b/spec/dummy/config/application.rb
new file mode 100644
index 0000000..56131c2
--- /dev/null
+++ b/spec/dummy/config/application.rb
@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+require_relative "boot"
+
+require "rails"
+require "active_record/railtie"
+require "action_controller/railtie"
+
+Bundler.require(*Rails.groups)
+
+require "transmutation"
+
+module Dummy
+  class Application < Rails::Application
+    config.root = File.expand_path("..", __dir__)
+
+    config.load_defaults Rails::VERSION::STRING.to_f
+
+    config.autoload_paths << config.root.join("app/serializers")
+
+    config.api_only = true
+    config.eager_load = false
+  end
+end
diff --git a/spec/dummy/config/boot.rb b/spec/dummy/config/boot.rb
new file mode 100644
index 0000000..45535e7
--- /dev/null
+++ b/spec/dummy/config/boot.rb
@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+ENV["BUNDLE_GEMFILE"] ||= File.expand_path("../../../Gemfile", __dir__)
+
+require "bundler/setup" if File.exist?(ENV["BUNDLE_GEMFILE"])
+$LOAD_PATH.unshift File.expand_path("../../../lib", __dir__)
diff --git a/spec/dummy/config/database.yml b/spec/dummy/config/database.yml
new file mode 100644
index 0000000..2cfeb7a
--- /dev/null
+++ b/spec/dummy/config/database.yml
@@ -0,0 +1,3 @@
+test:
+  adapter: sqlite3
+  database: ":memory:"
diff --git a/spec/dummy/config/environment.rb b/spec/dummy/config/environment.rb
new file mode 100644
index 0000000..e8173e0
--- /dev/null
+++ b/spec/dummy/config/environment.rb
@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+require_relative "application"
+
+Rails.application.initialize!
diff --git a/spec/dummy/config/environments/test.rb b/spec/dummy/config/environments/test.rb
new file mode 100644
index 0000000..56d781c
--- /dev/null
+++ b/spec/dummy/config/environments/test.rb
@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+Rails.application.configure do
+  config.eager_load = false
+  config.consider_all_requests_local = true
+  config.action_controller.perform_caching = false
+  config.action_controller.allow_forgery_protection = false
+  config.active_support.deprecation = :stderr
+end
diff --git a/spec/dummy/config/routes.rb b/spec/dummy/config/routes.rb
new file mode 100644
index 0000000..adbfe03
--- /dev/null
+++ b/spec/dummy/config/routes.rb
@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+Rails.application.routes.draw do
+  namespace :api do
+    namespace :v1 do
+      resources :users, only: %i[index show]
+      resources :posts, only: %i[index show]
+      resources :products, only: [:show]
+      resources :health, only: [:index] do
+        collection do
+          get :download
+        end
+      end
+    end
+  end
+end
diff --git a/spec/dummy/db/schema.rb b/spec/dummy/db/schema.rb
new file mode 100644
index 0000000..d4a49e4
--- /dev/null
+++ b/spec/dummy/db/schema.rb
@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+ActiveRecord::Schema.define(version: 0) do
+  create_table :users, force: true do |t|
+    t.string :first_name, null: false
+    t.string :last_name, null: false
+  end
+
+  create_table :posts, force: true do |t|
+    t.string :title, null: false
+    t.text :body, null: false
+    t.references :user, foreign_key: true
+    t.datetime :published_at
+  end
+
+  create_table :comments, force: true do |t|
+    t.text :body, null: false
+    t.references :user, foreign_key: true
+  end
+
+  create_table :products, force: true do |t|
+    t.string :name, null: false
+    t.string :description
+    t.integer :price_subunit, null: false
+    t.string :price_currency, null: false
+  end
+end
diff --git a/spec/dummy/test/fixtures/comments.yml b/spec/dummy/test/fixtures/comments.yml
new file mode 100644
index 0000000..54ddaa9
--- /dev/null
+++ b/spec/dummy/test/fixtures/comments.yml
@@ -0,0 +1,14 @@
+first_comment:
+  id: 1
+  body: "First!"
