diff --git a/src/app/helpers.rs b/src/app/helpers.rs
index 5e084275e..d1b3d3fb7 100644
--- a/src/app/helpers.rs
+++ b/src/app/helpers.rs
@@ -611,3 +611,78 @@ pub(super) fn build_window_icon() -> Option<Vec<u8>> {
     resvg::render(&tree, transform, &mut pixmap.as_mut());
     Some(pixmap.take())
 }
+
+#[cfg(test)]
+mod ucs_from_normal_tests {
+    use super::ucs_from_normal;
+    use glam::DVec3;
+
+    fn axes(ucs: &acadrust::tables::Ucs) -> (DVec3, DVec3, DVec3) {
+        let x = DVec3::new(ucs.x_axis.x, ucs.x_axis.y, ucs.x_axis.z);
+        let y = DVec3::new(ucs.y_axis.x, ucs.y_axis.y, ucs.y_axis.z);
+        (x, y, x.cross(y))
+    }
+
+    /// The contract `UCS FACE` depends on: the plane's Z is the face normal it
+    /// was handed. If this drifts, sketches tilt off the face they were
+    /// started on.
+    #[test]
+    fn z_axis_is_the_supplied_normal() {
+        for normal in [
+            DVec3::Z,
+            DVec3::NEG_Z,
+            DVec3::X,
+            DVec3::Y,
+            DVec3::new(1.0, 2.0, 3.0).normalize(),
+        ] {
+            let ucs = ucs_from_normal(DVec3::ZERO, normal).expect("a normal defines a plane");
+            let (_, _, z) = axes(&ucs);
+            assert!(
+                (z - normal).length() < 1e-9,
+                "expected Z {normal:?}, got {z:?}"
+            );
+        }
+    }
+
+    /// Right-handed and unit length, or every coordinate read off the plane is
+    /// skewed.
+    #[test]
+    fn axes_are_orthonormal() {
+        for normal in [DVec3::Z, DVec3::new(-2.0, 0.5, 1.0).normalize()] {
+            let ucs = ucs_from_normal(DVec3::ZERO, normal).unwrap();
+            let (x, y, z) = axes(&ucs);
+            for (name, axis) in [("X", x), ("Y", y), ("Z", z)] {
+                assert!((axis.length() - 1.0).abs() < 1e-9, "{name} is not unit");
+            }
+            assert!(x.dot(y).abs() < 1e-9, "X and Y are not perpendicular");
+            assert!(x.dot(z).abs() < 1e-9, "X and Z are not perpendicular");
+        }
+    }
+
+    /// The 1/64 branch exists so a normal along world Z does not cross-product
+    /// with itself. Both sides of that branch must still produce a usable
+    /// plane.
+    #[test]
+    fn a_normal_along_world_z_still_yields_a_plane() {
+        let ucs = ucs_from_normal(DVec3::ZERO, DVec3::Z).expect("world Z is a valid normal");
+        let (x, _, _) = axes(&ucs);
+        assert!(x.length() > 0.5, "X degenerated next to the world Z axis");
+    }
+
+    /// The same face must always give the same X axis, or a sketch would spin
+    /// each time the UCS was rebuilt from it.
+    #[test]
+    fn the_same_normal_is_reproducible() {
+        let normal = DVec3::new(0.3, -0.7, 0.2).normalize();
+        let first = axes(&ucs_from_normal(DVec3::ZERO, normal).unwrap());
+        let second = axes(&ucs_from_normal(DVec3::ZERO, normal).unwrap());
+        assert!((first.0 - second.0).length() < 1e-12);
+    }
+
+    /// A zero vector defines no plane; it must be refused rather than
+    /// producing a degenerate UCS that silently misplaces geometry.
