Fix loading project library without IfcProject

Per IFC4+, IfcContext is the abstract supertype of IfcProject and
IfcProjectLibrary; library-only files legitimately contain only
IfcProjectLibrary as their root context. Bonsai assumed an IfcProject
was always present at three crash sites: the parent-library enum
(reported in #8183), RefreshLibrary's tree view, and AddProjectLibrary.

Introduce tool.Project.get_root_context() that prefers IfcProject and
falls back to IfcProjectLibrary, and route the three sites through it.
get_parent_library() now returns None for a root IfcProjectLibrary;
get_project_hierarchy() and the EditProjectLibrary parent-swap branch
handle that. AddProjectLibrary creates the nested sub-library via
IfcRelNests when the root is an IfcProjectLibrary, matching the
existing convention for library-under-library nesting.

For the separate "Open IFC Project" path, abort with a friendly error
pointing users to Project Setup -> Project Library -> Select Library
File instead of letting set_units() crash deep in the importer.

Closes #8183.

Partly generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/module/project/data.py

@@ -162,8 +162,8 @@ class ProjectLibraryData:
         library_file = IfcStore.library_file
         if library_file is None or library_file.schema == "IFC2X3":
             return results
-        project = library_file.by_type("IfcProject")[0]
-        results.append((str(project.id()), f"IfcProject {project.Name or 'Unnamed'}", project.Description or ""))
+        root = tool.Project.get_root_context(library_file)
+        results.append((str(root.id()), f"{root.is_a()} {root.Name or 'Unnamed'}", root.Description or ""))
         for library_id, data in cls.data["project_libraries"].items():
             results.append((str(library_id), data["Name"] or "Unnamed", data["Description"] or ""))
         return results

src/bonsai/bonsai/bim/module/project/operator.py

@@ -281,9 +281,9 @@ class RefreshLibrary(bpy.types.Operator):
         elements = {e for e in elements if not tool.Project.is_element_assigned_to_project_library(e, rels)}
         self.props.add_library_project_library("Unassigned", len(elements), 0, False)
 
-        ifc_project = library_file.by_type("IfcProject")[0]
+        root_context = tool.Project.get_root_context(library_file)
         hierarchy = tool.Project.get_project_hierarchy(library_file)
-        tool.Project.load_project_libraries_to_ui(ifc_project, hierarchy)
+        tool.Project.load_project_libraries_to_ui(root_context, hierarchy)
         return {"FINISHED"}
 
 
@@ -763,7 +763,10 @@ class EditProjectLibrary(bpy.types.Operator):
         previous_parent_library = tool.Project.get_parent_library(project_library)
         new_parent_library = library_file.by_id(int(props.parent_library))
         if previous_parent_library != new_parent_library:
-            if previous_parent_library.is_a("IfcProject"):
+            if previous_parent_library is None:
+                # Edited library was a root in a library-only file; nest it under the new parent.
+                ifcopenshell.api.nest.assign_object(library_file, [project_library], new_parent_library)
+            elif previous_parent_library.is_a("IfcProject"):
                 # Then new one is IfcProjectLibrary.
                 ifcopenshell.api.nest.assign_object(library_file, [project_library], new_parent_library)
             else:  # Previous is IfcProjectLibrary.
@@ -804,9 +807,12 @@ class AddProjectLibrary(bpy.types.Operator):
         props = tool.Project.get_project_props()
         library_file = IfcStore.library_file
         assert library_file
-        project = library_file.by_type("IfcProject")[0]
+        root_context = tool.Project.get_root_context(library_file)
         project_library = ifcopenshell.api.root.create_entity(library_file, "IfcProjectLibrary")
-        ifcopenshell.api.project.assign_declaration(library_file, [project_library], project)
+        if root_context.is_a("IfcProject"):
+            ifcopenshell.api.project.assign_declaration(library_file, [project_library], root_context)
+        else:
+            ifcopenshell.api.nest.assign_object(library_file, [project_library], root_context)
         ProjectLibraryData.load()  # Update enum.
         props.selected_project_library = str(project_library.id())
         props.is_editing_project_library = True
@@ -1113,6 +1119,14 @@ class LoadProject(bpy.types.Operator, IFCFileSelector, ImportHelper):
                     f"Error loading IFC file from filepath '{filepath}'. See logs above in the system console for the details.",
                 )
                 return {"CANCELLED"}
+            if not tool.Ifc.get().by_type("IfcProject"):
+                self.report(
+                    {"ERROR"},
+                    "This file contains no IfcProject. It is likely an IFC project library — "
+                    "load it via Project Setup → Project Library → Select Library File instead.",
+                )
+                IfcStore.purge()
+                return {"CANCELLED"}
             props = tool.Project.get_project_props()
             props.is_loading = True
             props.total_elements = len(tool.Ifc.get().by_type("IfcElement"))