+  user_id: 1
+
+second_comment:
+  id: 2
+  body: "Second!"
+  user_id: 2
+
+third_comment:
+  id: 3
+  body: "Third!"
+  user_id: 1
diff --git a/spec/dummy/test/fixtures/posts.yml b/spec/dummy/test/fixtures/posts.yml
new file mode 100644
index 0000000..44c51a2
--- /dev/null
+++ b/spec/dummy/test/fixtures/posts.yml
@@ -0,0 +1,20 @@
+first_post:
+  id: 1
+  title: First post
+  body: "First!"
+  user_id: 1
+  published_at: 2025-01-01 00:00:00
+
+second_post:
+  id: 2
+  title: How does this work?
+  body: body
+  user_id: 2
+  published_at: 2025-01-01 00:00:00
+
+third_post:
+  id: 3
+  title: "Second post!?"
+  body: "Nope..."
+  user_id: 1
+  published_at: null
diff --git a/spec/dummy/test/fixtures/products.yml b/spec/dummy/test/fixtures/products.yml
new file mode 100644
index 0000000..f1c2353
--- /dev/null
+++ b/spec/dummy/test/fixtures/products.yml
@@ -0,0 +1,6 @@
+shoes:
+  id: 1
+  name: Shoes
+  description: A pair of shoes
+  price_subunit: 1000
+  price_currency: GBP
diff --git a/spec/dummy/test/fixtures/users.yml b/spec/dummy/test/fixtures/users.yml
new file mode 100644
index 0000000..6f7747e
--- /dev/null
+++ b/spec/dummy/test/fixtures/users.yml
@@ -0,0 +1,19 @@
+john:
+  id: 1
+  first_name: John
+  last_name: Doe
+
+jane:
+  id: 2
+  first_name: Jane
+  last_name: Doe
+
+adam:
+  id: 3
+  first_name: Adam
+  last_name: Smith
+
+eve:
+  id: 4
+  first_name: Eve
+  last_name: Smith
diff --git a/spec/lib/transmutation/serialization_spec.rb b/spec/lib/transmutation/serialization_spec.rb
index a8073c0..5300365 100644
--- a/spec/lib/transmutation/serialization_spec.rb
+++ b/spec/lib/transmutation/serialization_spec.rb
@@ -180,6 +180,10 @@ def initialize(id:)
         Transmutation.max_depth = 2
       end
 
+      after do
+        Transmutation.max_depth = 1
+      end
+
       it "returns the maximum depth of the serializer" do
         expect(Transmutation.max_depth).to eq(2)
       end
diff --git a/spec/rails_helper.rb b/spec/rails_helper.rb
new file mode 100644
index 0000000..c667a96
--- /dev/null
+++ b/spec/rails_helper.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+require "spec_helper"
+
+ENV["RAILS_ENV"] ||= "test"
+require_relative "dummy/config/environment"
+
+abort("The Rails environment is not running in test mode!") if Rails.env.production?
+
+require "rspec/rails"
+
+# Load the database schema into the in-memory SQLite database
+ActiveRecord::Schema.verbose = false
+load Rails.root.join("db/schema.rb").to_s
+
+RSpec.configure do |config|
+  config.include Rails.application.routes.url_helpers, type: :request
+
+  config.fixture_paths = [Rails.root.join("test/fixtures").to_s]
+  config.use_transactional_fixtures = true
+  config.global_fixtures = :all
+
+  config.infer_spec_type_from_file_location!
+
+  config.filter_rails_from_backtrace!
