diff --git a/app/service.py b/app/service.py
index 8022b93..590f99a 100644
--- a/app/service.py
+++ b/app/service.py
@@ -10,15 +10,18 @@ def list_tasks(status: str | None = None, q: str | None = None) -> list[dict[str
     """Return task records, optionally filtered by status and search text."""
     tasks = load_tasks()
     filtered: list[dict[str, Any]] = []
+    normalized_q = q.casefold() if q else None
 
     for task in tasks:
-        # Instructor note: intentional bug for the lab.
-        # This uses the literal string "status" instead of the query parameter value.
-        if status and task["status"] != "status":
+        if status and task["status"] != status:
+            continue
+
+        if normalized_q and not any(
+            normalized_q in task[field].casefold()
+            for field in ("title", "description")
+        ):
             continue
 
-        # Instructor note: partial feature for the lab.
-        # The route already accepts `q`, but search is not implemented yet.
         filtered.append(task)
 
     return filtered
@@ -51,14 +54,14 @@ def complete_task(task_id: int) -> dict[str, Any] | None:
     """Mark a task as completed."""
     tasks = load_tasks()
 
-    for task in tasks:
+    for index, task in enumerate(tasks):
         if task["id"] == task_id:
             updated_task = dict(task)
             updated_task["status"] = "done"
             updated_task["completed_at"] = datetime.now(timezone.utc).isoformat()
 
-            # Instructor note: intentional bug for the lab.
-            # The updated task is returned, but the stored list is never updated or saved.
+            tasks[index] = updated_task
+            save_tasks(tasks)
             return updated_task
 
     return None