diff --git a/examples/openapi.yaml b/examples/openapi.yaml
index d49713d4..89ccdf89 100644
--- a/examples/openapi.yaml
+++ b/examples/openapi.yaml
@@ -12,7 +12,7 @@ tags:
 paths:
   /:
     get:
-      operationId: things#index
+      operationId: example#root
       summary: Get metadata from the root of the API
       tags: ["Metadata"]
       responses:
diff --git a/examples/rack_handler.rb b/examples/rack_handler.rb
index 3e196545..9a23756f 100644
--- a/examples/rack_handler.rb
+++ b/examples/rack_handler.rb
@@ -10,7 +10,7 @@
 
   not_found = ->(_request) { [404, {}, []] }
   handlers = {
-    'things#index' => lambda do |_request|
+    'example#root' => lambda do |_request|
       [200, { Rack::CONTENT_TYPE => 'application/json' }, ['{"hello": "world"}']]
     end
   }
diff --git a/lib/openapi_first/test.rb b/lib/openapi_first/test.rb
index 0515e2b2..ec61d2a2 100644
--- a/lib/openapi_first/test.rb
+++ b/lib/openapi_first/test.rb
@@ -22,8 +22,8 @@ def initialize
         yield self
       end
 
-      def register(*)
-        Test.register(*)
+      def register(oad, as: :default)
+        Test.register(oad, as:)
       end
 
       attr_accessor :minimum_coverage, :coverage_formatter_options, :coverage_formatter
@@ -47,7 +47,7 @@ def handle_exit
         end
         return unless minimum_coverage > coverage
 
-        puts "API Coverage fails with exit 2, because API coverage of #{coverage}%" \
+        puts "API Coverage fails with exit 2, because API coverage of #{coverage}% " \
              "is below minimum of #{minimum_coverage}%!"
         exit 2
         # :nocov:
diff --git a/lib/openapi_first/test/coverage.rb b/lib/openapi_first/test/coverage.rb
index 2c35d631..ad593b73 100644
--- a/lib/openapi_first/test/coverage.rb
+++ b/lib/openapi_first/test/coverage.rb
@@ -53,11 +53,11 @@ def reset
         end
 
         def track_request(request, oad)
-          current_run[oad.key].track_request(request)
+          current_run[oad.key]&.track_request(request)
         end
 
         def track_response(response, _request, oad)
-          current_run[oad.key].track_response(response)
+          current_run[oad.key]&.track_response(response)
         end
 
         def result
diff --git a/lib/openapi_first/test/methods.rb b/lib/openapi_first/test/methods.rb
index 02138540..52f5ba5f 100644
--- a/lib/openapi_first/test/methods.rb
+++ b/lib/openapi_first/test/methods.rb
@@ -7,21 +7,54 @@ module OpenapiFirst
   module Test
     # Methods to use in integration tests
     module Methods
-      def self.[](*)
+      def self.included(base)
+        base.include(DefaultApiMethod)
+        base.include(AssertionMethod)
+      end
+
+      def self.[](application_under_test = nil, api: nil)
         mod = Module.new do
           def self.included(base)
-            OpenapiFirst::Test::Methods.included(base)
+            base.include OpenapiFirst::Test::Methods::AssertionMethod
           end
         end
-        mod.define_method(:app) { OpenapiFirst::Test.app(*) }
+
+        if api
+          mod.define_method(:openapi_first_default_api) { api }
+        else
+          mod.include(DefaultApiMethod)
+        end
+
+        if application_under_test
+          mod.define_method(:app) { OpenapiFirst::Test.app(application_under_test, api: openapi_first_default_api) }
+        end
+
         mod
       end
 