+end
diff --git a/spec/requests/api/v1/health_spec.rb b/spec/requests/api/v1/health_spec.rb
new file mode 100644
index 0000000..f453ac8
--- /dev/null
+++ b/spec/requests/api/v1/health_spec.rb
@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe "Api::V1::Health", type: :request do
+  describe "GET /api/v1/health" do
+    it "returns a JSON object without serialization", :aggregate_failures do
+      get api_v1_health_index_path
+
+      expect(response).to have_http_status(:ok)
+      expect(JSON.parse(response.body)).to eq({ "ok" => true })
+    end
+  end
+
+  describe "GET /api/v1/health/download" do
+    it "returns binary data using send_data", :aggregate_failures do
+      get download_api_v1_health_index_path
+
+      expect(response).to have_http_status(:ok)
+      expect(response.content_type).to include("text/plain")
+      expect(response.body).to eq("binary data content")
+    end
+
+    it "sets the correct Content-Disposition header" do
+      get download_api_v1_health_index_path
+
+      expect(response.headers["Content-Disposition"]).to include("report.txt")
+    end
+  end
+end
diff --git a/spec/requests/api/v1/posts_spec.rb b/spec/requests/api/v1/posts_spec.rb
new file mode 100644
index 0000000..4b1805e
--- /dev/null
+++ b/spec/requests/api/v1/posts_spec.rb
@@ -0,0 +1,37 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe "Api::V1::Posts", type: :request do
+  describe "GET /api/v1/posts" do
+    it "returns a list of posts serialized with Api::V1::PostSerializer", :aggregate_failures do
+      get api_v1_posts_path
+
+      expect(response).to have_http_status(:ok)
+      expect(JSON.parse(response.body)).to eq([
+                                                { "id" => 1, "title" => "First post" },
+                                                { "id" => 2, "title" => "How does this work?" },
+                                                { "id" => 3, "title" => "Second post!?" }
+                                              ])
+    end
+  end
+
+  describe "GET /api/v1/posts/:id" do
+    it "returns a post serialized with Api::V1::Detailed::PostSerializer", :aggregate_failures do
+      get api_v1_post_path(1)
+
+      expect(response).to have_http_status(:ok)
+      expect(JSON.parse(response.body)).to eq(
+        "id" => 1,
+        "title" => "First post",
+        "body" => "First!",
+        "user" => {
+          "id" => 1,
+          "first_name" => "John",
+          "last_name" => "Doe",
+          "full_name" => "John Doe"
+        }
+      )
+    end
+  end
+end
diff --git a/spec/requests/api/v1/products_spec.rb b/spec/requests/api/v1/products_spec.rb
new file mode 100644
index 0000000..8ab4ad5
--- /dev/null
+++ b/spec/requests/api/v1/products_spec.rb
@@ -0,0 +1,21 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe "Api::V1::Products", type: :request do
+  describe "GET /api/v1/products/:id" do
+    it "returns a product serialized with ProductSerializer", :aggregate_failures do
+      get api_v1_product_path(1)
+
+      expect(response).to have_http_status(:ok)
+      expect(JSON.parse(response.body)).to eq(
+        "id" => 1,
+        "name" => "Shoes",
+        "price" => {
+          "subunit" => 1000,
+          "currency" => "GBP"
+        }
+      )
+    end
+  end
+end
diff --git a/spec/requests/api/v1/users_spec.rb b/spec/requests/api/v1/users_spec.rb
new file mode 100644
index 0000000..4b02421
--- /dev/null
+++ b/spec/requests/api/v1/users_spec.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe "Api::V1::Users", type: :request do
+  describe "GET /api/v1/users" do
+    it "returns a list of users serialized with Api::V1::UserSerializer", :aggregate_failures do
