12 The range of negative values of $a$ that make the inequality
$$
\sin ^{2} x+a \cos x+a^{2} \geqslant 1+\cos x
$$

hold for all $x \in \mathbf{R}$ is $\qquad$
When $x=0$, $a+a^{2} \geqslant 2$, so $a \leqslant-2$ (since $a<0$).
Also, when $a \leqslant-2$, we have
$$
\begin{aligned}
a^{2}+a \cos x & \geqslant a^{2}+a \geqslant 2 \geqslant \cos ^{2} x+\cos x \\
& =1+\cos x-\sin ^{2} x,
\end{aligned}
$$

which means
$$
\sin ^{2} x+a \cos x+a^{2} \geqslant 1+\cos x.
$$

Therefore, the range of $a$ is $a \leqslant-2$.