-      def self.included(base)
-        if Test.minitest?(base)
-          base.include(OpenapiFirst::Test::MinitestHelpers)
-        else
-          base.include(OpenapiFirst::Test::PlainHelpers)
+      # Default methods
+      module DefaultApiMethod
+        # This is the default api that is used by assert_api_conform
+        # :default is the default name that is used if you don't pass an `api:` option to `OpenapiFirst::Test.register`
+        # This is overwritten if you pass an `api:` option to `include OpenapiFirst::Test::Methods[…]`
+        def openapi_first_default_api
+          klass = self.class
+          if klass.respond_to?(:metadata) && klass.metadata[:api]
+            klass.metadata[:api]
+          else
+            :default
+          end
+        end
+      end
+
+      # @visibility private
+      module AssertionMethod
+        def self.included(base)
+          if Test.minitest?(base)
+            base.include(OpenapiFirst::Test::MinitestHelpers)
+          else
+            base.include(OpenapiFirst::Test::PlainHelpers)
+          end
         end
       end
     end
diff --git a/lib/openapi_first/test/minitest_helpers.rb b/lib/openapi_first/test/minitest_helpers.rb
index 30bebe4c..a8f73281 100644
--- a/lib/openapi_first/test/minitest_helpers.rb
+++ b/lib/openapi_first/test/minitest_helpers.rb
@@ -5,7 +5,7 @@ module Test
     # Assertion methods for Minitest
     module MinitestHelpers
       # :nocov:
-      def assert_api_conform(status: nil, api: :default)
+      def assert_api_conform(status: nil, api: openapi_first_default_api)
         api = OpenapiFirst::Test[api]
         request = respond_to?(:last_request) ? last_request : @request
         response = respond_to?(:last_response) ? last_response : @response
diff --git a/lib/openapi_first/test/plain_helpers.rb b/lib/openapi_first/test/plain_helpers.rb
index 73170417..573153b7 100644
--- a/lib/openapi_first/test/plain_helpers.rb
+++ b/lib/openapi_first/test/plain_helpers.rb
@@ -5,7 +5,7 @@ module Test
     # Assertion methods to use when no known test framework was found
     # These methods just raise an exception if an error was found
     module PlainHelpers
-      def assert_api_conform(status: nil, api: :default)
+      def assert_api_conform(status: nil, api: openapi_first_default_api)
         api = OpenapiFirst::Test[api]
         # :nocov:
         request = respond_to?(:last_request) ? last_request : @request
diff --git a/spec/middlewares/request_validation/request_body_validation_spec.rb b/spec/middlewares/request_validation/request_body_validation_spec.rb
index ae6cbfe4..fd073cfe 100644
--- a/spec/middlewares/request_validation/request_body_validation_spec.rb
+++ b/spec/middlewares/request_validation/request_body_validation_spec.rb
@@ -212,12 +212,35 @@ def fixture_path(name)
       end
 
       it 'accepts an empty request body' do
+        header Rack::CONTENT_TYPE, 'application/json'
+        post path
+
+        expect(last_response.status).to be(200), last_response.body
+        expect(last_request.env[OpenapiFirst::REQUEST].parsed_body).to eq nil
+      end
+
+      it 'accepts an empty request body without content-type' do
+        post path
+
+        expect(last_response.status).to be(200), last_response.body
+        expect(last_request.env[OpenapiFirst::REQUEST].parsed_body).to eq nil
+      end
+
+      it 'accepts an unknown content-type and an empty request body' do
+        header Rack::CONTENT_TYPE, 'foo/bar'
         post path
 
         expect(last_response.status).to be(200), last_response.body
         expect(last_request.env[OpenapiFirst::REQUEST].parsed_body).to eq nil
       end
 
+      it 'returns 400 if content-type is unknown and request body is invalid' do
+        header Rack::CONTENT_TYPE, 'foo/bar'
+        post path, JSON.generate({ say: 'no ' })
+
+        expect(last_response.status).to be(400), last_response.body
+      end
+
       it 'returns 400 if request body is invalid' do
         header Rack::CONTENT_TYPE, 'application/json'
         post path, JSON.generate({ say: 'no ' })
diff --git a/spec/test/coverage_spec.rb b/spec/test/coverage_spec.rb
index 80e88b5f..43bce6b7 100644
--- a/spec/test/coverage_spec.rb
+++ b/spec/test/coverage_spec.rb
@@ -65,4 +65,18 @@
       specify { expect(result.coverage).to eq(50) }
     end
   end
+
+  describe '.track_request' do
+    it 'ignores unregistered OADs' do
+      oad = double(key: 'unknown')
+      described_class.track_request(double, oad)
+    end
+  end
+
+  describe '.track_response' do
+    it 'ignores unregistered OADs' do
+      oad = double(key: 'unknown')
+      described_class.track_response(double(:response), double(:request), oad)
+    end
+  end
 end
diff --git a/spec/test/methods_spec.rb b/spec/test/methods_spec.rb
index 63e2eccf..95b47f03 100644
--- a/spec/test/methods_spec.rb
+++ b/spec/test/methods_spec.rb
@@ -3,35 +3,16 @@
 require 'minitest'
 