+    #[test]
+    fn a_degenerate_normal_is_rejected() {
+        assert!(ucs_from_normal(DVec3::ZERO, DVec3::ZERO).is_none());
+    }
+}
diff --git a/tests/ucs_face_plane.rs b/tests/ucs_face_plane.rs
new file mode 100644
index 000000000..57f49f0a3
--- /dev/null
+++ b/tests/ucs_face_plane.rs
@@ -0,0 +1,80 @@
+//! `UCS FACE` turns a pick on a solid into a drawing plane. The plane is only
+//! as good as the face lookup underneath it, so this covers that half: given a
+//! body and a point on one of its faces, does the right face come back, and is
+//! its outward normal the one a sketch should be built on?
+//!
+//! The axis construction on top of that normal is unit-tested beside
+//! `ucs_from_normal` in `src/app/helpers.rs`, which is `pub(crate)` and so out
+//! of reach from here.
+
+use acadrust::{entities::Circle, types::Vector3, EntityType};
+use OpenCADStudio::scene::model::{presspull_model, solid_model};
+
+/// Extrude a disc into a puck 10 units tall, centred on the origin.
+fn puck() -> cadkernel::brep::Body {
+    let entity = EntityType::Circle(Circle::from_center_radius(
+        Vector3::new(0.0, 0.0, 0.0),
+        100.0,
+    ));
+    let body = presspull_model::extrusion_body(&entity, [0.0, 0.0, 10.0])
+        .expect("a closed circular profile extrudes");
+    assert!(body.validate().is_empty(), "extrusion produced a valid body");
+    body
+}
+
+/// The two flat ends point away from the material, in opposite directions.
+/// Getting this wrong is what would make a sketch land inside the solid, or
+/// extrude back into it.
+#[test]
+fn opposite_faces_report_opposite_outward_normals() {
+    let body = puck();
+
+    let top = solid_model::nearest_planar_face(&body, [0.0, 0.0, 10.0])
+        .expect("the top cap is planar");
+    let bottom = solid_model::nearest_planar_face(&body, [0.0, 0.0, 0.0])
+        .expect("the bottom cap is planar");
+    assert_ne!(top, bottom, "the caps are distinct faces");
+
+    let top_normal = solid_model::planar_face_normal(&body, top).expect("top cap has a normal");
+    let bottom_normal =
+        solid_model::planar_face_normal(&body, bottom).expect("bottom cap has a normal");
+
+    assert!(
+        (top_normal[2] - 1.0).abs() < 1e-9,
+        "top cap should point +Z, got {top_normal:?}"
+    );
+    assert!(
+        (bottom_normal[2] + 1.0).abs() < 1e-9,
+        "bottom cap should point -Z, got {bottom_normal:?}"
+    );
+}
+
+/// A pick anywhere on a cap resolves to that cap, not merely to the one
+/// nearest the origin. A sketch is started by clicking somewhere on a face,
+/// rarely at its centre.
+#[test]
+fn an_off_centre_pick_still_lands_on_its_own_face() {
+    let body = puck();
+    let centre = solid_model::nearest_planar_face(&body, [0.0, 0.0, 10.0]).unwrap();
+    let off_centre = solid_model::nearest_planar_face(&body, [60.0, -25.0, 10.0]).unwrap();
+    assert_eq!(centre, off_centre, "both picks are on the top cap");
+}
+
+/// The curved side is not a plane, so it cannot become a sketch plane. The
+/// lookup has to decline rather than hand back a nonsense normal.
+#[test]
+fn the_curved_side_is_not_offered_as_a_plane() {
+    let body = puck();
+    // Halfway up the barrel, on its surface.
+    let face = solid_model::nearest_planar_face(&body, [100.0, 0.0, 5.0]);
+    if let Some(face) = face {
+        // A planar cap may still be the nearest planar face; what must never
+        // happen is the barrel itself being reported as planar.
+        let normal = solid_model::planar_face_normal(&body, face)
+            .expect("whatever came back claims to be planar, so it has a normal");
+        assert!(
+            normal[2].abs() > 1e-9,
+            "a cap normal has Z; a barrel wall wrongly called planar would not"
+        );
+    }
+}