src/bonsai/bonsai/bim/module/project/prop.py

@@ -98,7 +98,8 @@ def is_editing_project_library_update(self: "BIMProjectProperties", context: bpy
         project_library = library_file.by_id(int(self.selected_project_library))
         self.project_library_attributes.clear()
         bonsai.bim.helper.import_attributes(project_library, self.project_library_attributes)
-        self.parent_library = str(tool.Project.get_parent_library(project_library).id())
+        if parent_library := tool.Project.get_parent_library(project_library):
+            self.parent_library = str(parent_library.id())
         ProjectLibraryData.load()  # Show edit icon in enum.
         return
 

src/bonsai/bonsai/tool/project.py

@@ -32,6 +32,7 @@ from typing import (
     NotRequired,
     Optional,
     TypedDict,
+    Union,
 )
 
 import bpy
@@ -376,12 +377,31 @@ class Project(bonsai.core.tool.Project):
             )
 
     @classmethod
-    def get_parent_library(cls, project_library: ifcopenshell.entity_instance) -> ifcopenshell.entity_instance:
+    def get_parent_library(
+        cls, project_library: ifcopenshell.entity_instance
+    ) -> Union[ifcopenshell.entity_instance, None]:
+        """Return the IfcContext that declares or nests ``project_library``.
+
+        Returns ``None`` when ``project_library`` is itself the root of a
+        library-only file (no IfcRelNests, no IfcRelDeclares).
+        """
         if nests := project_library.Nests:
-            # IfcProjectLibrary.
             return nests[0].RelatingObject
-        # IfcProject.
-        return project_library.HasContext[0].RelatingContext
+        if has_context := project_library.HasContext:
+            return has_context[0].RelatingContext
+        return None
+
+    @classmethod
+    def get_root_context(cls, ifc_file: ifcopenshell.file) -> ifcopenshell.entity_instance:
+        """Return the file's root IfcContext.
+
+        Prefers IfcProject if present, otherwise falls back to IfcProjectLibrary —
+        library-only files are valid per IFC4+ and contain no IfcProject. Caller is
+        responsible for the IFC2X3 guard; IfcContext does not exist in that schema.
+        """
+        if projects := ifc_file.by_type("IfcProject"):
+            return projects[0]
+        return ifc_file.by_type("IfcProjectLibrary")[0]
 
     @classmethod
     def get_project_hierarchy(cls, ifc_file: ifcopenshell.file) -> HiearchyDict:
@@ -401,6 +421,8 @@ class Project(bonsai.core.tool.Project):
             return hierarchy
         for project_library in ifc_file.by_type("IfcProjectLibrary"):
             parent_library = cls.get_parent_library(project_library)
+            if parent_library is None:
+                continue
             hierarchy[parent_library][project_library] = hierarchy[project_library]
         return hierarchy
 