 RSpec.describe OpenapiFirst::Test::Methods do
-  it 'can be included' do
-    minitest_class = Class.new(Minitest::Test) do
+  it 'includes PlainHelpers when included' do
+    test_class = Class.new do
       include OpenapiFirst::Test::Methods
     end
-    expect(minitest_class.included_modules).to include(OpenapiFirst::Test::MinitestHelpers)
-
-    other_class = Class.new do
-      include OpenapiFirst::Test::Methods
-    end
-    expect(other_class.included_modules).to include(OpenapiFirst::Test::PlainHelpers)
+    expect(test_class.included_modules).to include(OpenapiFirst::Test::PlainHelpers)
   end
 
-  it 'adds an app method that wraps the app' do
+  it 'raises OpenapiFirst::Error when assertion fails' do
     OpenapiFirst::Test.register('./examples/openapi.yaml')
-    myapp = ->(_env) { Rack::Response.new('hello').finish }
-    minitest_class = Class.new(Minitest::Test) do
-      include OpenapiFirst::Test::Methods[myapp]
-    end
-    expect(minitest_class.included_modules).to include(OpenapiFirst::Test::MinitestHelpers)
-
-    test_app = minitest_class.new(1).app
-    env = Rack::MockRequest.env_for('/')
-    expect(test_app.call(env)).to eq(Rack::Response.new('hello').finish)
-    expect(env[OpenapiFirst::REQUEST]).to be_valid
-  end
-
-  it 'detects wrong response status for Minitest' do
-    OpenapiFirst::Test.register('./examples/openapi.yaml')
-    minitest_class = Class.new(Minitest::Test) do
+    test_class = Class.new do
       include OpenapiFirst::Test::Methods
 
       def last_request = Rack::Request.new(Rack::MockRequest.env_for('/'))
@@ -39,21 +20,153 @@ def last_response = Rack::Response.new
     end
 
     expect do
-      minitest_class.new('hey').assert_api_conform(status: 444)
-    end.to raise_error(Minitest::Assertion)
+      test_class.new.assert_api_conform(status: 444)
+    end.to raise_error(OpenapiFirst::Error)
   end
 
-  it 'detects wrong response status for non Minitest' do
-    OpenapiFirst::Test.register('./examples/openapi.yaml')
-    minitest_class = Class.new do
+  context 'with RSpec' do
+    let(:app) do
+      lambda do |_|
+        res = Rack::Response.new(JSON.generate(hello: 'there'))
+        res.content_type = 'application/json'
+        res.finish
+      end
+    end
+
+    context 'with metadata', api: :v1 do
       include OpenapiFirst::Test::Methods
+      include Rack::Test::Methods
 
-      def last_request = Rack::Request.new(Rack::MockRequest.env_for('/'))
-      def last_response = Rack::Response.new
+      it 'targets that api when calling assert_api_conform' do
+        expect do
+          assert_api_conform(status: 200)
+        end.to raise_error(OpenapiFirst::Test::NotRegisteredError) do |ex|
+          expect(ex.message).to start_with("API description ':v1' not found.")
+        end
+      end
     end
 
