已知 $\alpha \in\left(\frac{\pi}{2}, \pi\right), \tan \left(\alpha+\frac{\pi}{4}\right)=\frac{1}{7}$, 那么 $\sin \alpha+\cos \alpha$ 的值为（）.
$\because \tan \left(\alpha+\frac{\pi}{4}\right)=\frac{\tan \alpha+1}{1-\tan \alpha}=\frac{1}{7}$,

$\therefore \tan \alpha=-\frac{3}{4}$,

又 $\because \alpha \in\left(\frac{\pi}{2}, \pi\right)$,

$\therefore \cos \alpha=-\frac{4}{5}, \sin \alpha=\frac{3}{5}$,

$\therefore \sin \alpha+\cos \alpha=-\frac{1}{5}$.

故选 A.