Add geometry.clip_solid API and fix half_space_solid docstring

clip_solid(file, item, location, normal) wraps Clipping.apply() to
clip any solid with a half-space plane. Normal points toward the
removed material — same convention as add_wall_representation clippings.

Clarifies that agreement_flag=False means the normal direction is the
void (removed) side.

src/ifcopenshell-python/ifcopenshell/api/geometry/__init__.py

@@ -26,6 +26,7 @@ geometry extrusions).
 from .. import wrap_usecases
 from .add_axis_representation import add_axis_representation
 from .add_boolean import add_boolean
+from .clip_solid import clip_solid
 from .add_door_representation import add_door_representation
 from .add_footprint_representation import add_footprint_representation
 from .add_mesh_representation import add_mesh_representation
@@ -61,6 +62,7 @@ wrap_usecases(__path__, __name__)
 __all__ = [
     "add_axis_representation",
     "add_boolean",
+    "clip_solid",
     "add_door_representation",
     "add_footprint_representation",
     "add_mesh_representation",

src/ifcopenshell-python/ifcopenshell/api/geometry/clip_solid.py

@@ -0,0 +1,62 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2026 Dion Moult <dion@thinkmoult.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+from __future__ import annotations
+
+from typing import Sequence
+
+import ifcopenshell.util.unit
+from ifcopenshell.util.data import Clipping
+
+
+def clip_solid(
+    file: ifcopenshell.file,
+    item: ifcopenshell.entity_instance,
+    location: Sequence[float],
+    normal: Sequence[float],
+) -> ifcopenshell.entity_instance:
+    """Clip a solid with a half-space plane, returning an IfcBooleanClippingResult.
+
+    Convenience wrapper around :class:`ifcopenshell.util.data.Clipping` for
+    use with any solid.  The ``normal`` points toward the **removed** material
+    (the void side); the kept region is on the opposite side.  This is the same
+    convention used by the ``clippings`` parameter of :func:`add_wall_representation`.
+
+    After clipping, set the parent ``IfcShapeRepresentation``
+    ``RepresentationType`` to ``"Clipping"``.
+
+    Example — trim an extruded solid to a lean-to slope (removed material is
+    above the slope)::
+
+        bcr = ifcopenshell.api.run(
+            "geometry.clip_solid", model,
+            item=extrusion,
+            location=[0.0, 0.0, 3.26],
+            normal=[0.419, 0.0, 0.908],  # points UP toward removed material
+        )
+
+    :param item: The solid to clip (``IfcSweptAreaSolid``, ``IfcSweptDiskSolid``,
+        or ``IfcBooleanClippingResult``).
+    :param location: A point on the clipping plane in the representation's
+        local coordinate system.
+    :param normal: Plane normal pointing toward the material to be removed.
+    :return: The resulting ``IfcBooleanClippingResult``.
+    """
+    unit_scale = ifcopenshell.util.unit.calculate_unit_scale(file)
+    clipping = Clipping(location=tuple(location), normal=tuple(normal))
+    return clipping.apply(file, item, unit_scale)

src/ifcopenshell-python/ifcopenshell/util/shape_builder.py

@@ -1038,7 +1038,7 @@ class ShapeBuilder:
     ) -> ifcopenshell.entity_instance:
         """
         :param plane: The IfcPlane representing the half space.
-        :param agreement_flag: False if +Z represents the void
+        :param agreement_flag: If False (default), the plane normal points toward the **removed** material (the void). The kept region is on the opposite side from the normal.
         :return: IfcHalfSpaceSolid
         """
         return self.file.createIfcHalfSpaceSolid(plane, AgreementFlag=agreement_flag)

src/ifcopenshell-python/test/api/geometry/test_clip_solid.py

@@ -0,0 +1,104 @@
+# IfcOpenShell - IFC toolkit and geometry engine
+# Copyright (C) 2026 Dion Moult <dion@thinkmoult.com>
+#
+# This file is part of IfcOpenShell.
+#
+# IfcOpenShell is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IfcOpenShell is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IfcOpenShell.  If not, see <http://www.gnu.org/licenses/>.
+
+import ifcopenshell.api.geometry
+import ifcopenshell.util.shape_builder
+import test.bootstrap
+
+
+class TestClipSolid(test.bootstrap.IFC4):
+    def make_extrusion(self):
+        builder = ifcopenshell.util.shape_builder.ShapeBuilder(self.file)
+        rect = builder.rectangle(size=(1.0, 1.0))
+        return builder.extrude(rect, magnitude=4.0)
+
+    def test_returns_boolean_clipping_result(self):
+        extrusion = self.make_extrusion()
+        result = ifcopenshell.api.geometry.clip_solid(
+            self.file,
+            item=extrusion,
+            location=[0.0, 0.0, 3.0],
+            normal=[0.0, 0.0, 1.0],
+        )
+        assert result.is_a("IfcBooleanClippingResult")
+        assert result.Operator == "DIFFERENCE"
+
+    def test_first_operand_is_the_item(self):
+        extrusion = self.make_extrusion()
+        result = ifcopenshell.api.geometry.clip_solid(
+            self.file,
+            item=extrusion,
+            location=[0.0, 0.0, 3.0],
+            normal=[0.0, 0.0, 1.0],
+        )
+        assert result.FirstOperand == extrusion
+
+    def test_second_operand_is_half_space_solid(self):
+        extrusion = self.make_extrusion()
+        result = ifcopenshell.api.geometry.clip_solid(
+            self.file,
+            item=extrusion,
+            location=[0.0, 0.0, 3.0],
+            normal=[0.0, 0.0, 1.0],
+        )
+        assert result.SecondOperand.is_a("IfcHalfSpaceSolid")
+
+    def test_clip_plane_location_matches(self):
+        extrusion = self.make_extrusion()
+        result = ifcopenshell.api.geometry.clip_solid(
+            self.file,
+            item=extrusion,
+            location=[0.0, 0.0, 3.0],
+            normal=[0.0, 0.0, 1.0],
+        )
+        plane = result.SecondOperand.BaseSurface
+        coords = plane.Position.Location.Coordinates
+        assert list(coords) == [0.0, 0.0, 3.0]
+
+    def test_chaining_two_clips(self):
+        extrusion = self.make_extrusion()
+        first_clip = ifcopenshell.api.geometry.clip_solid(
+            self.file,
+            item=extrusion,
+            location=[0.0, 0.0, 3.0],
+            normal=[0.0, 0.0, 1.0],
+        )
+        second_clip = ifcopenshell.api.geometry.clip_solid(
+            self.file,
+            item=first_clip,
+            location=[0.0, 0.0, 1.0],
+            normal=[0.0, 0.0, -1.0],
+        )
+        assert second_clip.is_a("IfcBooleanClippingResult")
+        assert second_clip.FirstOperand == first_clip
+        assert first_clip.FirstOperand == extrusion
+
+    def test_angled_clip_plane(self):
+        extrusion = self.make_extrusion()
+        result = ifcopenshell.api.geometry.clip_solid(
+            self.file,
+            item=extrusion,
+            location=[0.0, 0.0, 3.26],
+            normal=[0.419, 0.0, 0.908],
+        )
+        assert result.is_a("IfcBooleanClippingResult")
+        assert result.SecondOperand.is_a("IfcHalfSpaceSolid")
+
+
+class TestClipSolidIFC2X3(test.bootstrap.IFC2X3, TestClipSolid):
+    pass