diff --git a/backend/toolshed/api/files.py b/backend/toolshed/api/files.py
index 7c2435c..9b3b8d0 100644
--- a/backend/toolshed/api/files.py
+++ b/backend/toolshed/api/files.py
@@ -4,12 +4,31 @@ from rest_framework.decorators import api_view, permission_classes, authenticati
 from rest_framework.permissions import IsAuthenticated
 from rest_framework.response import Response
 
-from authentication.signature_auth import SignatureAuthenticationLocal
+from authentication.signature_auth import SignatureAuthentication, SignatureAuthenticationLocal
 from files.models import File
 from files.serializers import FileSerializer
 from toolshed.models import InventoryItem, WorkflowInstance
 
 
+def _get_authorized_item(identity, item_id):
+    """Look up an item by its owner-scoped id and confirm identity may act on it - either as its
+    personal owner (requires a local ToolshedUser account) or as a current member of its owning
+    group (works for a remote member too, since group membership is identity-level, see
+    docs/design-in-progress/groups-mvp.md). id is only unique within one owner/group's own items,
+    so the lookup itself must be scoped rather than a bare global get. Returns None if not found
+    or not authorized, the same shape InventoryItem.DoesNotExist handling around it already
+    expects."""
+    if identity.user.exists():
+        try:
+            return InventoryItem.objects.get(owner=identity.user.get(), id=item_id)
+        except InventoryItem.DoesNotExist:
+            pass
+    try:
+        return InventoryItem.objects.get(owner_group__in=identity.member_of_groups.all(), id=item_id)
+    except InventoryItem.DoesNotExist:
+        return None
+
+
 @api_view(['GET'])
 @permission_classes([IsAuthenticated])
 @authentication_classes([SignatureAuthenticationLocal])
@@ -19,35 +38,37 @@ def list_all_files(request, format=None):  # /files/
 
 
 def get_item_files(request, item_id):
-    try:
-        item = InventoryItem.objects.get(id=item_id, owner=request.user)
-        files = item.files.all()
-        return Response(FileSerializer(files, many=True).data)
-    except InventoryItem.DoesNotExist:
+    item = _get_authorized_item(request.user, item_id)
+    if item is None:
         return Response(status=status.HTTP_404_NOT_FOUND)
+    files = item.files.all()
+    return Response(FileSerializer(files, many=True).data)
 
 
 def post_item_file(request, item_id):
-    try:
-        item = InventoryItem.objects.get(id=item_id, owner=request.user)
-        if 'file_hash' in request.data:
-            # Attach a file the caller already staged on one of their own workflows, identified
-            # by its content hash (which the client already computed before ever uploading it),
-            # instead of re-uploading bytes that are already stored server-side.
-            try:
-                file = File.objects.get(hash=request.data['file_hash'], staged_by_workflows__owner=request.user)
-            except File.DoesNotExist:
-                return Response(status=status.HTTP_404_NOT_FOUND)
-            item.files.add(file)
-            return Response(FileSerializer(file).data, status=status.HTTP_201_CREATED)
-        serializer = FileSerializer(data=request.data)
-        if serializer.is_valid():
-            file = serializer.save()
-            item.files.add(file)
-            return Response(serializer.data, status=status.HTTP_201_CREATED)
-        return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
-    except InventoryItem.DoesNotExist:
+    item = _get_authorized_item(request.user, item_id)
+    if item is None:
         return Response(status=status.HTTP_404_NOT_FOUND)
+    if 'file_hash' in request.data:
+        # Attach a file the caller already staged on one of their own workflows, identified
+        # by its content hash (which the client already computed before ever uploading it),
+        # instead of re-uploading bytes that are already stored server-side. Workflows are
+        # always personally owned, so this only applies to a caller with a local account.
+        if not request.user.user.exists():
+            return Response(status=status.HTTP_404_NOT_FOUND)
+        try:
+            file = File.objects.get(hash=request.data['file_hash'],
+                                    staged_by_workflows__owner=request.user.user.get())
+        except File.DoesNotExist:
+            return Response(status=status.HTTP_404_NOT_FOUND)
+        item.files.add(file)
+        return Response(FileSerializer(file).data, status=status.HTTP_201_CREATED)
+    serializer = FileSerializer(data=request.data)
+    if serializer.is_valid():
+        file = serializer.save()
+        item.files.add(file)
+        return Response(serializer.data, status=status.HTTP_201_CREATED)
+    return Response(serializer.errors, status=status.HTTP_400_BAD_REQUEST)
 
 
 def get_staged_files(request, workflow_id):
@@ -77,7 +98,7 @@ def post_staged_file(request, workflow_id):
 
 @api_view(['POST', 'GET'])
 @permission_classes([IsAuthenticated])
-@authentication_classes([SignatureAuthenticationLocal])
+@authentication_classes([SignatureAuthentication])
 def item_files(request, item_id, format=None):  # /item_files/
     if request.method == 'GET':
         return get_item_files(request, item_id)
@@ -87,21 +108,21 @@ def item_files(request, item_id, format=None):  # /item_files/
 
