diff --git a/src/boolean/boolean_mesh.cpp b/src/boolean/boolean_mesh.cpp
index 51089ab..dfc6d28 100644
--- a/src/boolean/boolean_mesh.cpp
+++ b/src/boolean/boolean_mesh.cpp
@@ -86,7 +86,9 @@ bool is_point_inside_bvh(const core::Point3D& p,
 
     core::Vector3D dir(1.0, 0.371, 0.539);
     dir.normalize();
-    Ray3Dd ray(p, dir);
+    // Offset origin slightly to avoid exactly-on-face ambiguity
+    core::Point3D origin = p + dir * 1e-6;
+    Ray3Dd ray(origin, dir);
     auto results = bvh.query_ray(ray);
     int hits = 0;
     for (const auto& tri : results) {
@@ -96,7 +98,7 @@ bool is_point_inside_bvh(const core::Point3D& p,
         double a = e1.dot(h);
         if (std::abs(a) < 1e-12) continue;
         double f = 1.0 / a;
-        core::Vector3D s = p - tri.v(0);
+        core::Vector3D s = origin - tri.v(0);
         double u = f * s.dot(h);
         if (u < 0.0 || u > 1.0) continue;
         core::Vector3D q = s.cross(e1);