diff --git a/backend/migrations/versions/20260803_reliability_monitoring.py b/backend/migrations/versions/20260803_reliability_monitoring.py
index 21b3048..1cd3c74 100644
--- a/backend/migrations/versions/20260803_reliability_monitoring.py
+++ b/backend/migrations/versions/20260803_reliability_monitoring.py
@@ -105,9 +105,13 @@ def _create_missing_indexes(connection: Connection) -> None:
     existing_indexes = {
         index["name"] for index in inspector.get_indexes(table.name)
     }
-    for index in table.indexes:
-        if index.name not in existing_indexes:
-            index.create(bind=connection)
+    index = next(
+        item
+        for item in table.indexes
+        if item.name == "idx_ping_records_device_created_at"
+    )
+    if index.name not in existing_indexes:
+        index.create(bind=connection)
 
 
 def _upgrade_postgresql_alert_type(connection: Connection) -> None:
diff --git a/backend/tests/test_models_and_migration.py b/backend/tests/test_models_and_migration.py
index fb6d7fa..9e83318 100644
--- a/backend/tests/test_models_and_migration.py
+++ b/backend/tests/test_models_and_migration.py
@@ -4,7 +4,7 @@ import importlib
 from pathlib import Path
 
 import pytest
-from sqlalchemy import Column, Integer, Table, inspect, text
+from sqlalchemy import Column, Index, Integer, Table, inspect, text
 from sqlalchemy.ext.asyncio import AsyncSession, create_async_engine
 
 import app.models as models
@@ -249,3 +249,44 @@ async def test_migration_preserves_legacy_rows_and_owns_only_its_schema(
         if future_table in Base.metadata.tables.values():
             Base.metadata.remove(future_table)
         await engine.dispose()
+
+
+async def test_migration_does_not_create_a_future_ping_record_index(
+    tmp_path: Path,
+):
+    """Historical migration DDL stays limited to its named composite index."""
+    migration = importlib.import_module(
+        "migrations.versions.20260803_reliability_monitoring"
+    )
+    future_index = Index(
+        "idx_future_ping_record_round_num",
+        models.PingRecord.__table__.c.round_num,
+    )
+    engine = create_async_engine(
+        f"sqlite+aiosqlite:///{tmp_path / 'future-index.db'}"
+    )
+    try:
+        async with engine.begin() as connection:
+            await connection.execute(
+                text(
+                    "CREATE TABLE ping_records ("
+                    "id INTEGER PRIMARY KEY, device_id INTEGER NOT NULL, "
+                    "is_alive BOOLEAN NOT NULL, response_time_ms FLOAT, "
+                    "round_num INTEGER NOT NULL, created_at DATETIME)"
+                )
+            )
+            await connection.run_sync(migration.run_reliability_migration)
+            index_names = await connection.run_sync(
+                lambda sync_connection: {
+                    index["name"]
+                    for index in inspect(sync_connection).get_indexes(
+                        "ping_records"
+                    )
+                }
+            )
+
+        assert "idx_ping_records_device_created_at" in index_names
+        assert "idx_future_ping_record_round_num" not in index_names
+    finally:
+        models.PingRecord.__table__.indexes.remove(future_index)
+        await engine.dispose()