diff --git a/server/backend/internal/store/store.go b/server/backend/internal/store/store.go
index 51d2418..1d70f8e 100644
--- a/server/backend/internal/store/store.go
+++ b/server/backend/internal/store/store.go
@@ -3,12 +3,17 @@ package store
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"time"
 
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
+// ErrReorderMismatch wird von Reorder zurückgegeben, wenn die übergebene
+// ID-Liste nicht mit den tatsächlichen Items der Playlist übereinstimmt.
+var ErrReorderMismatch = errors.New("reorder: item list does not match playlist")
+
 // ------------------------------------------------------------------
 // Domain types
 // ------------------------------------------------------------------
@@ -531,20 +536,46 @@ func (s *PlaylistStore) ScreenSlugByItemID(ctx context.Context, itemID string) (
 }
 
 // Reorder sets order_index for each item ID in the given slice order.
+// Returns ErrReorderMismatch if the number of provided IDs does not match
+// the number of items in the playlist, or if any ID does not belong to it.
 func (s *PlaylistStore) Reorder(ctx context.Context, playlistID string, itemIDs []string) error {
+	seen := make(map[string]struct{}, len(itemIDs))
+	for _, id := range itemIDs {
+		if _, dup := seen[id]; dup {
+			return fmt.Errorf("%w: duplicate id %s", ErrReorderMismatch, id)
+		}
+		seen[id] = struct{}{}
+	}
+
 	tx, err := s.pool.Begin(ctx)
 	if err != nil {
 		return err
 	}
 	defer tx.Rollback(ctx) //nolint:errcheck
 
+	var count int
+	if err := tx.QueryRow(ctx,
+		`select count(*) from playlist_items where playlist_id=$1`, playlistID,
+	).Scan(&count); err != nil {
+		return err
+	}
+	if count != len(itemIDs) {
+		return fmt.Errorf("%w: got %d ids, playlist has %d items",
+			ErrReorderMismatch, len(itemIDs), count)
+	}
+
 	for i, id := range itemIDs {
-		if _, err := tx.Exec(ctx,
+		tag, err := tx.Exec(ctx,
 			`update playlist_items set order_index=$1 where id=$2 and playlist_id=$3`,
 			i, id, playlistID,
-		); err != nil {
+		)
+		if err != nil {
 			return err
 		}
+		if tag.RowsAffected() != 1 {
+			return fmt.Errorf("%w: id %s not found in playlist %s",
+				ErrReorderMismatch, id, playlistID)
+		}
 	}
 	return tx.Commit(ctx)
 }