6. For any real number $x$, it always holds that
$$
\log _{a}(\sin x+\cos x)^{2} \geqslant-2 \text {. }
$$

Then the range of values for $a$ is $\qquad$
$6.01$  when,
$$
\begin{array}{l}
{\left[\sqrt{2} \sin \left(x+\frac{\pi}{4}\right)\right]^{2} \geqslant \frac{1}{a^{2}}} \\
\Rightarrow a^{2} \geqslant \frac{1}{2 \sin ^{2}\left(x+\frac{\pi}{4}\right)} .
\end{array}
$$

Since $\frac{1}{2 \sin ^{2}\left(x+\frac{\pi}{4}\right)}$ has no upper bound, therefore, $a<1$. At this point,
$$
\begin{array}{l}
2 \sin ^{2}\left(x+\frac{\pi}{4}\right) \leqslant \frac{1}{a^{2}} \Rightarrow a^{2} \leqslant \frac{1}{2 \sin ^{2}\left(x+\frac{\pi}{4}\right)} \\
\Rightarrow a^{2} \leqslant \frac{1}{2} \Rightarrow 0<a \leqslant \frac{\sqrt{2}}{2} .
\end{array}
$$