ZLayer<HttpClient, Nothing, HttpClient> mockLayer = ZLayer.fromService(_ => mockHttpClient); MockHttpClient.when(_.get(anyString())).thenReturn(Task.succeed("Mocked Data")); ZIO<HttpClient, Throwable, String> result = ApiClient.getData().provideLayer(mockLayer); assert(result == "Mocked Data"); <dependency> <groupId>dev.zio</groupId> <artifactId>zio-mock_2.12</artifactId> <version>1.0.3</version> <scope>test</scope> </dependency> @RunWith(MockitoJUnitRunner.class)