 @api_view(['DELETE'])
 @permission_classes([IsAuthenticated])
-@authentication_classes([SignatureAuthenticationLocal])
+@authentication_classes([SignatureAuthentication])
 def delete_item_file(request, item_id, file_id, format=None):  # /item_files/
-    try:
-        item = InventoryItem.objects.get(id=item_id, owner=request.user)
-        file = item.files.get(id=file_id)
-        item.files.remove(file_id)
-        if file.connected_items.count() == 0 and file.profile_picture_users.count() == 0 \
-                and file.staged_by_workflows.count() == 0:
-            file.file.delete(save=False)
-            file.delete()
-        return Response(status=status.HTTP_204_NO_CONTENT)
-    except InventoryItem.DoesNotExist:
+    item = _get_authorized_item(request.user, item_id)
+    if item is None:
         return Response(status=status.HTTP_404_NOT_FOUND)
+    try:
+        file = item.files.get(id=file_id)
     except File.DoesNotExist:
         return Response(status=status.HTTP_404_NOT_FOUND)
+    item.files.remove(file_id)
+    if file.connected_items.count() == 0 and file.profile_picture_users.count() == 0 \
+            and file.staged_by_workflows.count() == 0:
+        file.file.delete(save=False)
+        file.delete()
+    return Response(status=status.HTTP_204_NO_CONTENT)
 
 
 @api_view(['POST', 'GET'])
diff --git a/backend/toolshed/api/inventory.py b/backend/toolshed/api/inventory.py
index cd065f1..c0c119c 100644
--- a/backend/toolshed/api/inventory.py
+++ b/backend/toolshed/api/inventory.py
@@ -35,6 +35,12 @@ class InventoryItemViewSet(viewsets.ModelViewSet):
     serializer_class = InventoryItemSerializer
     authentication_classes = [SignatureAuthentication]
     permission_classes = [IsAuthenticated]
+    # Detail routes address an item by its owner-scoped id, not the internal row id - the
+    # router still names the URL capture group 'pk', so keep that as lookup_url_kwarg and just
+    # change which model field it's matched against. get_queryset() below is always already
+    # scoped to the requester's own items/groups, so this can't cross into another owner's ids.
+    lookup_field = 'id'
+    lookup_url_kwarg = 'pk'
 
     def get_queryset(self):
         # A KnownIdentity acting purely as a group member (e.g. a remote member on a group
diff --git a/backend/toolshed/migrations/0013_owneritemsequence.py b/backend/toolshed/migrations/0013_owneritemsequence.py
new file mode 100644
index 0000000..24ba6fa
--- /dev/null
+++ b/backend/toolshed/migrations/0013_owneritemsequence.py
@@ -0,0 +1,37 @@
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('authentication', '0005_group_groupinviteincoming_groupinvite_and_more'),
+        ('toolshed', '0012_inventoryitem_owner_group_alter_inventoryitem_owner'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='OwnerItemSequence',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('last_id', models.PositiveIntegerField(default=0)),
+                ('owner', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE,
+                                            related_name='+', to=settings.AUTH_USER_MODEL)),
+                ('owner_group', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.CASCADE,
+                                                  related_name='+', to='authentication.group')),
+            ],
+        ),
+        migrations.AddConstraint(
+            model_name='owneritemsequence',
+            constraint=models.UniqueConstraint(condition=models.Q(('owner__isnull', False)), fields=('owner',),
+                                               name='owneritemsequence_unique_owner'),
+        ),
+        migrations.AddConstraint(
+            model_name='owneritemsequence',
+            constraint=models.UniqueConstraint(condition=models.Q(('owner_group__isnull', False)),
+                                               fields=('owner_group',),
+                                               name='owneritemsequence_unique_owner_group'),
+        ),
+    ]
diff --git a/backend/toolshed/migrations/0014_rename_id_inventoryitem_internal_id.py b/backend/toolshed/migrations/0014_rename_id_inventoryitem_internal_id.py
new file mode 100644
index 0000000..4867623
--- /dev/null
+++ b/backend/toolshed/migrations/0014_rename_id_inventoryitem_internal_id.py
@@ -0,0 +1,21 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('toolshed', '0013_owneritemsequence'),
+    ]
+
+    operations = [
+        migrations.RenameField(
+            model_name='inventoryitem',
+            old_name='id',
+            new_name='internal_id',
+        ),
+        migrations.AlterField(
+            model_name='inventoryitem',
+            name='internal_id',
+            field=models.AutoField(primary_key=True, serialize=False),
+        ),
+    ]
diff --git a/backend/toolshed/migrations/0015_inventoryitem_id.py b/backend/toolshed/migrations/0015_inventoryitem_id.py
new file mode 100644
index 0000000..d34c34b
--- /dev/null
+++ b/backend/toolshed/migrations/0015_inventoryitem_id.py
@@ -0,0 +1,16 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('toolshed', '0014_rename_id_inventoryitem_internal_id'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='inventoryitem',
+            name='id',
+            field=models.PositiveIntegerField(editable=False, null=True),
+        ),
+    ]
diff --git a/backend/toolshed/migrations/0016_backfill_inventoryitem_id.py b/backend/toolshed/migrations/0016_backfill_inventoryitem_id.py
new file mode 100644
index 0000000..0c02816
--- /dev/null
+++ b/backend/toolshed/migrations/0016_backfill_inventoryitem_id.py
@@ -0,0 +1,42 @@
+from django.db import migrations
+
+
+def collapse_ids(apps, schema_editor):
+    """Collapse each owner/owner_group's InventoryItem ids from the sparse global range they
+    had before this migration down to a continuous 1..N range, in original creation order
+    (internal_id order), including soft-deleted rows since they still occupy a slot in that
+    scope's history. Then seed OwnerItemSequence so future allocation continues right after."""
+    InventoryItem = apps.get_model('toolshed', 'InventoryItem')
+    OwnerItemSequence = apps.get_model('toolshed', 'OwnerItemSequence')
+
+    scope = None
+    next_id = 0
+    counts = {}
+    for item in InventoryItem.objects.order_by('owner_id', 'owner_group_id', 'internal_id'):
+        key = (item.owner_id, item.owner_group_id)
+        if key != scope:
+            scope = key
+            next_id = 0
+        next_id += 1
+        item.id = next_id
+        item.save(update_fields=['id'])
+        counts[key] = next_id
+
+    for (owner_id, owner_group_id), count in counts.items():
+        OwnerItemSequence.objects.update_or_create(
+            owner_id=owner_id, owner_group_id=owner_group_id, defaults={'last_id': count})
+
+
+def noop_reverse(apps, schema_editor):
+    pass
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('toolshed', '0015_inventoryitem_id'),
+    ]
+
+    operations = [
+        migrations.RunPython(collapse_ids, noop_reverse),
+    ]
diff --git a/backend/toolshed/migrations/0017_inventoryitem_id_not_null_and_unique.py b/backend/toolshed/migrations/0017_inventoryitem_id_not_null_and_unique.py
new file mode 100644
index 0000000..a019156
--- /dev/null
+++ b/backend/toolshed/migrations/0017_inventoryitem_id_not_null_and_unique.py
@@ -0,0 +1,21 @@
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('toolshed', '0016_backfill_inventoryitem_id'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='inventoryitem',
+            name='id',
+            field=models.PositiveIntegerField(editable=False),
+        ),
+        migrations.AddConstraint(
+            model_name='inventoryitem',
+            constraint=models.UniqueConstraint(fields=('owner', 'owner_group', 'id'),
+                                               name='inventoryitem_unique_owner_scoped_id'),
+        ),
+    ]
diff --git a/backend/toolshed/models.py b/backend/toolshed/models.py
index 5ac047d..d02de2b 100644
--- a/backend/toolshed/models.py
+++ b/backend/toolshed/models.py
@@ -1,4 +1,4 @@
-from django.db import models
+from django.db import models, transaction
 from django.core.validators import MinValueValidator, MaxValueValidator
 from django_softdelete.models import SoftDeleteModel
 from rest_framework.exceptions import ValidationError
@@ -82,6 +82,31 @@ class Tag(models.Model):
         return f"{self.origin}#tag:{self.name}"
 
