Due QTBUG-106933, which seems is an optimization feature, not a bug, description of QtConcurrent::run(QThreadPool *pool, Function function, ...) should be changed. Currrently there is: "Runs function in a separate thread. The thread is taken from the QThreadPool pool." It should be changed to something like: "Runs function in a separate thread taken from the QThreadPool pool or current thread, depending on <something>" Sorry I don't known details, it is quite useless for my task, but I think that information is rather important for other programmers.