diff --git a/Gemfile.lock b/Gemfile.lock
index 7068eed..748a59d 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -97,4 +97,4 @@ DEPENDENCIES
   simplecov (~> 0.22)
 
 BUNDLED WITH
-   2.5.22
+   4.0.10
diff --git a/lib/ruby_llm/test/errors/no_response_provided_error.rb b/lib/ruby_llm/test/errors/no_response_provided_error.rb
index 63c9c97..3753900 100644
--- a/lib/ruby_llm/test/errors/no_response_provided_error.rb
+++ b/lib/ruby_llm/test/errors/no_response_provided_error.rb
@@ -13,8 +13,7 @@ class NoResponseProvidedError < StandardError
 
         def initialize(messages)
           @messages = messages
-
-          super("No test response provided for the following request:\n#{messages.last.content}")
+          super("No test response provided for the following request:\n#{messages.last&.content}")
         end
       end
     end
diff --git a/test/ruby_llm/test/complete_parameters_test.rb b/test/ruby_llm/test/complete_parameters_test.rb
new file mode 100644
index 0000000..3695368
--- /dev/null
+++ b/test/ruby_llm/test/complete_parameters_test.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module RubyLLM
+  module Test
+    class CompleteParametersTest < Minitest::Test
+      def test_with_block
+        params_with_block = CompleteParameters.new(
+          messages: [], tools: [], temperature: 0.5, model: "test-model", params: {}, headers: {}, schema: nil,
+          thinking: nil, tool_prefs: nil,
+          block: -> { "test block" }
+        )
+
+        assert_predicate params_with_block, :block_received?
+      end
+
+      def test_without_block
+        params_without_block = CompleteParameters.new(
+          messages: [], tools: [], temperature: 0.5, model: "test-model", params: {}, headers: {}, schema: nil,
+          thinking: nil, tool_prefs: nil,
+          block: nil
+        )
+
+        refute_predicate params_without_block, :block_received?
+      end
+    end
+  end
+end
diff --git a/test/ruby_llm/test/errors/no_response_provided_error_test.rb b/test/ruby_llm/test/errors/no_response_provided_error_test.rb
new file mode 100644
index 0000000..b3558f3
--- /dev/null
+++ b/test/ruby_llm/test/errors/no_response_provided_error_test.rb
@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module RubyLLM
+  module Test
+    module Errors
+      class NoResponseProvidedErrorTest < Minitest::Test
+        def test_error_message_includes_last_message_content
+          messages = [
+            Message.new(role: :user, content: "What is the capital of France?"),
+            Message.new(role: :assistant, content: "The capital of France is Paris.")
+          ]
+
+          error = NoResponseProvidedError.new(messages)
+
+          assert_includes error.message, messages.last.content
+        end
+
+        def test_handles_empty_messages
+          error = NoResponseProvidedError.new([])
+
+          assert_includes error.message, "No test response provided for the following request:"
+        end
+      end
+    end
+  end
+end
diff --git a/test/ruby_llm/test/test_provider_test.rb b/test/ruby_llm/test/test_provider_test.rb
new file mode 100644
index 0000000..208ef7b
--- /dev/null
+++ b/test/ruby_llm/test/test_provider_test.rb
@@ -0,0 +1,119 @@
+# frozen_string_literal: true
+
+require "test_helper"
+
+module RubyLLM
+  module Test
+    class TestProviderTest < Minitest::Test
+      def setup
+        RubyLLM::Test.reset
+      end
+
+      def test_provider_returns_stubbed_response
+        RubyLLM::Test.stub_response("stubbed response")
+
+        provider = TestProvider.new(nil, RubyLLM::Test)
+        response = provider.complete([], tools: [], temperature: 0.5, model: "test-model", params: {}, headers: {})
+
+        assert_kind_of Message, response
+        assert_equal "stubbed response", response.content
+      end
+
+      def test_when_response_not_stubbed
+        provider = TestProvider.new(nil, RubyLLM::Test)
+        message = Message.new(role: :user, content: "What is the capital of France?")
+
+        exception = assert_raises(Errors::NoResponseProvidedError) do
+          provider.complete([ message ], tools: [], temperature: 0.5, model: "test-model", params: {}, headers: {})
+        end
+
+        assert_includes exception.message, message.content
+      end
+
+      def test_when_stub_is_a_message
+        stubbed_message = Message.new(role: :assistant, content: "This is a stubbed message.")
+        RubyLLM::Test.stub_response(stubbed_message)
+
+        provider = TestProvider.new(nil, RubyLLM::Test)
+        response = provider.complete([], tools: [], temperature: 0.5, model: "test-model", params: {}, headers: {})
+
+        assert_equal stubbed_message, response
+      end
+
+      def test_hash_stub_is_converted_to_json_message
+        stubbed_hash = { answer: "42" }
+        RubyLLM::Test.stub_response(stubbed_hash)
+
+        provider = TestProvider.new(nil, RubyLLM::Test)
+        response = provider.complete([], tools: [], temperature: 0.5, model: "test-model", params: {}, headers: {})
+
+        assert_kind_of Message, response
+        assert_equal stubbed_hash.to_json, response.content
+      end
+    end
+
+    class TestProviderLastCallTest < Minitest::Test
+      def setup
+        RubyLLM::Test.reset
+        RubyLLM::Test.stub_response("stubbed response")
+
+        initialize_arguments
+        provider = TestProvider.new(nil, RubyLLM::Test)
+        provider.complete(@messages, tools: @tools, temperature: @temperature, model: @model, params: @params,
+                                     headers: @headers, schema: @schema, thinking: @thinking, tool_prefs: @tool_prefs)
+
+        @last_call = provider.last_call
+      end
+
+      def test_last_call_captures_messages
+        assert_equal @messages, @last_call.messages
+      end
+
+      def test_last_call_captures_tools
+        assert_equal @tools, @last_call.tools
+      end
+
+      def test_last_call_captures_temperature
+        assert_equal @temperature, @last_call.temperature
+      end
+
+      def test_last_call_captures_model
+        assert_equal @model, @last_call.model
+      end
+
+      def test_last_call_captures_params
+        assert_equal @params, @last_call.params
+      end
+
+      def test_last_call_captures_headers
+        assert_equal @headers, @last_call.headers
+      end
+
+      def test_last_call_captures_schema
+        assert_equal @schema, @last_call.schema
+      end
+
+      def test_last_call_captures_thinking
+        assert_equal @thinking, @last_call.thinking
+      end
+
+      def test_last_call_captures_tool_prefs
+        assert_equal @tool_prefs, @last_call.tool_prefs
+      end
+
+      private
+
+      def initialize_arguments
+        @messages = [ Message.new(role: :user, content: "Hello") ]
+        @tools = %w[tool1 tool2]
+        @temperature = 0.7
+        @model = "test-model"
+        @params = { param1: "value1" }
+        @headers = { "Authorization" => "Bearer token" }
+        @schema = { type: "object" }
+        @thinking = "thinking process"
+        @tool_prefs = { prefer_tool1: true }
+      end
+    end
+  end
+end
diff --git a/test/ruby_llm/test_test.rb b/test/ruby_llm/test_test.rb
index 5f11544..bf79376 100644
--- a/test/ruby_llm/test_test.rb
+++ b/test/ruby_llm/test_test.rb
@@ -4,8 +4,42 @@
 