 
+class OwnerItemSequence(models.Model):
+    """Tracks the last InventoryItem id handed out to a given owner or owner_group, so ids can
+    be allocated sequentially and without gaps within that scope (see InventoryItem.create_for_owner).
+    Exactly one of owner/owner_group is set, mirroring InventoryItem's own owner/owner_group split."""
+    owner = models.ForeignKey(ToolshedUser, on_delete=models.CASCADE, null=True, blank=True, related_name='+')
+    owner_group = models.ForeignKey(Group, on_delete=models.CASCADE, null=True, blank=True, related_name='+')
+    last_id = models.PositiveIntegerField(default=0)
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(fields=['owner'], condition=models.Q(owner__isnull=False),
+                                    name='owneritemsequence_unique_owner'),
+            models.UniqueConstraint(fields=['owner_group'], condition=models.Q(owner_group__isnull=False),
+                                    name='owneritemsequence_unique_owner_group'),
+        ]
+
+    @classmethod
+    def allocate(cls, *, owner=None, owner_group=None):
+        with transaction.atomic():
+            seq, _ = cls.objects.select_for_update().get_or_create(owner=owner, owner_group=owner_group)
+            seq.last_id += 1
+            seq.save(update_fields=['last_id'])
+            return seq.last_id
+
+
 class InventoryItem(SoftDeleteModel):
     AVAILABILITY_POLICY_CHOICES = (
         ('sell', 'Sell'),
@@ -91,6 +116,11 @@ class InventoryItem(SoftDeleteModel):
         ('private', 'Private'),
     )
 
+    internal_id = models.AutoField(primary_key=True)
+    # The externally visible identifier: sequential and gapless within owner/owner_group's own
+    # items (see OwnerItemSequence), never the internal_id above. Always allocate through
+    # create_for_owner rather than InventoryItem.objects.create() directly.
+    id = models.PositiveIntegerField(editable=False)
     published = models.BooleanField(default=False)
     name = models.CharField(max_length=255, null=True, blank=True)
     description = models.TextField(null=True, blank=True)
@@ -108,12 +138,26 @@ class InventoryItem(SoftDeleteModel):
     storage_location = models.ForeignKey('StorageLocation', on_delete=models.SET_NULL, null=True, blank=True,
                                          related_name='inventory_items')
 
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(fields=['owner', 'owner_group', 'id'],
+                                    name='inventoryitem_unique_owner_scoped_id'),
+        ]
+
     def clean(self):
         if (self.name is None or self.name == "") and self.files.count() == 0:
             raise ValidationError("Name or at least one file must be set")
         if (self.owner is None) == (self.owner_group is None):
             raise ValidationError("Exactly one of owner or owner_group must be set")
 
