diff --git a/Plant3D2026/P_Rec_Annot/src/PRecAnnot.Abstractions/Contracts.cs b/Plant3D2026/P_Rec_Annot/src/PRecAnnot.Abstractions/Contracts.cs
index 8e3285f..64255be 100644
--- a/Plant3D2026/P_Rec_Annot/src/PRecAnnot.Abstractions/Contracts.cs
+++ b/Plant3D2026/P_Rec_Annot/src/PRecAnnot.Abstractions/Contracts.cs
@@ -21,6 +21,8 @@ public readonly record struct Point2Dto(double X, double Y);
 public readonly record struct Point3Dto(double X, double Y, double Z);
 public readonly record struct Vector2Dto(double X, double Y);
 public sealed record Bounds2Dto(Point2Dto Minimum, Point2Dto Maximum);
+public sealed record Bounds3Dto(Point3Dto Minimum, Point3Dto Maximum);
+public enum ExtentsIntersectionType { DoNotIntersect, Intersect, ExtentsMatch, FirstExtentsInsideSecondExtents, SecondExtentsInsideFirstExtents }
 
 public sealed record LineNumberAnnotationSettings
 {
diff --git a/Plant3D2026/P_Rec_Annot/src/PRecAnnot.Core/ExtentsIntersectionClassifier.cs b/Plant3D2026/P_Rec_Annot/src/PRecAnnot.Core/ExtentsIntersectionClassifier.cs
new file mode 100644
index 0000000..55393c4
--- /dev/null
+++ b/Plant3D2026/P_Rec_Annot/src/PRecAnnot.Core/ExtentsIntersectionClassifier.cs
@@ -0,0 +1,18 @@
+using PRecAnnot.Abstractions;
+
+namespace PRecAnnot.Core;
+
+public static class ExtentsIntersectionClassifier
+{
+    // Source-backed port of ExtentsIntersect.Commands without Autodesk types.
+    public static ExtentsIntersectionType Classify(Bounds3Dto first, Bounds3Dto second)
+    {
+        if (Same(first, second)) return ExtentsIntersectionType.ExtentsMatch;
+        if (Contains(first, second)) return ExtentsIntersectionType.SecondExtentsInsideFirstExtents;
+        if (Contains(second, first)) return ExtentsIntersectionType.FirstExtentsInsideSecondExtents;
+        return Overlaps(first, second) ? ExtentsIntersectionType.Intersect : ExtentsIntersectionType.DoNotIntersect;
+    }
+    private static bool Contains(Bounds3Dto outer, Bounds3Dto inner) => outer.Minimum.X <= inner.Minimum.X && outer.Minimum.Y <= inner.Minimum.Y && outer.Minimum.Z <= inner.Minimum.Z && outer.Maximum.X >= inner.Maximum.X && outer.Maximum.Y >= inner.Maximum.Y && outer.Maximum.Z >= inner.Maximum.Z;
+    private static bool Overlaps(Bounds3Dto a, Bounds3Dto b) => a.Minimum.X <= b.Maximum.X && a.Maximum.X >= b.Minimum.X && a.Minimum.Y <= b.Maximum.Y && a.Maximum.Y >= b.Minimum.Y && a.Minimum.Z <= b.Maximum.Z && a.Maximum.Z >= b.Minimum.Z;
+    private static bool Same(Bounds3Dto a, Bounds3Dto b) => a == b;
+}
diff --git a/Plant3D2026/P_Rec_Annot/tests/PRecAnnot.Core.Tests/ExtentsIntersectionClassifierTests.cs b/Plant3D2026/P_Rec_Annot/tests/PRecAnnot.Core.Tests/ExtentsIntersectionClassifierTests.cs
new file mode 100644
index 0000000..7038f7e
--- /dev/null
+++ b/Plant3D2026/P_Rec_Annot/tests/PRecAnnot.Core.Tests/ExtentsIntersectionClassifierTests.cs
@@ -0,0 +1,13 @@
+using PRecAnnot.Abstractions;
+using PRecAnnot.Core;
+namespace PRecAnnot.Core.Tests;
+public sealed class ExtentsIntersectionClassifierTests
+{
+    [Theory]
+    [InlineData(0, 0, 2, 2, ExtentsIntersectionType.ExtentsMatch)]
+    [InlineData(0, 0, 4, 4, ExtentsIntersectionType.FirstExtentsInsideSecondExtents)]
+    [InlineData(3, 3, 5, 5, ExtentsIntersectionType.DoNotIntersect)]
+    public void ClassifiesPlanarExtents(double min, double minY, double max, double maxY, ExtentsIntersectionType expected) => Assert.Equal(expected, ExtentsIntersectionClassifier.Classify(Box(0, 0, 2, 2), Box(min, minY, max, maxY)));
+    [Fact] public void TouchingExtents_Intersect() => Assert.Equal(ExtentsIntersectionType.Intersect, ExtentsIntersectionClassifier.Classify(Box(0, 0, 2, 2), Box(2, 1, 3, 2)));
+    private static Bounds3Dto Box(double x1, double y1, double x2, double y2) => new(new(x1, y1, 0), new(x2, y2, 1));
+}