diff --git a/backend/tests/test_task_repository.py b/backend/tests/test_task_repository.py
new file mode 100644
index 0000000000..101e2e0a67
--- /dev/null
+++ b/backend/tests/test_task_repository.py
@@ -0,0 +1,35 @@
+import pytest
+from oasst_backend.task_repository import validate_frontend_message_id
+from oasst_shared.exceptions.oasst_api_error import OasstError, OasstErrorCode
+
+
+def test_validate_frontend_message_id_valid():
+    """Test that a valid string ID passes validation."""
+    validate_frontend_message_id("valid-id-123")
+
+
+def test_validate_frontend_message_id_empty():
+    """Test that an empty string raises an error."""
+    with pytest.raises(OasstError) as excinfo:
+        validate_frontend_message_id("")
+
+    assert excinfo.value.error_code == OasstErrorCode.INVALID_FRONTEND_MESSAGE_ID
+    assert "message_id must not be empty" in str(excinfo.value)
+
+
+def test_validate_frontend_message_id_none():
+    """Test that None raises an error."""
+    with pytest.raises(OasstError) as excinfo:
+        validate_frontend_message_id(None)
+
+    assert excinfo.value.error_code == OasstErrorCode.INVALID_FRONTEND_MESSAGE_ID
+    assert "message_id must be string" in str(excinfo.value)
+
+
+def test_validate_frontend_message_id_wrong_type():
+    """Test that a non-string type raises an error."""
+    with pytest.raises(OasstError) as excinfo:
+        validate_frontend_message_id(12345)
+
+    assert excinfo.value.error_code == OasstErrorCode.INVALID_FRONTEND_MESSAGE_ID
+    assert "message_id must be string" in str(excinfo.value)