+    @classmethod
+    def create_for_owner(cls, *, owner=None, owner_group=None, **kwargs):
+        """The only supported way to create an InventoryItem: allocates the next id for this
+        owner/owner_group scope and creates the item with it, atomically."""
+        with transaction.atomic():
+            next_id = OwnerItemSequence.allocate(owner=owner, owner_group=owner_group)
+            return cls.objects.create(owner=owner, owner_group=owner_group, id=next_id, **kwargs)
+
 
 class ItemProperty(models.Model):
     property = models.ForeignKey(Property, on_delete=models.CASCADE)
diff --git a/backend/toolshed/offlinedata.py b/backend/toolshed/offlinedata.py
index f0704cb..10cde6e 100644
--- a/backend/toolshed/offlinedata.py
+++ b/backend/toolshed/offlinedata.py
@@ -638,7 +638,7 @@ def import_inventory(user, data, available_files):
                 except (TypeError, ValueError):
                     owned_quantity = 1
 
-                item = InventoryItem.objects.create(
+                item = InventoryItem.create_for_owner(
                     owner=user,
                     name=name or None,
                     description=row.get('description', '') or '',
diff --git a/backend/toolshed/serializers.py b/backend/toolshed/serializers.py
index da31585..3cf9a03 100644
--- a/backend/toolshed/serializers.py
+++ b/backend/toolshed/serializers.py
@@ -200,6 +200,7 @@ class InventoryItemSerializer(serializers.ModelSerializer):
         model = InventoryItem
         fields = ['id', 'name', 'description', 'owner', 'owner_group', 'category', 'availability_policy',
                   'owned_quantity', 'tags', 'tags_input', 'properties', 'files', 'storage_location']
+        read_only_fields = ['id']
 
     def get_tags(self, obj):
         return [tag.name for tag in obj.tags.all()]
@@ -216,7 +217,7 @@ class InventoryItemSerializer(serializers.ModelSerializer):
         tags = validated_data.pop('tags', [])
         props = validated_data.pop('itemproperty_set', [])
         files = validated_data.pop('files', [])
-        item = InventoryItem.objects.create(**validated_data)
+        item = InventoryItem.create_for_owner(**validated_data)
         for tag in tags:
             item.tags.add(tag, through_defaults={})
         for prop in props:
diff --git a/backend/toolshed/tests/fixtures.py b/backend/toolshed/tests/fixtures.py
index 79b573d..942de7e 100644
--- a/backend/toolshed/tests/fixtures.py
+++ b/backend/toolshed/tests/fixtures.py
@@ -35,10 +35,10 @@ class InventoryTestMixin(CategoryTestMixin, TagTestMixin, PropertyTestMixin):
     def prepare_inventory(self):
         self.f['local_user1'].friends.add(self.f['local_user2'].public_identity)
 
-        self.f['item1'] = InventoryItem.objects.create(
+        self.f['item1'] = InventoryItem.create_for_owner(
             owner=self.f['local_user1'], owned_quantity=1, name='test1', description='test', category=self.f['cat1'],
             availability_policy='friends')
-        self.f['item2'] = InventoryItem.objects.create(
+        self.f['item2'] = InventoryItem.create_for_owner(
             owner=self.f['local_user1'], owned_quantity=1, name='test2', description='test2', category=self.f['cat1'],
             availability_policy='friends')
         self.f['item2'].tags.add(self.f['tag1'], through_defaults={})
diff --git a/backend/toolshed/tests/test_files.py b/backend/toolshed/tests/test_files.py
index ba9deec..a6877e5 100644
--- a/backend/toolshed/tests/test_files.py
+++ b/backend/toolshed/tests/test_files.py
@@ -1,7 +1,7 @@
 from django.test import Client
-from authentication.tests import SignatureAuthClient, UserTestMixin, ToolshedTestCase
+from authentication.tests import SignatureAuthClient, UserTestMixin, GroupTestMixin, ToolshedTestCase
 from files.tests import FilesTestMixin
-from toolshed.models import File
+from toolshed.models import File, InventoryItem
 
