diff --git a/tests/test_dao_presence.py b/tests/test_dao_presence.py
index 85e381d..0d86cb8 100644
--- a/tests/test_dao_presence.py
+++ b/tests/test_dao_presence.py
@@ -718,13 +718,15 @@ def test_override_remove_then_recreate_overlapping(tmp_path) -> None:
 
 def test_override_create_racing_conflicts_serialize(tmp_path) -> None:
     """Two racing creates for the same child and dates: exactly one
-    lands, the other is rejected with the overlap error (the check and
-    the INSERT share one transaction under the connection lock)."""
+    lands, the other is rejected with the overlap error.  DETERMINISTIC
+    gate: task A is paused at its INSERT — inside its open transaction,
+    after its overlap probe — so task B is created while A provably
+    holds the connection lock and must queue; B then re-runs the probe
+    against A's committed row and is rejected."""
     import asyncio as asyncio_module
+    from unittest.mock import MagicMock
 
     async def _main():
-        from unittest.mock import AsyncMock, MagicMock
-
         from custom_components.nestquest.dao_children import ChildrenDao
         from custom_components.nestquest.dao_presence import (
             PresenceOverridesDao,
@@ -732,10 +734,24 @@ def test_override_create_racing_conflicts_serialize(tmp_path) -> None:
         from custom_components.nestquest.db import NestQuestDatabase
         from custom_components.nestquest.migrations import apply_migrations
 
+        armed = {"active": False, "gated": False}
+        gate_open = asyncio_module.Event()
+        a_started = asyncio_module.Event()
+
         hass = MagicMock()
-        hass.async_add_executor_job = AsyncMock(
-            side_effect=(lambda fn, *a: fn(*a))
-        )
+
+        async def _gated_executor(fn, *args):
+            if armed["active"]:
+                # Gate the FIRST _execute after arming: task A's INSERT,
+                # which runs inside its open transaction (BEGIN already
+                # executed) after the overlap probe.
+                if getattr(fn, "__name__", "") == "_execute" and not armed["gated"]:
+                    armed["gated"] = True
+                    a_started.set()
+                    await gate_open.wait()
+            return fn(*args)
+
+        hass.async_add_executor_job = _gated_executor
         database = NestQuestDatabase(hass)
         await database.open(tmp_path / "override-race.db")
         try:
@@ -743,26 +759,32 @@ def test_override_create_racing_conflicts_serialize(tmp_path) -> None:
             children = ChildrenDao(database)
             overrides = PresenceOverridesDao(database)
             child = await children.create("Ada", NOW)
-            results = await asyncio_module.gather(
-                overrides.create(
-                    child.id, "2026-07-20", "2026-07-24", True
-                ),
-                overrides.create(
-                    child.id, "2026-07-22", "2026-07-26", False
-                ),
-                return_exceptions=True,
+
+            armed["active"] = True
+            task_a = asyncio_module.ensure_future(
+                overrides.create(child.id, "2026-07-20", "2026-07-24", True)
             )
-            outcomes = sorted(
-                "ok" if not isinstance(r, BaseException) else type(r).__name__
-                for r in results
+            await a_started.wait()
+            armed["active"] = False
+            task_b = asyncio_module.ensure_future(
+                overrides.create(child.id, "2026-07-22", "2026-07-26", False)
             )
-            assert outcomes == ["ValueError", "ok"], (
-                f"exactly one create must land: {outcomes}"
+            await asyncio_module.sleep(0)
+            gate_open.set()
+            record_a, error_b = await asyncio_module.gather(
+                task_a, task_b, return_exceptions=True
             )
+            assert isinstance(record_a, object) and not isinstance(
+                record_a, BaseException
+            ), f"A must land: {record_a!r}"
+            assert isinstance(error_b, ValueError), (
+                f"B must be rejected with the overlap error: {error_b!r}"
+            )
+            assert "overlaps existing" in str(error_b)
             listed = await overrides.list_by_child_and_range(
                 child.id, "2026-01-01", "2026-12-31"
             )
-            assert len(listed) == 1
+            assert [row.id for row in listed] == [record_a.id]
         finally:
             await database.close()