diff --git a/apps/backend/app/repositories/reading.py b/apps/backend/app/repositories/reading.py
index d251f55..d005d16 100644
--- a/apps/backend/app/repositories/reading.py
+++ b/apps/backend/app/repositories/reading.py
@@ -13,11 +13,12 @@ class ReadingRepository:
 
     async def latest_by_site(self) -> Sequence[Reading]:
         # `.distinct(site_id)` compile en `DISTINCT ON (site_id)` sous PostgreSQL : une seule
-        # ligne par site, la plus récente grâce à l'ordre composite qui suit.
+        # ligne par site, la plus récente grâce à l'ordre composite qui suit. `reading_id` départage
+        # les égalités de timestamp, que `uq_reading_source` autorise à `source` différente.
         requete = (
             select(Reading)
             .distinct(Reading.site_id)
-            .order_by(Reading.site_id, Reading.timestamp.desc())
+            .order_by(Reading.site_id, Reading.timestamp.desc(), Reading.reading_id.desc())
         )
         return (await self._session.execute(requete)).scalars().all()
 
diff --git a/apps/backend/tests/repositories/test_reading.py b/apps/backend/tests/repositories/test_reading.py
index dd2f2df..4f12df0 100644
--- a/apps/backend/tests/repositories/test_reading.py
+++ b/apps/backend/tests/repositories/test_reading.py
@@ -88,6 +88,26 @@ async def test_latest_by_site_returns_one_row_per_site(session: AsyncSession) ->
     assert identifiants == {premier, second}
 
 
+async def test_latest_by_site_breaks_a_timestamp_tie_on_the_last_written_reading(
+    session: AsyncSession,
+) -> None:
+    site = await creer_site(session)
+    depot = ReadingRepository(session)
+    horodatage = datetime(2026, 9, 15, tzinfo=UTC)
+    await creer_lecture(
+        session, site_id=site.site_id, timestamp=horodatage, source="api_history", consumption_kw=10
+    )
+    derniere = await creer_lecture(
+        session, site_id=site.site_id, timestamp=horodatage, source="api_current", consumption_kw=42
+    )
+
+    resultats = await depot.latest_by_site()
+    retenues = [r.reading_id for r in resultats if r.site_id == site.site_id]
+    await session.rollback()
+
+    assert retenues == [derniere.reading_id]
+
+
 async def test_latest_for_site_returns_the_most_recent_reading(session: AsyncSession) -> None:
     site = await creer_site(session)
     depot = ReadingRepository(session)