Aims. We study the 12C/13C isotopic ratio in the disk of the central molecular zone and in the halo to trace gas accretion toward the Galactic center region in the Milky Way. Methods. Using the IRAM 30m telescope, we observed the J = 1−0 rotational transition of HCO+, HCN, HNC, and their 13C isotopic substitutions in order to measure the 12C/13C isotopic ratio. We observed 9 positions selected throughout the Galactic center region, including clouds at high latitude, locations where the X1 and X2 orbits associated with the barred potential are expected to intersect, and typical Galactic center molecular clouds. Results. We find a systematically higher 12C/13C isotopic ratio (>40) toward the halo and the X1 orbits than for the Galactic center molecular clouds (20–25). Our results point to molecular gas that has undergone a different degree of nuclear processing than observed in the gas towards the inner Galactic center region. Conclusions. The high isotopic ratios are consistent with the accretion of the gas from the halo and from the outskirts of the Galactic disk.FONDAPFONDAP