diff --git a/src/entities/curve.rs b/src/entities/curve.rs
index 6a083fca0..343f0bb2a 100644
--- a/src/entities/curve.rs
+++ b/src/entities/curve.rs
@@ -110,7 +110,7 @@ pub fn entity_curve(entity: &EntityType) -> Option<PlanarCurve> {
 /// Exact spatial source geometry for commands that traverse nonplanar curves.
 /// Curve construction and arc-length calculations remain in the kernel.
 pub fn entity_spatial_measurement(entity: &EntityType) -> Option<cadkernel::space::ArcLengthCurve3> {
-    use cadkernel::space::{ArcLengthCurve3, NurbsCurve3, Parameterization};
+    use cadkernel::space::{ArcLengthCurve3, NurbsCurve3};
     let curve = match entity {
         EntityType::Polyline3D(polyline) => {
             if polyline.flags.spline_fit {
@@ -132,24 +132,7 @@ pub fn entity_spatial_measurement(entity: &EntityType) -> Option<cadkernel::spac
         }
         EntityType::Spline(spline) => {
             if crate::entities::spline::uses_fit_method(spline) {
-                let mut points: Vec<_> = spline.fit_points.iter().copied().map(xyz).collect();
-                let parameterization = match spline.knot_parameterization {
-                    1 => Parameterization::Centripetal, 2 => Parameterization::Uniform,
-                    _ => Parameterization::Chord,
-                };
-                if spline.flags.periodic {
-                    NurbsCurve3::interpolate_periodic(&points, parameterization)?
-                } else {
-                    if spline.flags.closed && points.first() != points.last() {
-                        if let Some(first) = points.first().copied() { points.push(first); }
-                    }
-                    let tangent = |value: Vector3| {
-                        let value = xyz(value);
-                        (Vec3::from(value).length_squared() > 1e-18).then_some(value)
-                    };
-                    NurbsCurve3::interpolate_fit(&points, tangent(spline.begin_tangent),
-                        tangent(spline.end_tangent), parameterization)?
-                }
+                crate::entities::spline::fit_nurbs3(spline)?
             } else {
                 let controls: Vec<_> = spline.control_points.iter().copied().map(xyz).collect();
                 let weights = if spline.weights.is_empty() { vec![1.0; controls.len()] }
diff --git a/src/entities/spline.rs b/src/entities/spline.rs
index 5c143ea47..13bb5415c 100644
--- a/src/entities/spline.rs
+++ b/src/entities/spline.rs
@@ -21,6 +21,40 @@ pub(crate) fn shows_fit_points(spline: &Spline) -> bool {
     uses_fit_method(spline) && (!spline.cv_frame_visible || spline.flags.periodic)
 }
 
+pub(crate) fn fit_nurbs3(spline: &Spline) -> Option<NurbsCurve3> {
+    if !uses_fit_method(spline) {
+        return None;
+    }
+    let mut points: Vec<_> = spline.fit_points.iter()
+        .map(|point| [point.x, point.y, point.z]).collect();
+    let parameterization = match spline.knot_parameterization {
+        1 => Parameterization::Centripetal,
+        2 => Parameterization::Uniform,
+        _ => Parameterization::Chord,
+    };
+    if spline.flags.periodic {
+        return NurbsCurve3::interpolate_periodic(&points, parameterization);
+    }
+    if spline.flags.closed && points.first() != points.last() {
+        if let Some(first) = points.first().copied() { points.push(first); }
+    }
+    let tangent = |value: acadrust::types::Vector3| {
+        let value = [value.x, value.y, value.z];
+        (value.iter().map(|component| component * component).sum::<f64>() > 1e-18)
+            .then_some(value)
+    };
+    NurbsCurve3::interpolate_fit(&points, tangent(spline.begin_tangent),
+        tangent(spline.end_tangent), parameterization)
+}
+
+/// Conservative world bounds of a fit-method spline. Fit points are interpolation
+/// constraints, not control-hull vertices, so their box can exclude the curve.
+/// The shared kernel reconstructs the same spatial interpolation used by the
+/// curve consumers, then its control hull bounds every point of that curve.
+pub(crate) fn fit_geometry_bounds(spline: &Spline) -> Option<cadkernel::brep::Aabb> {
+    let curve = fit_nurbs3(spline)?;
+    cadkernel::brep::Aabb::around(curve.control_points().iter().copied())
+}
 fn to_render(spl: &Spline) -> RenderEntity {
     let n = spl.control_points.len();
     if n < 2 {
@@ -1095,3 +1129,33 @@ impl crate::entities::traits::Transformable for Spline {
         apply_transform(self, t);
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn fit_bounds_accept_closed_curves_with_unset_tangents() {
+        let mut spline = Spline::default();
+        spline.degree = 3;
+        spline.flags.closed = true;
+        spline.control_points.push(acadrust::types::Vector3::ZERO);
+        spline.fit_points = vec![
+            acadrust::types::Vector3::new(0.0, 0.0, 0.0),
+            acadrust::types::Vector3::new(4.0, 0.0, 1.0),
+            acadrust::types::Vector3::new(4.0, 3.0, 2.0),
+            acadrust::types::Vector3::new(0.0, 3.0, 1.0),
+        ];
+
+        let bounds = fit_geometry_bounds(&spline).unwrap();
+        for point in &spline.fit_points {
+            for (value, min, max) in [
+                (point.x, bounds.min[0], bounds.max[0]),
+                (point.y, bounds.min[1], bounds.max[1]),
+                (point.z, bounds.min[2], bounds.max[2]),
+            ] {
+                assert!(value >= min - 1e-9 && value <= max + 1e-9);
+            }
+        }
+    }
+}
diff --git a/src/scene/convert/tess.rs b/src/scene/convert/tess.rs
index a84d3d60d..db78e6930 100644
--- a/src/scene/convert/tess.rs
+++ b/src/scene/convert/tess.rs
@@ -1731,6 +1731,11 @@ fn entity_bounds(e: &acadrust::EntityType) -> ([f64; 3], [f64; 3]) {
             return (bounds.min, bounds.max);
         }
     }
+    if let acadrust::EntityType::Spline(spline) = e {
+        if let Some(bounds) = crate::entities::spline::fit_geometry_bounds(spline) {
+            return (bounds.min, bounds.max);
+        }
+    }
     let bounds = e.as_entity().bounding_box();
     (
         [bounds.min.x, bounds.min.y, bounds.min.z],