 from toolshed.tests import InventoryTestMixin
 
@@ -156,3 +156,60 @@ class FileApiTestCase(UserTestMixin, FilesTestMixin, InventoryTestMixin, Toolshe
         self.assertEqual(reply.json()[0]['files'][0]['mime_type'], 'text/plain')
         self.assertEqual(reply.json()[0]['files'][1]['mime_type'], 'text/plain')
         self.assertEqual(reply.json()[1]['files'][0]['mime_type'], 'text/plain')
+
+
+class GroupOwnedFileApiTestCase(UserTestMixin, GroupTestMixin, FilesTestMixin, ToolshedTestCase):
+    def setUp(self):
+        super().setUp()
+        self.prepare_users()
+        self.prepare_groups()
+        self.prepare_files()
+        self.f['group1'].members.add(self.f['local_user2'].public_identity)
+        self.f['group_item'] = InventoryItem.create_for_owner(
+            owner_group=self.f['group1'], owned_quantity=1, name='group-drill', availability_policy='private')
+        self.f['group_item'].files.add(self.f['test_file1'])
+
+    def test_get_group_item_files(self):
+        response = client.get(f"/api/item_files/{self.f['group_item'].id}/", self.f['local_user1'])
+        self.assertEqual(response.status_code, 200)
+        self.assertEqual(len(response.json()), 1)
+
+    def test_other_member_can_post_file(self):
+        response = client.post(f"/api/item_files/{self.f['group_item'].id}/", self.f['local_user2'],
+                               {'data': self.f['encoded_content4'], 'mime_type': 'text/plain'})
+        self.assertEqual(response.status_code, 201)
+        self.assertEqual(self.f['group_item'].files.count(), 2)
+
+    def test_remote_member_without_local_account_can_post_file(self):
+        self.f['group1'].members.add(self.f['ext_user1'].public_identity)
+        response = client.post(f"/api/item_files/{self.f['group_item'].id}/", self.f['ext_user1'],
+                               {'data': self.f['encoded_content4'], 'mime_type': 'text/plain'})
+        self.assertEqual(response.status_code, 201)
+        self.assertEqual(self.f['group_item'].files.count(), 2)
+
+    def test_remote_member_without_local_account_can_get_files(self):
+        self.f['group1'].members.add(self.f['ext_user1'].public_identity)
+        response = client.get(f"/api/item_files/{self.f['group_item'].id}/", self.f['ext_user1'])
+        self.assertEqual(response.status_code, 200)
+
+    def test_non_member_cannot_post_file(self):
+        response = client.post(f"/api/item_files/{self.f['group_item'].id}/", self.f['ext_user1'],
+                               {'data': self.f['encoded_content4'], 'mime_type': 'text/plain'})
+        self.assertEqual(response.status_code, 404)
+        self.assertEqual(self.f['group_item'].files.count(), 1)
+
+    def test_non_member_cannot_get_files(self):
+        response = client.get(f"/api/item_files/{self.f['group_item'].id}/", self.f['ext_user1'])
+        self.assertEqual(response.status_code, 404)
+
+    def test_other_member_can_delete_file(self):
+        response = client.delete(f"/api/item_files/{self.f['group_item'].id}/{self.f['test_file1'].id}/",
+                                 self.f['local_user2'])
+        self.assertEqual(response.status_code, 204)
+        self.assertEqual(self.f['group_item'].files.count(), 0)
+
+    def test_non_member_cannot_delete_file(self):
+        response = client.delete(f"/api/item_files/{self.f['group_item'].id}/{self.f['test_file1'].id}/",
+                                 self.f['ext_user1'])
+        self.assertEqual(response.status_code, 404)
+        self.assertEqual(self.f['group_item'].files.count(), 1)
diff --git a/backend/toolshed/tests/test_inventory.py b/backend/toolshed/tests/test_inventory.py
index d572b52..dc608ba 100644
--- a/backend/toolshed/tests/test_inventory.py
+++ b/backend/toolshed/tests/test_inventory.py
@@ -1,7 +1,8 @@
+from authentication.models import Group
 from authentication.tests import SignatureAuthClient, UserTestMixin, GroupTestMixin, ToolshedTestCase
 from files.tests import FilesTestMixin
 from toolshed.models import InventoryItem, Category
-from toolshed.tests import InventoryTestMixin
+from toolshed.tests import InventoryTestMixin, CategoryTestMixin, TagTestMixin, PropertyTestMixin, LocationTestMixin
 
 client = SignatureAuthClient()
 
@@ -232,7 +233,7 @@ class InventoryApiTestCase(UserTestMixin, InventoryTestMixin, ToolshedTestCase):
         self.assertEqual(reply.status_code, 403)
 
     def test_get_shared_item_private(self):
-        private_item = InventoryItem.objects.create(
+        private_item = InventoryItem.create_for_owner(
             owner=self.f['local_user1'], owned_quantity=1, name='secret', availability_policy='private')
         reply = client.get('/api/inventory_items/testuser1@example.com/' + str(private_item.id) + '/',
                            self.f['local_user2'])
@@ -336,11 +337,17 @@ class TestInventoryItemWithFileApiTestCase(UserTestMixin, FilesTestMixin, Invent
         self.assertEqual(reply.status_code, 400)
 
 
-class GroupOwnedInventoryApiTestCase(UserTestMixin, GroupTestMixin, ToolshedTestCase):
+class GroupOwnedInventoryApiTestCase(UserTestMixin, GroupTestMixin, CategoryTestMixin, TagTestMixin,
+                                     PropertyTestMixin, FilesTestMixin, LocationTestMixin, ToolshedTestCase):
     def setUp(self):
         super().setUp()
         self.prepare_users()
         self.prepare_groups()
+        self.prepare_categories()
+        self.prepare_tags()
+        self.prepare_properties()
+        self.prepare_files()
+        self.prepare_locations()
         self.f['group1'].members.add(self.f['local_user2'].public_identity)
 
