diff --git a/src/agents/sandbox/fs-bridge-mutation-helper.ts b/src/agents/sandbox/fs-bridge-mutation-helper.ts
index 39372dfd586..19beb8bad26 100644
--- a/src/agents/sandbox/fs-bridge-mutation-helper.ts
+++ b/src/agents/sandbox/fs-bridge-mutation-helper.ts
@@ -146,6 +146,14 @@ export const SANDBOX_PINNED_MUTATION_PYTHON = [
   "        try:",
   "            for child in os.listdir(src_dir_fd):",
   "                move_entry(src_dir_fd, child, temp_dir_fd, child)",
+  "        except Exception:",
+  "            # Rollback: move children from temp_dir back to source to preserve original tree.",
+  "            # Without this, a size-check failure on a later child leaves earlier children stranded",
+  "            # in the hidden temp dir and the source partially deleted.",
+  "            for child in os.listdir(temp_dir_fd):",
+  "                move_entry(temp_dir_fd, child, src_dir_fd, child)",
+  "            remove_tree(dst_parent_fd, temp_dir_name)",
+  "            raise",
   "        finally:",
   "            os.close(src_dir_fd)",
   "            os.close(temp_dir_fd)",