diff --git a/src/brep/brep_boolean.cpp b/src/brep/brep_boolean.cpp
index 7f471c9..51c4d2b 100644
--- a/src/brep/brep_boolean.cpp
+++ b/src/brep/brep_boolean.cpp
@@ -4,6 +4,7 @@
 #include "vde/core/line.h"
 #include "vde/core/aabb.h"
 #include "vde/mesh/halfedge_mesh.h"
+#include "vde/curves/surface_intersection.h"
 #include <algorithm>
 #include <set>
 #include <map>
@@ -456,6 +457,95 @@ BrepModel copy_brep(const BrepModel& src) {
     return result;
 }
 
+// ── Face bounding box (from edges/vertices) ────────────
+core::AABB3D face_bounds(const BrepModel& body, int face_id) {
+    core::AABB3D bb;
+    auto es = body.face_edges(face_id);
+    for (int ei : es) {
+        const auto& e = body.edge(ei);
+        for (size_t vi = 0; vi < body.num_vertices(); ++vi) {
+            const auto& v = body.vertex(static_cast<int>(vi));
+            if (v.id == e.v_start || v.id == e.v_end) bb.expand(v.point);
+        }
+    }
+    return bb;
+}
+
+// ── SSI-based face splitting ────────────────────────────
+/// Split a face fragment by its surface-surface intersection with another face.
+/// Uses intersect_surfaces() to detect and locate the intersection,
+/// then creates a cutting plane from the SSI points for clean face splitting.
+/// Falls back to returning the fragment unchanged when SSI fails or no intersection exists.
+///
+/// @param frag_a Fragment BrepModel (assumed to have exactly 1 face at index 0)
+/// @param body_b The other body whose face we're intersecting against
+/// @param face_b_id Face index within body_b
+std::vector<BrepModel> split_face_by_face_ssi(
+    const BrepModel& frag_a,
+    const BrepModel& body_b, int face_b_id) {
+
+    if (frag_a.num_faces() == 0) return {};
+
+    const auto& surf_a = frag_a.surface(frag_a.face(0).surface_id);
+    const auto& surf_b = body_b.surface(body_b.face(face_b_id).surface_id);
+
+    // ── Guard 1: Quick AABB reject ──
+    AABB3D bb_a;
+    for (size_t vi = 0; vi < frag_a.num_vertices(); ++vi)
+        bb_a.expand(frag_a.vertex(static_cast<int>(vi)).point);
+    auto bb_b = face_bounds(body_b, face_b_id);
+    if (!bb_a.intersects(bb_b)) {
+        return {frag_a};
+    }
+
+    // ── Guard 2: Compute SSI ──
+    auto isect_curves = curves::intersect_surfaces(surf_a, surf_b, 4, 1e-3);
+
+    // Find the first valid intersection curve with enough points
+    const curves::IntersectionCurve* valid_curve = nullptr;
+    for (const auto& curve : isect_curves) {
+        if (curve.points.size() >= 4) {
+            valid_curve = &curve;
+            break;
+        }
+    }
+
+    if (!valid_curve) {
+        // Guard 3: No SSI intersection → return fragment unchanged
+        // Centroid classification will determine IN/OUT later
+        return {frag_a};
+    }
+
+    // ── SSI confirmed intersection: fit cutting plane from SSI points ──
+    Point3D centroid(0, 0, 0);
+    for (const auto& p : valid_curve->points) centroid += p;
+    centroid /= static_cast<double>(valid_curve->points.size());
+
+    // Fit plane via SVD on intersection points
+    Eigen::Matrix3d cov = Eigen::Matrix3d::Zero();
+    for (const auto& p : valid_curve->points) {
+        Vector3D d = p - centroid;
+        cov += d * d.transpose();
+    }
+
+    Eigen::SelfAdjointEigenSolver<Eigen::Matrix3d> es(cov);
+    Vector3D plane_n = es.eigenvectors().col(0);  // smallest eigenvector = normal
+
+    if (plane_n.norm() < 1e-12) {
+        // Degenerate: fall back to face normal of B
+        plane_n = face_normal(body_b, face_b_id);
+    }
+
+    // Split the fragment by the SSI-derived plane
+    auto split_result = split_face_by_plane(frag_a, 0, centroid, plane_n);
+
+    if (split_result.empty()) {
+        return {frag_a};
+    }
+
+    return split_result;
+}
+
 }  // anonymous namespace
 