     def create_group_item(self, name='drill'):
@@ -416,4 +423,162 @@ class GroupOwnedInventoryApiTestCase(UserTestMixin, GroupTestMixin, ToolshedTest
         self.create_group_item()
         reply = client.get('/api/inventory_items/?group={}'.format(self.f['group1'].id), self.f['ext_user1'])
         self.assertEqual(reply.status_code, 200)
-        self.assertEqual(len(reply.json()), 0)
\ No newline at end of file
+        self.assertEqual(len(reply.json()), 0)
+
+    def test_create_group_owned_item_with_full_fields(self):
+        # Parity with InventoryApiTestCase.test_post_new_item - tags/properties/category must
+        # attach to a group-owned item exactly the same way they do for a personal one.
+        reply = client.post('/api/inventory_items/', self.f['local_user1'], {
+            'availability_policy': 'rent',
+            'category': 'cat2',
+            'name': 'drill',
+            'description': 'test',
+            'owned_quantity': 3,
+            'tags': ['tag1', 'tag2'],
+            'properties': [{'name': 'prop1', 'value': 'value1'}, {'name': 'prop2', 'value': 'value2'}],
+            'owner_group': self.f['group1'].id,
+        })
+        self.assertEqual(reply.status_code, 201)
+        item = InventoryItem.objects.get(name='drill')
+        self.assertIsNone(item.owner)
+        self.assertEqual(item.owner_group, self.f['group1'])
+        self.assertEqual(item.availability_policy, 'rent')
+        self.assertEqual(item.category, Category.objects.get(name='cat2'))
+        self.assertEqual(item.owned_quantity, 3)
+        self.assertEqual([t for t in item.tags.all()], [self.f['tag1'], self.f['tag2']])
+        self.assertEqual([p for p in item.properties.all()], [self.f['prop1'], self.f['prop2']])
+        self.assertEqual([p.value for p in item.itemproperty_set.all()], ['value1', 'value2'])
+
+    def test_create_group_owned_item_empty_fails(self):
+        # Parity with InventoryApiTestCase.test_post_new_item_empty - clean()'s name-or-files
+        # validation must still apply to group-owned items.
+        reply = client.post('/api/inventory_items/', self.f['local_user1'], {
+            'availability_policy': 'private', 'owned_quantity': 1, 'owner_group': self.f['group1'].id,
+        })
+        self.assertEqual(reply.status_code, 400)
+        self.assertEqual(InventoryItem.objects.count(), 0)
+
+    def test_create_group_owned_item_nonexistent_group(self):
+        reply = client.post('/api/inventory_items/', self.f['local_user1'], {
+            'name': 'drill', 'owned_quantity': 1, 'availability_policy': 'private', 'owner_group': 999999,
+        })
+        self.assertEqual(reply.status_code, 404)
+        self.assertEqual(InventoryItem.objects.count(), 0)
+
+    def test_group_items_listed_for_nonexistent_group_query_param(self):
+        reply = client.get('/api/inventory_items/?group=999999', self.f['local_user1'])
+        self.assertEqual(reply.status_code, 200)
+        self.assertEqual(len(reply.json()), 0)
+
+    def test_put_group_item(self):
+        # Parity with InventoryApiTestCase.test_put_item - full replace, by a different member
+        # than the one who created it, exercising the group_items_id -> _is_authorized branch
+        # in perform_update for a PUT (not just PATCH).
+        item_id = self.create_group_item().json()['id']
+        reply = client.put('/api/inventory_items/{}/'.format(item_id), self.f['local_user2'], {
+            'availability_policy': 'sell',
+            'name': 'drill-4000',
+            'description': 'new description',
+            'owned_quantity': 100,
+            'tags': ['tag1', 'tag3'],
+            'properties': [{'name': 'prop1', 'value': 'value5'}],
+        })
+        self.assertEqual(reply.status_code, 200)
+        item = InventoryItem.objects.get(id=item_id)
+        self.assertEqual(item.owner_group, self.f['group1'])
+        self.assertEqual(item.availability_policy, 'sell')
+        self.assertEqual(item.name, 'drill-4000')
+        self.assertEqual(item.description, 'new description')
+        self.assertEqual(item.owned_quantity, 100)
+        self.assertEqual([t for t in item.tags.all()], [self.f['tag1'], self.f['tag3']])
+        self.assertEqual([p.value for p in item.itemproperty_set.all()], ['value5'])
+
+    def test_patch_group_item_clears_fields(self):
+        # Parity with InventoryApiTestCase.test_patch_item2 - clearing category/tags/properties.
+        reply = client.post('/api/inventory_items/', self.f['local_user1'], {
+            'name': 'drill', 'owned_quantity': 1, 'availability_policy': 'private',
+            'category': 'cat1', 'tags': ['tag1'],
+            'owner_group': self.f['group1'].id,
+        })
+        item_id = reply.json()['id']
+        reply = client.patch('/api/inventory_items/{}/'.format(item_id), self.f['local_user2'], {
+            'category': None, 'tags': [], 'properties': []
+        })
+        self.assertEqual(reply.status_code, 200)
