diff --git a/docs/realtime-csg-doctrine.md b/docs/realtime-csg-doctrine.md
index 4815fa0..e6953d8 100644
--- a/docs/realtime-csg-doctrine.md
+++ b/docs/realtime-csg-doctrine.md
@@ -265,6 +265,12 @@ Algorithm migration steps:
 
 ## Acceptance Tests To Add
 
+- Where `vg_csg` exposes the same behavior as public RealtimeCSG/demo APIs,
+  maintain exact observable parity fixtures. These should compare public
+  behavior such as category vocabulary, brush plane/polygon counts, split
+  counts, visible/reversed semantics, bounds early-outs, and mesh output
+  invariants. Do not use vague "looks right" tests where a public seam has a
+  countable contract.
 - Two overlapping additive boxes produce no duplicate coplanar surface.
 - Subtraction is implemented as `!(!A || B)` and matches direct cases.
 - Intersection is implemented as `!(!A || !B)` and matches direct cases.
diff --git a/src/brush.rs b/src/brush.rs
index 2f04216..1004932 100644
--- a/src/brush.rs
+++ b/src/brush.rs
@@ -13,6 +13,14 @@ pub enum BrushOp {
     Intersect,
 }
 
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub enum PolygonCategory {
+    Inside,
+    Outside,
+    Aligned,
+    ReverseAligned,
+}
+
 #[derive(Clone, Copy, Debug, PartialEq)]
 pub struct Aabb {
     pub min: Vec3,
diff --git a/src/convex.rs b/src/convex.rs
index a00d810..f19235f 100644
--- a/src/convex.rs
+++ b/src/convex.rs
@@ -330,6 +330,35 @@ fn polygon_normal(vertices: &[Vec3]) -> Vec3 {
 mod tests {
     use super::*;
 
+    fn assert_box_parity(solid: &ConvexSolid, material: MaterialId) {
+        assert_eq!(solid.polygons.len(), 6);
+        assert_eq!(solid.clip_planes.len(), 6);
+        assert!(
+            solid
+                .polygons
+                .iter()
+                .all(|polygon| polygon.vertices.len() == 4)
+        );
+        assert!(
+            solid
+                .polygons
+                .iter()
+                .all(|polygon| polygon.material == material)
+        );
+    }
+
+    #[test]
+    fn parity_box_brush_from_planes_has_six_quad_polygons() {
+        let solid = ConvexSolid::box_from_center_size(
+            Vec3::ZERO,
+            Vec3::splat(2.0),
+            Quat::IDENTITY,
+            MaterialId(9),
+        );
+
+        assert_box_parity(&solid, MaterialId(9));
+    }
+
     #[test]
     fn subtract_center_box_emits_six_convex_fragments() {
         let source = ConvexSolid::box_from_center_size(
diff --git a/src/lib.rs b/src/lib.rs
index dae2d44..c47e809 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -14,7 +14,7 @@ mod mesh;
 mod primitives;
 
 pub use assembler::{Assembler, BuildOutput, BuildReport, BuildWarning};
-pub use brush::{Aabb, Brush, BrushId, BrushOp, MaterialId, Primitive};
+pub use brush::{Aabb, Brush, BrushId, BrushOp, MaterialId, PolygonCategory, Primitive};
 pub use convex::{ConvexPolygon, ConvexSolid, Plane};
 pub use dsl::LevelDsl;
 pub use mesh::TriangleMesh;
diff --git a/tests/realtime_csg_parity.rs b/tests/realtime_csg_parity.rs
new file mode 100644
index 0000000..9116e42
--- /dev/null
+++ b/tests/realtime_csg_parity.rs
@@ -0,0 +1,50 @@
+use bevy_math::{Quat, Vec3};
+use vg_csg::{Aabb, Assembler, ConvexSolid, MaterialId, PolygonCategory};
+
+#[test]
+fn parity_polygon_category_vocabulary_matches_public_demo() {
+    let categories = [
+        PolygonCategory::Inside,
+        PolygonCategory::Outside,
+        PolygonCategory::Aligned,
+        PolygonCategory::ReverseAligned,
+    ];
+
+    assert_eq!(categories.len(), 4);
+}
+
+#[test]
+fn parity_box_brush_observable_counts_match_realtime_csg_demo() {
+    let solid = ConvexSolid::box_from_center_size(
+        Vec3::ZERO,
+        Vec3::splat(2.0),
+        Quat::IDENTITY,
+        MaterialId(3),
+    );
+
+    assert_eq!(solid.clip_planes.len(), 6);
+    assert_eq!(solid.polygons.len(), 6);
+    assert_eq!(
+        solid
+            .polygons
+            .iter()
+            .map(|polygon| polygon.vertices.len())
+            .sum::<usize>(),
+        24
+    );
+}
+
+#[test]
+fn parity_center_box_subtraction_has_six_surviving_regions() {
+    let mut asm = Assembler::new();
+    asm.solid_box(
+        "source",
+        Aabb::from_center_size(Vec3::ZERO, Vec3::splat(4.0)),
+        MaterialId(1),
+    );
+    asm.cut_box("void", Aabb::from_center_size(Vec3::ZERO, Vec3::splat(2.0)));
+
+    let output = asm.build();
+    assert_eq!(output.report.emitted_convex_fragments, 6);
+    assert_eq!(output.report.warnings.len(), 0);
+}