+      get api_v1_users_path
+
+      expect(response).to have_http_status(:ok)
+      expect(response.content_type).to include("application/json")
+      expect(JSON.parse(response.body)).to eq([
+                                                { "id" => 1, "full_name" => "John Doe" },
+                                                { "id" => 2, "full_name" => "Jane Doe" },
+                                                { "id" => 3, "full_name" => "Adam Smith" },
+                                                { "id" => 4, "full_name" => "Eve Smith" }
+                                              ])
+    end
+  end
+
+  describe "GET /api/v1/users/:id" do
+    let(:expected_json) do
+      {
+        "id" => 1,
+        "first_name" => "John",
+        "last_name" => "Doe",
+        "full_name" => "John Doe",
+        "posts" => [
+          { "id" => 1, "title" => "First post" },
+          { "id" => 3, "title" => "Second post!?" }
+        ],
+        "comments" => [
+          { "id" => 1, "body" => "First!" },
+          { "id" => 3, "body" => "Third!" }
+        ],
+        "published_posts" => [
+          { "id" => 1, "title" => "First post" }
+        ]
+      }
+    end
+
+    it "returns a user serialized with Api::V1::Detailed::UserSerializer", :aggregate_failures do
+      get api_v1_user_path(1)
+
+      expect(response).to have_http_status(:ok)
+      expect(JSON.parse(response.body)).to eq(expected_json)
+    end
+  end
+end
diff --git a/spec/system/dummy/app.rb b/spec/system/dummy/app.rb
deleted file mode 100644
index 1e9e4ec..0000000
--- a/spec/system/dummy/app.rb
+++ /dev/null
@@ -1,10 +0,0 @@
-# frozen_string_literal: true
-
-require "json"
-require "zeitwerk"
-
-loader = Zeitwerk::Loader.new
-loader.push_dir(__dir__)
-loader.collapse("#{__dir__}/*")
-loader.ignore(__FILE__)
-loader.setup
diff --git a/spec/system/dummy/controllers/base_controller.rb b/spec/system/dummy/controllers/base_controller.rb
deleted file mode 100644
index 8e01103..0000000
--- a/spec/system/dummy/controllers/base_controller.rb
+++ /dev/null
@@ -1,20 +0,0 @@
-# frozen_string_literal: true
-
-class BaseController
-  def render(json: nil, body: nil, **options)
-    return body if body
-
-    JSON.parse(JSON.generate(json))
-  end
-
-  # Simulates Rails' send_data method which internally calls render
-  # See: https://api.rubyonrails.org/classes/ActionController/DataStreaming.html
-  #
-  # Note: Rails calls render with a Hash (not keyword arguments):
-  #   render options.slice(:status, :content_type).merge(body: data)
-  #
-  # We simulate this exactly to catch any issues with Hash vs kwargs handling
-  def send_data(data, options = {})
-    render(**options.slice(:status, :content_type).merge(body: data))
-  end
-end
diff --git a/spec/system/dummy/models/comment.rb b/spec/system/dummy/models/comment.rb
deleted file mode 100644
index 9d85469..0000000
--- a/spec/system/dummy/models/comment.rb
+++ /dev/null
@@ -1,23 +0,0 @@
-# frozen_string_literal: true
-
-class Comment
-  attr_accessor :id, :body, :user_id
-
-  def initialize(id:, body:, user_id: nil)
-    self.id = id
-    self.body = body
-    self.user_id = user_id
-  end
-
-  # =====
-  # Finder methods
-  # =====
-
-  def self.all
-    [
-      Comment.new(id: 1, body: "First!", user_id: 1),
-      Comment.new(id: 2, body: "Second!", user_id: 2),
-      Comment.new(id: 3, body: "Third!", user_id: 1)
-    ]
-  end
-end
diff --git a/spec/system/dummy/models/post.rb b/spec/system/dummy/models/post.rb
deleted file mode 100644
index a5a0ac5..0000000
--- a/spec/system/dummy/models/post.rb
+++ /dev/null
@@ -1,49 +0,0 @@
-# frozen_string_literal: true