+        item = InventoryItem.objects.get(id=item_id)
+        self.assertEqual(item.category, None)
+        self.assertEqual([t for t in item.tags.all()], [])
+
+    def test_group_item_storage_location(self):
+        reply = client.post('/api/inventory_items/', self.f['local_user1'], {
+            'name': 'drill', 'owned_quantity': 1, 'availability_policy': 'private',
+            'storage_location': self.f['loc1'].id, 'owner_group': self.f['group1'].id,
+        })
+        self.assertEqual(reply.status_code, 201)
+        item = InventoryItem.objects.get(name='drill')
+        self.assertEqual(item.storage_location, self.f['loc1'])
+
+    def test_post_group_item_with_file_id(self):
+        # Parity with TestInventoryItemWithFileApiTestCase.test_post_item_with_file_id.
+        reply = client.post('/api/inventory_items/', self.f['local_user1'], {
+            'name': 'drill', 'owned_quantity': 1, 'availability_policy': 'private',
+            'files': [self.f['test_file1'].id], 'owner_group': self.f['group1'].id,
+        })
+        self.assertEqual(reply.status_code, 201)
+        item = InventoryItem.objects.get(name='drill')
+        self.assertEqual([f for f in item.files.all()], [self.f['test_file1']])
+
+    def test_post_group_item_with_encoded_file(self):
+        reply = client.post('/api/inventory_items/', self.f['local_user1'], {
+            'name': 'drill', 'owned_quantity': 1, 'availability_policy': 'private',
+            'files': [{'data': self.f['encoded_content3'], 'mime_type': 'text/plain'}],
+            'owner_group': self.f['group1'].id,
+        })
+        self.assertEqual(reply.status_code, 201)
+        item = InventoryItem.objects.get(name='drill')
+        self.assertEqual([f for f in item.files.all()], [self.f['test_file3']])
+
+    def test_group_items_excluded_from_search(self):
+        # Group-owned items are only ever reachable via the group's own detail page for MVP
+        # (see docs/design-in-progress/groups-mvp.md) - search must not surface them, same as
+        # the main Inventory list already doesn't.
+        self.create_group_item(name='searchable-drill')
+        InventoryItem.create_for_owner(owner=self.f['local_user1'], owned_quantity=1, name='searchable-personal')
+        reply = client.get('/api/search/?query=searchable', self.f['local_user1'])
+        self.assertEqual(reply.status_code, 200)
+        names = [item['name'] for item in reply.json()]
+        self.assertEqual(names, ['searchable-personal'])
+
+
+class InventoryItemIdAllocationTestCase(UserTestMixin, ToolshedTestCase):
+    """InventoryItem.id is sequential and gapless within each owner/owner_group's own items,
+    never reused, and allocated independently per scope - see OwnerItemSequence."""
+
+    def setUp(self):
+        super().setUp()
+        self.prepare_users()
+
+    def test_ids_are_sequential_and_independent_per_owner(self):
+        user1_items = [InventoryItem.create_for_owner(owner=self.f['local_user1'], name=f'u1-{i}')
+                       for i in range(3)]
+        user2_items = [InventoryItem.create_for_owner(owner=self.f['local_user2'], name=f'u2-{i}')
+                       for i in range(2)]
+        self.assertEqual([item.id for item in user1_items], [1, 2, 3])
+        self.assertEqual([item.id for item in user2_items], [1, 2])
+
+    def test_deleted_item_id_is_never_reused(self):
+        item1 = InventoryItem.create_for_owner(owner=self.f['local_user1'], name='first')
+        item2 = InventoryItem.create_for_owner(owner=self.f['local_user1'], name='second')
+        self.assertEqual((item1.id, item2.id), (1, 2))
+        item2.delete()  # soft delete - item2's row (and its id) stays in the table
+        item3 = InventoryItem.create_for_owner(owner=self.f['local_user1'], name='third')
+        self.assertEqual(item3.id, 3)
+        self.assertFalse(InventoryItem.objects.filter(owner=self.f['local_user1'], id=2).exists())
+        self.assertTrue(InventoryItem.global_objects.filter(owner=self.f['local_user1'], id=2).exists())
+
+    def test_group_scope_has_independent_sequence(self):
+        group = Group.objects.create(name='alloc-test-group', domain=self.f['example_com'].name)
+        personal_item = InventoryItem.create_for_owner(owner=self.f['local_user1'], name='personal')
+        group_item = InventoryItem.create_for_owner(owner_group=group, name='group-owned')
+        self.assertEqual(personal_item.id, 1)
+        self.assertEqual(group_item.id, 1)
\ No newline at end of file
diff --git a/backend/toolshed/tests/test_locations.py b/backend/toolshed/tests/test_locations.py
index 5f9eef9..438775d 100644
--- a/backend/toolshed/tests/test_locations.py
+++ b/backend/toolshed/tests/test_locations.py
@@ -111,7 +111,7 @@ class LocationApiTestCase(UserTestMixin, InventoryTestMixin, LocationTestMixin,
         self.assertEqual(StorageLocation.objects.filter(id=self.f['loc4'].id).count(), 0)
 
