To find the sum of the fractions \( \frac{12}{31} + \frac{13}{31} \), we can add the numerators since they have the same denominator.
\[ \frac{12 + 13}{31} = \frac{25}{31} \]
So, the sum of the fractions \( \frac{12}{31} + \frac{13}{31} \) is \( \frac{25}{31} \).