src/bonsai/test/bim/module/project/test_project_library_data.py

@@ -0,0 +1,121 @@
+# Bonsai - OpenBIM Blender Add-on
+# Copyright (C) 2026
+#
+# This file is part of Bonsai.
+#
+# Bonsai is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Bonsai 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 General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with Bonsai.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file was generated with the assistance of an AI coding tool.
+
+import ifcopenshell
+import ifcopenshell.api.nest
+import ifcopenshell.api.project
+import ifcopenshell.api.root
+import pytest
+
+import bonsai.tool as tool
+from bonsai.bim.ifc import IfcStore
+from bonsai.bim.module.project.data import ProjectLibraryData
+from test.bim.bootstrap import NewIfc
+
+pytestmark = pytest.mark.project
+
+
+def _make_library_only_file(*, with_child: bool = False) -> ifcopenshell.file:
+    """Build a minimal IFC4 file containing only an IfcProjectLibrary (no IfcProject).
+
+    Per IFC4+, a file must contain at least one IfcContext; IfcProjectLibrary is a
+    valid root on its own. ``with_child=True`` nests a sub-library under the root via
+    IfcRelNests, mirroring real authored library files.
+    """
+    library_file = ifcopenshell.api.project.create_file(version="IFC4")
+    root = ifcopenshell.api.root.create_entity(library_file, ifc_class="IfcProjectLibrary", name="RootLib")
+    if with_child:
+        child = ifcopenshell.api.root.create_entity(library_file, ifc_class="IfcProjectLibrary", name="ChildLib")
+        ifcopenshell.api.nest.assign_object(library_file, [child], root)
+    return library_file
+
+
+class TestLibraryOnlyFile(NewIfc):
+    def test_get_root_context_returns_project_library_when_no_project(self):
+        library_file = _make_library_only_file()
+        assert not library_file.by_type("IfcProject")
+
+        root = tool.Project.get_root_context(library_file)
+
+        assert root.is_a("IfcProjectLibrary")
+        assert root.Name == "RootLib"
+
+    def test_get_parent_library_returns_none_for_root_library(self):
+        library_file = _make_library_only_file()
+        root = library_file.by_type("IfcProjectLibrary")[0]
+
+        assert tool.Project.get_parent_library(root) is None
+
+    def test_get_project_hierarchy_skips_root_library(self):
+        library_file = _make_library_only_file(with_child=True)
+        root = next(lib for lib in library_file.by_type("IfcProjectLibrary") if lib.Name == "RootLib")
+        child = next(lib for lib in library_file.by_type("IfcProjectLibrary") if lib.Name == "ChildLib")
+
+        hierarchy = tool.Project.get_project_hierarchy(library_file)
+
+        assert root in hierarchy
+        assert child in hierarchy[root]
+
+    def test_project_library_data_loads_without_crash(self):
+        IfcStore.library_file = _make_library_only_file()
+        try:
+            ProjectLibraryData.is_loaded = False
+            ProjectLibraryData.load()
+            assert ProjectLibraryData.is_loaded
+            enum = ProjectLibraryData.data["parent_libraries_enum"]
+            assert len(enum) == 1
+            assert enum[0][1].startswith("IfcProjectLibrary ")
+        finally:
+            IfcStore.library_file = None
+            ProjectLibraryData.is_loaded = False
+
+    def test_refresh_library_succeeds_on_library_only_file(self):
+        import bpy
+
+        IfcStore.library_file = _make_library_only_file(with_child=True)
+        try:
+            result = bpy.ops.bim.refresh_library()
+            assert result == {"FINISHED"}
+        finally:
+            IfcStore.library_file = None
+            ProjectLibraryData.is_loaded = False
+
+    def test_add_project_library_nests_under_root_when_no_project(self):
+        import bpy
+
+        IfcStore.library_file = _make_library_only_file()
+        library_file = IfcStore.library_file
+        try:
+            root = library_file.by_type("IfcProjectLibrary")[0]
+            before = set(library_file.by_type("IfcProjectLibrary"))
+
+            result = bpy.ops.bim.add_project_library()
+
+            assert result == {"FINISHED"}
+            after = set(library_file.by_type("IfcProjectLibrary"))
+            new_libraries = after - before
+            assert len(new_libraries) == 1
+            new_library = next(iter(new_libraries))
+            assert new_library.Nests
+            assert new_library.Nests[0].RelatingObject == root
+            assert not new_library.HasContext
+        finally:
+            IfcStore.library_file = None
+            ProjectLibraryData.is_loaded = False