diff --git a/backend/prompt_studio/prompt_studio_core_v2/migrations/0011_repair_ownerless_custom_tools.py b/backend/prompt_studio/prompt_studio_core_v2/migrations/0011_repair_ownerless_custom_tools.py
new file mode 100644
index 0000000000..c29e09380c
--- /dev/null
+++ b/backend/prompt_studio/prompt_studio_core_v2/migrations/0011_repair_ownerless_custom_tools.py
@@ -0,0 +1,34 @@
+"""UN-3057: grant an OWNER row to custom tools left ownerless by the clone path.
+
+The Prompt Studio clone helper never created the OWNER ``ResourceMembership``
+that UN-2202 made authoritative, so every project cloned after
+``0009_absorb_shared_users`` ran has no owner: visible (the clone copies the
+parent's ``shared_to_org``) but unmanageable by anyone except an org admin.
+The helper is fixed going forward; this repairs the rows already written.
+
+Idempotent and non-destructive — only resources with zero OWNER rows are
+touched, so it is safe to re-run and reverses to a no-op.
+"""
+
+from django.db import migrations
+from tenant_account_v2.migrations._membership_backfill import (
+    repair_ownerless_owner_rows,
+)
+
+APP_LABEL = "prompt_studio_core_v2"
+MODEL_NAME = "CustomTool"
+
+
+def _forward(apps, schema_editor):
+    repair_ownerless_owner_rows(apps, APP_LABEL, MODEL_NAME)
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("prompt_studio_core_v2", "0010_customtool_custtool_org_modified_idx"),
+        ("tenant_account_v2", "0005_resource_membership"),
+    ]
+
+    operations = [
+        migrations.RunPython(_forward, migrations.RunPython.noop),
+    ]
diff --git a/backend/prompt_studio/prompt_studio_core_v2/tests/test_ownerless_owner_repair.py b/backend/prompt_studio/prompt_studio_core_v2/tests/test_ownerless_owner_repair.py
new file mode 100644
index 0000000000..da9143e748
--- /dev/null
+++ b/backend/prompt_studio/prompt_studio_core_v2/tests/test_ownerless_owner_repair.py
@@ -0,0 +1,86 @@
+"""Repair of ownerless ``CustomTool`` rows (UN-3057).
+
+The Prompt Studio clone path created projects without the OWNER
+``ResourceMembership`` that UN-2202 made authoritative, so every project cloned
+after the UN-2202 backfill ran is ownerless: visible, but unmanageable by anyone
+except an org admin. Fixing the clone helper stops new breakage; these already
+broken rows need a repair pass.
+
+Exercises the migration helper against the real models (``django.apps.apps``
+satisfies the ``apps.get_model`` interface the migration passes in), so the
+behaviour is pinned without driving the migration executor.
+"""
+
+from __future__ import annotations
+
+import secrets
+
+from account_v2.models import Organization, User
+from django.apps import apps as django_apps
+from django.test import TestCase
+from permissions.roles import ResourceRole
+from tenant_account_v2.migrations._membership_backfill import (
+    repair_ownerless_owner_rows,
+)
+
+from prompt_studio.prompt_studio_core_v2.models import CustomTool
+
+APP_LABEL = "prompt_studio_core_v2"
+MODEL_NAME = "CustomTool"
+
+
+def _make_user(email: str) -> User:
+    return User.objects.create_user(
+        username=email, email=email, password=secrets.token_urlsafe()
+    )
+
+
+class RepairOwnerlessOwnerRowsTests(TestCase):
+    def setUp(self) -> None:
+        self.org = Organization.objects.create(
+            name="org-a", display_name="Org A", organization_id="org-a"
+        )
+        self.creator = _make_user("creator@example.com")
+        self.other = _make_user("other@example.com")
+
+    def _tool(self, name: str, creator: User | None) -> CustomTool:
+        return CustomTool.objects.create(
+            tool_name=name,
+            description="",
+            organization=self.org,
+            created_by=creator,
+        )
+
+    def _repair(self) -> int:
+        return repair_ownerless_owner_rows(django_apps, APP_LABEL, MODEL_NAME)
+
+    def _owner_ids(self, tool: CustomTool) -> set:
+        return set(
+            tool.memberships.filter(role=ResourceRole.OWNER).values_list(
+                "user_id", flat=True
+            )
+        )
+
+    def test_ownerless_tool_gets_an_owner_row_for_its_creator(self) -> None:
+        tool = self._tool("cloned-project", self.creator)
+        self.assertEqual(self._owner_ids(tool), set())
+
+        self._repair()
+
+        self.assertEqual(self._owner_ids(tool), {self.creator.id})
+
+    def test_tool_that_already_has_an_owner_is_left_alone(self) -> None:
+        """A creator deliberately replaced by a co-owner must not be re-added."""