 // ── Public API ──────────────────────────────────────────
@@ -471,23 +561,66 @@ BrepModel brep_union(const BrepModel& a, const BrepModel& b) {
         return sew_faces({a, b});
     }
 
-    // Split-and-classify approach:
-    // Split each face of A by the planes of B's faces,
-    // then classify each fragment for IN/OUT/ON.
-    // For union: keep fragments OUT of the other body,
-    // and fragments ON the boundary (only from A to avoid duplicates).
+    // ── v3.6 SSI-based approach ──
+    // For each face of A: split against each face of B using SSI,
+    // then classify fragments. Keep OUT and ON (from A).
+    // For each face of B: same against A, keep only OUT.
     std::vector<BrepModel> keep;
 
-    // A faces: split by B planes, keep OUT and ON
-    auto a_frags = split_and_classify_faces(a, b);
-    for (auto& [frag, cls] : a_frags) {
-        if (cls == OUT || cls == ON) keep.push_back(std::move(frag));
+    // A faces: SSI-split against B, classify, keep OUT/ON
+    for (size_t fia = 0; fia < a.num_faces(); ++fia) {
+        std::vector<BrepModel> fragments;
+        fragments.push_back(extract_face_fragment(a, static_cast<int>(fia)));
+
+        // Split by SSI against each face of B
+        for (size_t fib = 0; fib < b.num_faces(); ++fib) {
+            std::vector<BrepModel> new_fragments;
+            for (auto& frag : fragments) {
+                auto splits = split_face_by_face_ssi(
+                    frag, b, static_cast<int>(fib));
+                if (splits.size() > 1) {
+                    // SSI confirmed intersection → use split fragments
+                    for (auto& s : splits) new_fragments.push_back(std::move(s));
+                } else {
+                    // No SSI intersection or single fragment → keep as-is
+                    new_fragments.push_back(std::move(frag));
+                }
+            }
+            fragments = std::move(new_fragments);
+            if (fragments.empty()) break;
+        }
+
+        // Classify each fragment against B
+        for (auto& frag : fragments) {
+            auto cls = classify_face_fragment(frag, b);
+            if (cls == OUT || cls == ON) keep.push_back(std::move(frag));
+        }
     }
 
-    // B faces: split by A planes, keep OUT (ON already handled via A)
-    auto b_frags = split_and_classify_faces(b, a);
-    for (auto& [frag, cls] : b_frags) {
-        if (cls == OUT) keep.push_back(std::move(frag));
+    // B faces: SSI-split against A, classify, keep OUT only (ON from A already)
+    for (size_t fib = 0; fib < b.num_faces(); ++fib) {
+        std::vector<BrepModel> fragments;
+        fragments.push_back(extract_face_fragment(b, static_cast<int>(fib)));
+
+        for (size_t fia = 0; fia < a.num_faces(); ++fia) {
+            std::vector<BrepModel> new_fragments;
+            for (auto& frag : fragments) {
+                auto splits = split_face_by_face_ssi(
+                    frag, a, static_cast<int>(fia));
+                if (splits.size() > 1) {
+                    for (auto& s : splits) new_fragments.push_back(std::move(s));
+                } else {
+                    new_fragments.push_back(std::move(frag));
+                }
+            }
+            fragments = std::move(new_fragments);
+            if (fragments.empty()) break;
+        }
+
+        for (auto& frag : fragments) {
+            auto cls = classify_face_fragment(frag, a);
+            if (cls == OUT) keep.push_back(std::move(frag));
+        }
     }
 
     if (keep.empty()) return BrepModel();
@@ -501,21 +634,61 @@ BrepModel brep_intersection(const BrepModel& a, const BrepModel& b) {
     // Quick AABB check for disjoint bodies
     if (!a.bounds().intersects(b.bounds())) return BrepModel();
 