-    expect do
-      minitest_class.new.assert_api_conform(status: 444)
-    end.to raise_error(OpenapiFirst::Error)
+    context 'with an [api:] option', api: :v2 do
+      include OpenapiFirst::Test::Methods[api: :v1]
+
+      it 'targets the api from the argument when calling assert_api_conform' do
+        expect do
+          assert_api_conform(status: 200)
+        end.to raise_error(OpenapiFirst::Test::NotRegisteredError) do |ex|
+          expect(ex.message).to start_with("API description ':v1' not found.")
+        end
+      end
+    end
+
+    context 'with an [Application] argument and metadata', api: :v2 do
+      include OpenapiFirst::Test::Methods[->(_) { Rack::Response.new('hey').finish }]
+      include Rack::Test::Methods
+
+      it 'targets that api when calling the app' do
+        OpenapiFirst::Test.register('./examples/openapi.yaml', as: :v2)
+
+        get('/')
+
+        expect(last_request.env[OpenapiFirst::REQUEST].operation_id).to eq('example#root')
+      end
+    end
+  end
+
+  context 'with Minitest' do
+    it 'includes MinitestHelpers when included' do
+      minitest_class = Class.new(Minitest::Test) do
+        include OpenapiFirst::Test::Methods
+      end
+      expect(minitest_class.included_modules).to include(OpenapiFirst::Test::MinitestHelpers)
+    end
+
+    it 'raises Minitest::Assertion when assertion fails' do
+      OpenapiFirst::Test.register('./examples/openapi.yaml')
+      minitest_class = Class.new(Minitest::Test) do
+        include OpenapiFirst::Test::Methods
+
+        def last_request = Rack::Request.new(Rack::MockRequest.env_for('/'))
+        def last_response = Rack::Response.new
+      end
+
+      expect do
+        minitest_class.new('hey').assert_api_conform(status: 444)
+      end.to raise_error(Minitest::Assertion)
+    end
+  end
+
+  context 'with [arguments]' do
+    it 'adds an app method that wraps the default API' do
+      OpenapiFirst::Test.register('./examples/openapi.yaml')
+      myapp = ->(_env) { Rack::Response.new('hello').finish }
+
+      minitest_class = Class.new(Minitest::Test) do
+        include OpenapiFirst::Test::Methods[myapp]
+      end
+
+      expect(minitest_class.included_modules).to include(OpenapiFirst::Test::MinitestHelpers)
+      test_app = minitest_class.new(1).app
+      env = Rack::MockRequest.env_for('/')
+      expect(test_app.call(env)).to eq(Rack::Response.new('hello').finish)
+      expect(env[OpenapiFirst::REQUEST]).to be_valid
+    end
+
+    it 'adds an app method that wraps the app for a specific API' do
+      OpenapiFirst::Test.register('./examples/openapi.yaml', as: :v1)
+      myapp = ->(_env) { Rack::Response.new('hello').finish }
+
+      minitest_class = Class.new(Minitest::Test) do
+        include OpenapiFirst::Test::Methods[myapp, api: :v1]
+      end
+
+      expect(minitest_class.included_modules).to include(OpenapiFirst::Test::MinitestHelpers)
+      test_app = minitest_class.new(1).app
+      env = Rack::MockRequest.env_for('/')
+      expect(test_app.call(env)).to eq(Rack::Response.new('hello').finish)
+      expect(env[OpenapiFirst::REQUEST]).to be_valid
+    end
+
+    it 'adds an assert_api_conform method that targets the specified API' do
+      OpenapiFirst::Test.register('./examples/openapi.yaml', as: :v1)
+      test_class = Class.new do
+        include OpenapiFirst::Test::Methods[api: :v1]
+
+        def last_request = Rack::Request.new(Rack::MockRequest.env_for('/'))
+        def last_response = Rack::Response.new
+      end
+
+      expect(test_class.new.openapi_first_default_api).to eq(:v1)
+
+      expect do
+        test_class.new.assert_api_conform(status: 444)
+      end.to raise_error(OpenapiFirst::Error)
+    end
+
+    it 'adds an assert_api_conform method that still can target another API' do
+      OpenapiFirst::Test.register('./examples/openapi.yaml', as: :v1)
+      test_class = Class.new do
+        include OpenapiFirst::Test::Methods[api: :v1]
+
+        def last_request = Rack::Request.new(Rack::MockRequest.env_for('/'))
+        def last_response = Rack::Response.new
+      end
+
+      expect do
+        test_class.new.assert_api_conform(status: 444, api: :other)
+      end.to raise_error(OpenapiFirst::Test::NotRegisteredError) do |ex|
+        expect(ex.message).to start_with("API description ':other' not found.")
+      end
+    end
+
+    it 'does not add an app method if app is nil' do
+      OpenapiFirst::Test.register('./examples/openapi.yaml', as: :v1)
+
+      minitest_class = Class.new(Minitest::Test) do
+        include OpenapiFirst::Test::Methods[api: :v1]
+      end
+
+      expect(minitest_class.included_modules).to include(OpenapiFirst::Test::MinitestHelpers)
+      expect(minitest_class.new(1).respond_to?(:app)).to eq(false)
+    end
   end
 end
diff --git a/spec/test_spec.rb b/spec/test_spec.rb
index 218848f7..61f4a226 100644
--- a/spec/test_spec.rb
+++ b/spec/test_spec.rb
@@ -24,8 +24,10 @@
       expect(described_class[:default].filepath).to eq('./examples/openapi.yaml')
     end
 
-    it 'can register an OAD with a custom name' do
+    it 'can register multiple OADs' do
+      described_class.register('./spec/data/dice.yaml')
       described_class.register('./examples/openapi.yaml', as: :mine)
+      expect(described_class[:default].filepath).to eq('./spec/data/dice.yaml')
       expect(described_class[:mine].filepath).to eq('./examples/openapi.yaml')
     end