+        tool = self._tool("handed-over-project", self.creator)
+        tool.memberships.create(user=self.other, role=ResourceRole.OWNER)
+
+        self._repair()
+
+        self.assertEqual(self._owner_ids(tool), {self.other.id})
+
+    def test_tool_with_no_creator_is_skipped(self) -> None:
+        tool = self._tool("orphan-project", None)
+
+        self._repair()
+
+        self.assertEqual(self._owner_ids(tool), set())
diff --git a/backend/tenant_account_v2/migrations/_membership_backfill.py b/backend/tenant_account_v2/migrations/_membership_backfill.py
index 00235b60f6..74020cdf86 100644
--- a/backend/tenant_account_v2/migrations/_membership_backfill.py
+++ b/backend/tenant_account_v2/migrations/_membership_backfill.py
@@ -68,3 +68,57 @@ def backfill_memberships(apps, app_label: str, model_name: str) -> None:
         skipped,
         skipped_org,
     )
+
+
+def repair_ownerless_owner_rows(apps, app_label: str, model_name: str) -> int:
+    """Give ``created_by`` an OWNER row on resources that have no owner at all.
+
+    UN-3057: the Prompt Studio clone path created ``CustomTool`` rows without
+    the OWNER row that UN-2202 made authoritative, so projects cloned after
+    :func:`backfill_memberships` ran are ownerless — visible (the clone copies
+    the parent's ``shared_to_org``), but unmanageable by anyone but an org
+    admin. This repairs what that backfill could not have seen.
+
+    Only resources with *zero* OWNER rows are touched, so a creator who was
+    deliberately replaced by a co-owner is not resurrected. Null creator or
+    null organization means there is nothing to grant, so those are skipped.
+    Idempotent: a second run finds no ownerless rows.
+    """
+    Resource = apps.get_model(app_label, model_name)  # NOSONAR
+    Membership = apps.get_model("tenant_account_v2", "ResourceMembership")  # NOSONAR
+    ContentType = apps.get_model("contenttypes", "ContentType")  # NOSONAR
+
+    content_type = ContentType.objects.get_for_model(Resource)
+    owned_ids = set(
+        Membership.objects.filter(content_type=content_type, role=OWNER).values_list(
+            "object_id", flat=True
+        )
+    )
+
+    # ``_base_manager``: several resources' default manager is org-scoped by
+    # ``UserContext`` (unset here → it would filter every row out and silently
+    # repair nothing). Same guard as ``tenant_account_v2.signals``.
+    repaired = skipped = 0
+    for resource in Resource._base_manager.exclude(created_by=None).iterator():
+        if resource.organization_id is None:
+            skipped += 1
+            continue
+        object_id = str(resource.pk)
+        if object_id in owned_ids:
+            continue
+        _, created = Membership.objects.get_or_create(
+            content_type=content_type,
+            object_id=object_id,
+            user_id=resource.created_by_id,
+            defaults={"role": OWNER, "organization_id": resource.organization_id},
+        )
+        repaired += int(created)
+
+    logger.info(
+        "%s.%s ownerless repair: owners granted=%s (skipped %s null-org)",
+        app_label,
+        model_name,
+        repaired,
+        skipped,
+    )
+    return repaired