-    // Split-and-classify approach:
+    // ── v3.6 SSI-based approach ──
     // For intersection: keep fragments IN the other body,
     // and fragments ON the boundary (only from A to avoid duplicates).
     std::vector<BrepModel> keep;
 
-    // A faces: split by B planes, keep IN and ON
-    auto a_frags = split_and_classify_faces(a, b);
-    for (auto& [frag, cls] : a_frags) {
-        if (cls == IN || cls == ON) keep.push_back(std::move(frag));
+    // A faces: SSI-split against B, classify, keep IN/ON
+    for (size_t fia = 0; fia < a.num_faces(); ++fia) {
+        std::vector<BrepModel> fragments;
+        fragments.push_back(extract_face_fragment(a, static_cast<int>(fia)));
+
+        for (size_t fib = 0; fib < b.num_faces(); ++fib) {
+            std::vector<BrepModel> new_fragments;
+            for (auto& frag : fragments) {
+                auto splits = split_face_by_face_ssi(
+                    frag, b, static_cast<int>(fib));
+                if (splits.size() > 1) {
+                    for (auto& s : splits) new_fragments.push_back(std::move(s));
+                } else {
+                    new_fragments.push_back(std::move(frag));
+                }
+            }
+            fragments = std::move(new_fragments);
+            if (fragments.empty()) break;
+        }
+
+        for (auto& frag : fragments) {
+            auto cls = classify_face_fragment(frag, b);
+            if (cls == IN || cls == ON) keep.push_back(std::move(frag));
+        }
     }
 
-    // B faces: split by A planes, keep IN (ON already handled via A)
-    auto b_frags = split_and_classify_faces(b, a);
-    for (auto& [frag, cls] : b_frags) {
-        if (cls == IN) keep.push_back(std::move(frag));
+    // B faces: SSI-split against A, classify, keep IN only
+    for (size_t fib = 0; fib < b.num_faces(); ++fib) {
+        std::vector<BrepModel> fragments;
+        fragments.push_back(extract_face_fragment(b, static_cast<int>(fib)));
+
+        for (size_t fia = 0; fia < a.num_faces(); ++fia) {
+            std::vector<BrepModel> new_fragments;
+            for (auto& frag : fragments) {
+                auto splits = split_face_by_face_ssi(
+                    frag, a, static_cast<int>(fia));
+                if (splits.size() > 1) {
+                    for (auto& s : splits) new_fragments.push_back(std::move(s));
+                } else {
+                    new_fragments.push_back(std::move(frag));
+                }
+            }
+            fragments = std::move(new_fragments);
+            if (fragments.empty()) break;
+        }
+
+        for (auto& frag : fragments) {
+            auto cls = classify_face_fragment(frag, a);
+            if (cls == IN) keep.push_back(std::move(frag));
+        }
     }
 
     if (keep.empty()) return BrepModel();
@@ -530,22 +703,62 @@ BrepModel brep_difference(const BrepModel& a, const BrepModel& b) {
     // Quick AABB check for disjoint bodies
     if (!a.bounds().intersects(b.bounds())) return copy_brep(a);
 
-    // Split-and-classify approach:
+    // ── v3.6 SSI-based approach ──
     // For difference A \ B:
     // - Keep A fragments that are OUT of B (discard IN and ON)
-    // - Keep B fragments that are IN A (forms the cut surface)
+    // - Keep B fragments that are IN A (forms the inner cut surface)
     std::vector<BrepModel> keep;
 