-
-class Post
-  attr_accessor :id, :title, :body, :user_id, :published_at
-
-  def initialize(id:, title:, body:, user_id: nil, published_at: nil)
-    self.id = id
-    self.title = title
-    self.body = body
-    self.user_id = user_id
-    self.published_at = published_at
-  end
-
-  def user
-    User.find(user_id) if user_id
-  end
-
-  # =====
-  # Finder methods
-  # =====
-
-  def self.all
-    [
-      Post.new(id: 1, title: "First post", body: "First!", user_id: 1, published_at: Time.now),
-      Post.new(id: 2, title: "How does this work?", body: "body", user_id: 2, published_at: Time.now),
-      Post.new(id: 3, title: "Second post!?", body: "Nope...", user_id: 1, published_at: nil)
-    ]
-  end
-
-  def self.find(id)
-    all.find { |post| post.id == id }
-  end
-
-  # =====
-  # JSON serialization
-  # =====
-
-  def to_json(options = {})
-    as_json(options).to_json
-  end
-
-  def as_json(_options = {})
-    {
-      id:,
-      title: first_name,
-      body: last_name
-    }
-  end
-end
diff --git a/spec/system/dummy/models/product.rb b/spec/system/dummy/models/product.rb
deleted file mode 100644
index 994d312..0000000
--- a/spec/system/dummy/models/product.rb
+++ /dev/null
@@ -1,49 +0,0 @@
-# frozen_string_literal: true
-
-class Product
-  attr_accessor :id, :name, :description, :price_subunit, :price_currency
-
-  def initialize(id:, name:, description:, price_subunit:, price_currency:)
-    self.id = id
-    self.name = name
-    self.description = description
-    self.price_subunit = price_subunit
-    self.price_currency = price_currency
-  end
-
-  def price
-    Money.new(price_subunit, price_currency)
-  end
-
-  # =====
-  # Finder methods
-  # =====
-
-  def self.all
-    [
-      Product.new(id: 1, name: "Shoes", description: "A pair of shoes", price_subunit: 1000, price_currency: "GBP")
-    ]
-  end
-
-  def self.find(id)
-    all.find { |user| user.id == id }
-  end
-
-  # =====
-  # JSON serialization
-  # =====
-
-  def to_json(options = {})
-    as_json(options).to_json
-  end
-
-  def as_json(_options = {})
-    {
-      id:,
-      name:,
-      description:,
-      price_subunit:,
-      price_currency:
-    }
-  end
-end
diff --git a/spec/system/dummy/models/user.rb b/spec/system/dummy/models/user.rb
deleted file mode 100644
index 859dcb5..0000000
--- a/spec/system/dummy/models/user.rb
+++ /dev/null
@@ -1,52 +0,0 @@
-# frozen_string_literal: true
-
-class User
-  attr_accessor :id, :first_name, :last_name
-
-  def initialize(id:, first_name:, last_name:)
-    self.id = id
-    self.first_name = first_name
-    self.last_name = last_name
-  end
-
-  def posts
-    Post.all.select { |post| post.user_id == id }
-  end
-
-  def comments
-    Comment.all.select { |comment| comment.user_id == id }
-  end
-
-  # =====
-  # Finder methods
-  # =====
-
-  def self.all
-    [
-      User.new(id: 1, first_name: "John", last_name: "Doe"),
-      User.new(id: 2, first_name: "Jane", last_name: "Doe"),
-      User.new(id: 3, first_name: "Adam", last_name: "Smith"),
-      User.new(id: 4, first_name: "Eve", last_name: "Smith")
-    ]
-  end
-
-  def self.find(id)
-    all.find { |user| user.id == id }
-  end
-
-  # =====
-  # JSON serialization
-  # =====
-
-  def to_json(options = {})
-    as_json(options).to_json
-  end
-
-  def as_json(_options = {})
-    {
-      id:,
-      first_name:,
-      last_name:
-    }
-  end
-end
diff --git a/spec/system/rendering_spec.rb b/spec/system/rendering_spec.rb
deleted file mode 100644
index 02627d1..0000000
--- a/spec/system/rendering_spec.rb
+++ /dev/null
@@ -1,136 +0,0 @@
-# frozen_string_literal: true
-
-require_relative "dummy/app"
