For now, real-time test discovery only works for the xUnit, NUnit, and MSTest frameworks. Real-time test discovery only works for the managed languages (C# and Visual Basic), since it's built using the Roslyn compiler. What languages and test frameworks can use Real Time Test Discovery? For more information about test discovery behavior for dynamically defined tests such as theories or custom traits, see Dynamic test discovery. It uses a Roslyn analyzer to find tests and populate Test Explorer in real time, without requiring you to build your project. This feature is called Real-time test discovery. Tests now appear in Test Explorer while I type, without having to build my project. This symbol appears if dynamically defined tests are detected in your project. The '+' (plus) symbol indicates that more tests may be discovered after a build when assembly-based discovery runs. What does the '+' (plus) symbol that appears in the top line of Test Explorer mean? Real-time test discovery improves responsiveness, but still allows you to get complete and precise results after a build. This setting means real-time test discovery finds as many tests as it can while you're editing, and assembly-based discovery allows dynamically defined tests to appear after a build. In Visual Studio 2017 version 15.6 and later, assembly-based discovery (the traditional discoverer) runs only after builds. A build is required for those tests to be accurately found. It can’t discover tests that use theories, custom adapters, custom traits, #ifdef statements, and more because they're defined at run time. Real-time test discovery is source-based test discovery. Build your project to run assembly-based discovery.