 module RubyLLM
   class TestTest < Minitest::Test
-    def test_that_it_has_a_version_number
-      refute_nil ::RubyLLM::Test::VERSION
+    def test_stub_response
+      RubyLLM::Test.reset
+      RubyLLM::Test.stub_response("test response")
+
+      assert_equal "test response", RubyLLM::Test.next_response
+    end
+
+    def test_stub_responses
+      RubyLLM::Test.reset
+      RubyLLM::Test.stub_responses("response 1", "response 2")
+
+      assert_equal "response 1", RubyLLM::Test.next_response
+      assert_equal "response 2", RubyLLM::Test.next_response
+    end
+
+    def test_with_responses
+      RubyLLM::Test.reset
+      RubyLLM::Test.stub_response("initial response")
+
+      RubyLLM::Test.with_responses("temp response 1", "temp response 2") do
+        assert_equal "temp response 1", RubyLLM::Test.next_response
+        assert_equal "temp response 2", RubyLLM::Test.next_response
+      end
+
+      # Ensure original responses are restored after the block
+      assert_equal "initial response", RubyLLM::Test.next_response
+    end
+
+    def test_responses_empty
+      RubyLLM::Test.reset
+
+      assert_predicate RubyLLM::Test, :responses_empty?
+
+      RubyLLM::Test.stub_response("not empty anymore")
+
+      refute_predicate RubyLLM::Test, :responses_empty?
     end
   end
 end
diff --git a/test/test_helper.rb b/test/test_helper.rb
index cca02d9..0e40b00 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -1,6 +1,11 @@
 # frozen_string_literal: true
 
-$LOAD_PATH.unshift File.expand_path("../lib", __dir__)
-require "ruby_llm/test"
+require "simplecov"
+
+SimpleCov.start do
+  add_filter %r{^/test/}
+  enable_coverage :branch
+end
 
+require "ruby_llm/test"
 require "minitest/autorun"