-
-RSpec.describe "Rendering" do
-  describe "Api::V1::UsersController" do
-    subject(:controller) { Api::V1::UsersController.new }
-
-    describe "#index" do
-      let(:expected_json) do
-        [
-          { "id" => 1, "full_name" => "John Doe" },
-          { "id" => 2, "full_name" => "Jane Doe" },
-          { "id" => 3, "full_name" => "Adam Smith" },
-          { "id" => 4, "full_name" => "Eve Smith" }
-        ]
-      end
-
-      it "returns a list of users serialized with the Api::V1::UserSerializer" do
-        expect(controller.index).to eq(expected_json)
-      end
-    end
-
-    describe "#show" do
-      let(:expected_json) do
-        {
-          "id" => 1,
-          "first_name" => "John",
-          "last_name" => "Doe",
-          "full_name" => "John Doe",
-          "posts" => [
-            { "id" => 1, "title" => "First post" },
-            { "id" => 3, "title" => "Second post!?" }
-          ],
-          "comments" => [
-            { "id" => 1, "body" => "First!" },
-            { "id" => 3, "body" => "Third!" }
-          ],
-          "published_posts" => [
-            { "id" => 1, "title" => "First post" }
-          ]
-        }
-      end
-
-      it "returns a user serialized with the Api::V1::Detailed::UserSerializer" do
-        expect(controller.show(1)).to eq(expected_json)
-      end
-    end
-  end
-
-  describe "Api::V1::PostsController" do
-    subject(:controller) { Api::V1::PostsController.new }
-
-    describe "#index" do
-      let(:expected_json) do
-        [
-          { "id" => 1, "title" => "First post" },
-          { "id" => 2, "title" => "How does this work?" },
-          { "id" => 3, "title" => "Second post!?" }
-        ]
-      end
-
-      it "returns a list of posts serialized with the Api::V1::PostSerializer" do
-        expect(controller.index).to eq(expected_json)
-      end
-    end
-
-    describe "#show" do
-      let(:expected_json) do
-        {
-          "id" => 1,
-          "title" => "First post",
-          "body" => "First!",
-          "user" => { "id" => 1, "first_name" => "John", "last_name" => "Doe", "full_name" => "John Doe" }
-        }
-      end
-
-      it "returns a post serialized with the Api::V1::Detailed::PostSerializer" do
-        expect(controller.show(1)).to eq(expected_json)
-      end
-    end
-  end
-
-  describe "Api::V1::ProductsController" do
-    subject(:controller) { Api::V1::ProductsController.new }
-
-    describe "#show" do
-      let(:expected_json) do
-        {
-          "id" => 1,
-          "name" => "Shoes",
-          "price" => {
-            "subunit" => 1000,
-            "currency" => "GBP"
-          }
-        }
-      end
-
-      it "returns a product serialized with the ProductSerializer" do
-        expect(controller.show(1)).to eq(expected_json)
-      end
-    end
-  end
-
-  describe "Api::V1::HealthController" do
-    subject(:controller) { Api::V1::HealthController.new }
-
-    describe "#index" do
-      let(:expected_json) do
-        {
-          "ok" => true
-        }
-      end
-
-      it "returns a JSON object" do
-        expect(controller.index).to eq(expected_json)
-      end
-    end
-
-    describe "#download" do
-      it "returns binary data without serialization" do
-        expect(controller.download).to eq("binary data content")
-      end
-
-      it "does not call serialize" do
-        expect(controller).not_to receive(:serialize)
-        controller.download
-      end
-
-      it "does not call lookup_serializer" do
-        expect(controller).not_to receive(:lookup_serializer)
-        controller.download
-      end
-    end
-  end
-end