     def test_delete_location_with_items_sets_null(self):
-        item = InventoryItem.objects.create(
+        item = InventoryItem.create_for_owner(
             owner=self.f['local_user1'], name='located_item', storage_location=self.f['loc3'])
         reply = client.delete('/api/storage_locations/' + str(self.f['loc3'].id) + '/', self.f['local_user1'])
         self.assertEqual(reply.status_code, 204)
diff --git a/backend/toolshed/tests/test_offlinedata.py b/backend/toolshed/tests/test_offlinedata.py
index 499e597..655cd38 100644
--- a/backend/toolshed/tests/test_offlinedata.py
+++ b/backend/toolshed/tests/test_offlinedata.py
@@ -28,11 +28,11 @@ class _DeleteTestDataMixin(UserTestMixin, CategoryTestMixin, LocationTestMixin):
         self.f['orphan_file'] = File.objects.create(
             file=ContentFile(b'orphan', 'orphan'), mime_type='text/plain', hash='orphan')
 
-        self.f['item1'] = InventoryItem.objects.create(
+        self.f['item1'] = InventoryItem.create_for_owner(
             owner=self.f['local_user1'], owned_quantity=1, name='item1', category=self.f['cat1'])
         self.f['item1'].files.add(self.f['orphan_file'])
 
-        self.f['item_other_user'] = InventoryItem.objects.create(
+        self.f['item_other_user'] = InventoryItem.create_for_owner(
             owner=self.f['local_user2'], owned_quantity=1, name='item2', category=self.f['cat1'])
         self.f['item_other_user'].files.add(self.f['shared_file'])
 
@@ -136,7 +136,7 @@ class ImportInventoryPropertiesTestCase(UserTestMixin, CategoryTestMixin, TagTes
         self.prepare_properties()
 
     def test_property_values_with_comma_and_equals_round_trip(self):
-        item = InventoryItem.objects.create(owner=self.f['local_user1'], name='widget')
+        item = InventoryItem.create_for_owner(owner=self.f['local_user1'], name='widget')
         ItemProperty.objects.create(inventory_item=item, property=self.f['prop1'], value='10cm, 20cm')
         ItemProperty.objects.create(inventory_item=item, property=self.f['prop2'], value='a=b')
 
@@ -167,7 +167,7 @@ class ImportInventoryPropertiesTestCase(UserTestMixin, CategoryTestMixin, TagTes
         self.assertEqual(values, {'prop1': 'value1', 'prop2': 'value2'})
 
     def test_item_without_properties_imports_cleanly(self):
-        item = InventoryItem.objects.create(owner=self.f['local_user1'], name='bare item')
+        item = InventoryItem.create_for_owner(owner=self.f['local_user1'], name='bare item')
 
         csv_bytes = b''.join(rows_to_csv(list(inventory_rows(self.f['local_user1']))))
 
@@ -179,7 +179,7 @@ class ImportInventoryPropertiesTestCase(UserTestMixin, CategoryTestMixin, TagTes
         self.assertEqual(list(new_item.itemproperty_set.all()), [])
 
     def test_category_and_tags_round_trip(self):
-        item = InventoryItem.objects.create(
+        item = InventoryItem.create_for_owner(
             owner=self.f['local_user1'], name='cat and tags item', category=self.f['cat1'])
         item.tags.add(self.f['tag1'], self.f['tag2'], through_defaults={})
 
@@ -206,7 +206,7 @@ class ImportInventoryPropertiesTestCase(UserTestMixin, CategoryTestMixin, TagTes
         self.assertFalse(InventoryItem.objects.filter(owner=self.f['local_user1'], name='ghost widget').exists())
 
     def test_property_value_with_quote_character_round_trips(self):
-        item = InventoryItem.objects.create(owner=self.f['local_user1'], name='quoted widget')
+        item = InventoryItem.create_for_owner(owner=self.f['local_user1'], name='quoted widget')
         ItemProperty.objects.create(inventory_item=item, property=self.f['prop1'], value='12" screen')
 
         csv_bytes = b''.join(rows_to_csv(list(inventory_rows(self.f['local_user1']))))
@@ -237,7 +237,7 @@ class ExportImportApiRoundTripTestCase(UserTestMixin, CategoryTestMixin, TagTest
     def test_export_then_import_preserves_category_tags_and_properties(self):
         import base64
 
-        item = InventoryItem.objects.create(
+        item = InventoryItem.create_for_owner(
             owner=self.f['local_user1'], name='drill', description='cordless drill',
             category=self.f['cat1'], availability_policy='friends', owned_quantity=2)
         item.tags.add(self.f['tag1'], self.f['tag2'], through_defaults={})
diff --git a/frontend/src/label.js b/frontend/src/label.js
index d095b0a..62ddca0 100644
--- a/frontend/src/label.js
+++ b/frontend/src/label.js
@@ -197,6 +197,18 @@ function relation(node, ownAxis, wantWidth, pxPerMm) {
     if (combinesAsWidth === wantWidth) {
         return {a, b};
     }
+    if (a === 0) {
+        // Every child is a fixed size (a === 0) in the combining direction - e.g. a row that's
+        // just one crisp QR leaf, with no scale-free (text) sibling to invert against. Inverting
+        // "width = b" for an a of 0 would divide by zero: a constant width genuinely doesn't
+        // determine a height, since nothing here actually scales with it. Ask each child directly
+        // for its own size in the wanted direction instead (every one of them must be similarly
+        // fixed, since only a fixed leaf ever contributes a === 0), and take the largest - the
+        // shared dimension has to fit whichever child needs the most room, with any child that
+        // ends up with room to spare centered within it (see drawQrLeaf).
+        const otherParts = node.map(child => relation(child, axis, wantWidth, pxPerMm));
+        return {a: 0, b: Math.max(...otherParts.map(p => p.b))};
+    }
     return {a: 1 / a, b: -b / a};				// invert: solve the affine relation the other way
 }