diff --git a/database/music_database.py b/database/music_database.py
index f047e1d9..20e6b217 100644
--- a/database/music_database.py
+++ b/database/music_database.py
@@ -992,6 +992,39 @@ class MusicDatabase:
             raise
 
         self._init_manual_library_match_table()
+        self._backfill_mirrored_track_source_ids()
+
+    def _backfill_mirrored_track_source_ids(self) -> int:
+        """One-time, idempotent: assign a stable source_track_id to mirrored tracks
+        that have none (file-import / iTunes-only playlists imported before #901), so
+        their existing Find & Add matches start sticking without a manual re-import.
+        Only touches empty-id rows, so it's a no-op once they're filled."""
+        from core.playlists.source_refs import stable_source_track_id
+        updated = 0
+        try:
+            with self._get_connection() as conn:
+                cursor = conn.cursor()
+                cursor.execute("""
+                    SELECT id, track_name, artist_name, album_name
+                    FROM mirrored_playlist_tracks
+                    WHERE source_track_id IS NULL OR source_track_id = ''
+                """)
+                rows = cursor.fetchall()
+                for r in rows:
+                    sid = stable_source_track_id({
+                        'track_name': r['track_name'], 'artist_name': r['artist_name'],
+                        'album_name': r['album_name']})
+                    if sid:
+                        cursor.execute(
+                            "UPDATE mirrored_playlist_tracks SET source_track_id = ? WHERE id = ?",
+                            (sid, r['id']))
+                        updated += 1
+                conn.commit()
+            if updated:
+                logger.info("Backfilled stable source_track_id on %d mirrored tracks (#901)", updated)
+        except Exception as e:
+            logger.error("mirrored track source_id backfill failed: %s", e)
+        return updated
 
     # Bump when the schema's generation meaningfully changes. Stamped into
     # PRAGMA user_version as a backstop indicator; nothing GATES on it yet.
diff --git a/tests/database/test_mirrored_playlists.py b/tests/database/test_mirrored_playlists.py
index ff563e68..c1d0b6b6 100644
--- a/tests/database/test_mirrored_playlists.py
+++ b/tests/database/test_mirrored_playlists.py
@@ -92,3 +92,36 @@ def test_native_ids_still_used_verbatim(tmp_path):
                              profile_id=1)
     rows = db.get_mirrored_playlist_tracks(pid)
     assert rows[0]["source_track_id"] == "spotify123"         # native id untouched
+
+
+def test_backfill_fills_existing_empty_ids_idempotently(tmp_path):
+    # #901 backfill: a file-import playlist mirrored BEFORE the fix has empty-id rows.
+    # The backfill assigns the SAME stable ids a fresh import would, so existing
+    # Find & Add matches start working without a re-import.
+    db = MusicDatabase(str(tmp_path / "music.db"))
+    pid = db.mirror_playlist(source="file", source_playlist_id="old", name="Old",
+                             tracks=[{"track_name": "Slow Ride", "artist_name": "Foghat"}], profile_id=1)
+    # simulate a pre-fix row: blank out the id
+    with db._get_connection() as conn:
+        conn.execute("UPDATE mirrored_playlist_tracks SET source_track_id = '' WHERE playlist_id = ?", (pid,))
+        conn.commit()
+
+    n = db._backfill_mirrored_track_source_ids()
+    assert n == 1
+    rows = db.get_mirrored_playlist_tracks(pid)
+    from core.playlists.source_refs import stable_source_track_id
+    assert rows[0]["source_track_id"] == stable_source_track_id(
+        {"track_name": "Slow Ride", "artist_name": "Foghat"})   # same id a fresh import gives
+
+    # idempotent — second run touches nothing
+    assert db._backfill_mirrored_track_source_ids() == 0
+
+
+def test_backfill_leaves_native_ids_untouched(tmp_path):
+    db = MusicDatabase(str(tmp_path / "music.db"))
+    pid = db.mirror_playlist(source="spotify", source_playlist_id="sp", name="Sp",
+                             tracks=[{"track_name": "S", "artist_name": "A", "source_track_id": "spotify123"}],
+                             profile_id=1)
+    db._backfill_mirrored_track_source_ids()
+    rows = db.get_mirrored_playlist_tracks(pid)
+    assert rows[0]["source_track_id"] == "spotify123"