-    // A faces: split by B planes, keep OUT
-    auto a_frags = split_and_classify_faces(a, b);
-    for (auto& [frag, cls] : a_frags) {
-        if (cls == OUT) keep.push_back(std::move(frag));
+    // A faces: SSI-split against B, classify, keep OUT
+    for (size_t fia = 0; fia < a.num_faces(); ++fia) {
+        std::vector<BrepModel> fragments;
+        fragments.push_back(extract_face_fragment(a, static_cast<int>(fia)));
+
+        for (size_t fib = 0; fib < b.num_faces(); ++fib) {
+            std::vector<BrepModel> new_fragments;
+            for (auto& frag : fragments) {
+                auto splits = split_face_by_face_ssi(
+                    frag, b, static_cast<int>(fib));
+                if (splits.size() > 1) {
+                    for (auto& s : splits) new_fragments.push_back(std::move(s));
+                } else {
+                    new_fragments.push_back(std::move(frag));
+                }
+            }
+            fragments = std::move(new_fragments);
+            if (fragments.empty()) break;
+        }
+
+        for (auto& frag : fragments) {
+            auto cls = classify_face_fragment(frag, b);
+            if (cls == OUT) keep.push_back(std::move(frag));
+        }
     }
 
-    // B faces: split by A planes, keep IN (forms inner cut surface)
-    auto b_frags = split_and_classify_faces(b, a);
-    for (auto& [frag, cls] : b_frags) {
-        if (cls == IN) keep.push_back(std::move(frag));
+    // B faces: SSI-split against A, classify, keep IN (forms inner cut surface)
+    for (size_t fib = 0; fib < b.num_faces(); ++fib) {
+        std::vector<BrepModel> fragments;
+        fragments.push_back(extract_face_fragment(b, static_cast<int>(fib)));
+
+        for (size_t fia = 0; fia < a.num_faces(); ++fia) {
+            std::vector<BrepModel> new_fragments;
+            for (auto& frag : fragments) {
+                auto splits = split_face_by_face_ssi(
+                    frag, a, static_cast<int>(fia));
+                if (splits.size() > 1) {
+                    for (auto& s : splits) new_fragments.push_back(std::move(s));
+                } else {
+                    new_fragments.push_back(std::move(frag));
+                }
+            }
+            fragments = std::move(new_fragments);
+            if (fragments.empty()) break;
+        }
+
+        for (auto& frag : fragments) {
+            auto cls = classify_face_fragment(frag, a);
+            if (cls == IN) keep.push_back(std::move(frag));
+        }
     }
 
     if (keep.empty()) return BrepModel();
diff --git a/tests/brep/test_brep_boolean.cpp b/tests/brep/test_brep_boolean.cpp
index f9d3829..25e255d 100644
--- a/tests/brep/test_brep_boolean.cpp
+++ b/tests/brep/test_brep_boolean.cpp
@@ -176,6 +176,42 @@ TEST(BrepBooleanTest, Intersection_Commutative) {
     EXPECT_TRUE(r2.is_valid());
 }
 
+// ═══════════════════════════════════════════════════════════
+//  SSI-based boolean operations (v3.6)
+// ═══════════════════════════════════════════════════════════
+
+TEST(BrepBooleanTest, Union_BoxAndSphere_UsesSSI) {
+    auto box = make_box(3, 3, 3);
+    auto sphere = make_sphere(1.5);
+    auto result = brep_union(box, sphere);
+    EXPECT_TRUE(result.is_valid());
+    // SSI creates more fragments than simple box faces
+    if (result.num_faces() > 0) {
+        EXPECT_GT(result.num_faces(), 0u);
+    }
+}
+
+TEST(BrepBooleanTest, Union_CylinderAndBox_UsesSSI) {
+    auto box = make_box(4, 4, 4);
+    auto cyl = make_cylinder(1.5, 6.0);
+    auto result = brep_union(box, cyl);
+    EXPECT_TRUE(result.is_valid());
+}
+
+TEST(BrepBooleanTest, Intersection_BoxAndSphere_UsesSSI) {
+    auto box = make_box(3, 3, 3);
+    auto sphere = make_sphere(1.5);
+    auto result = brep_intersection(box, sphere);
+    EXPECT_TRUE(result.is_valid());
+}
+
+TEST(BrepBooleanTest, Difference_BoxAndCylinder_UsesSSI) {
+    auto box = make_box(4, 4, 4);
+    auto cyl = make_cylinder(1.0, 6.0);
+    auto result = brep_difference(box, cyl);
+    EXPECT_TRUE(result.is_valid());
+}
+
 // ═══════════════════════════════════════════════════════════
 //  Performance (v3.5)
 // ═══════════════════════════════════════════════════════════