diff --git a/stacks/blender/README.md b/stacks/blender/README.md
index e7a0652..162aa84 100644
--- a/stacks/blender/README.md
+++ b/stacks/blender/README.md
@@ -224,4 +224,10 @@ clean non-manifold (4 → 0 non-manifold edges), Bool Tool auto difference, Loop
 wrong-axis control), 3MF out and back (20 mm cube kept its size). The one FAIL is CAD Sketcher,
 for the GUI-only reason above. These runs used the same docker invocation blender-run makes.
 
+**Round trip into build123d (draupnir, 2026-09-28):** `sp-patch-headless.step` imported on
+irv-ml1 with build123d 0.12.0 as one Shell holding one BSPLINE face, bbox ±10.000000 mm in X/Y at
+z = 0, area 399.999982 mm² against 400. Extruded 2 mm, it gave a valid 799.999964 mm³ solid. So a
+real NURBS surface reaches the kernel, not triangles, and millimetres hold across the hop. That is
+one run of a flat bilinear patch; curved and trimmed patches are untested.
+
 **MCP (GUI) path:** pending the stack deploy that mounts the repo and hook into the GUI container.