diff --git a/obelisk/cpp/obelisk_cpp/CMakeLists.txt b/obelisk/cpp/obelisk_cpp/CMakeLists.txt
index 641624df..8eeadff6 100644
--- a/obelisk/cpp/obelisk_cpp/CMakeLists.txt
+++ b/obelisk/cpp/obelisk_cpp/CMakeLists.txt
@@ -40,7 +40,9 @@ if(USE_MUJOCO)
   find_package(Eigen3 REQUIRED)
 
   include(FetchContent)
-  set(MUJOCO_VERSION "3.1.6" CACHE STRING "mujoco version")
+  # >= 3.5.0 required: mj_multiRay's angular culling in <= 3.4 wrongly drops
+  # large slabs near the ray origin (below-ground lidar returns at slab seams)
+  set(MUJOCO_VERSION "3.8.1" CACHE STRING "mujoco version")
   set(COMP_ARCH "x86_64" CACHE STRING "computer architecture")
 
   FetchContent_Declare(
diff --git a/obelisk/cpp/obelisk_cpp/include/obelisk_mujoco_sim_robot.h b/obelisk/cpp/obelisk_cpp/include/obelisk_mujoco_sim_robot.h
index 68799cba..e025dd24 100644
--- a/obelisk/cpp/obelisk_cpp/include/obelisk_mujoco_sim_robot.h
+++ b/obelisk/cpp/obelisk_cpp/include/obelisk_mujoco_sim_robot.h
@@ -1539,16 +1539,18 @@ namespace obelisk {
                 Eigen::Matrix<double, Eigen::Dynamic, 3, Eigen::RowMajor> dirs_rm = dirs_w;
                 std::vector<int> geomids(num_rays, -1);
                 const mjtNum cutoff = iface.get_max_distance() > 0.0 ? iface.get_max_distance() : mjMAXVAL;
+                // mujoco >= 3.5: ray functions take an optional surface-normal
+                // output (nullptr = not needed)
                 mj_multiRay(this->model_, this->data_, origin.data(), dirs_rm.data(),
                             iface.get_geom_group_mask(), 1, -1, geomids.data(), dists.data(),
-                            num_rays, cutoff);
+                            nullptr, num_rays, cutoff);
             } else {
                 int geom_id[1] = { -1 };
                 for (int ii = 0; ii < num_rays; ++ii) {
                     Eigen::Vector3d ray_origin = starts_w.row(ii).transpose();
                     Eigen::Vector3d direction  = dirs_w.row(ii).transpose();
                     dists[ii] = mj_ray(this->model_, this->data_, ray_origin.data(), direction.data(),
-                                       iface.get_geom_group_mask(), 1, -1, geom_id);
+                                       iface.get_geom_group_mask(), 1, -1, geom_id, nullptr);
                 }
             }
         }