diff --git a/backend/tests/test_support_bundle.py b/backend/tests/test_support_bundle.py
index fa5f3240ef..a107b009c1 100644
--- a/backend/tests/test_support_bundle.py
+++ b/backend/tests/test_support_bundle.py
@@ -902,6 +902,20 @@ def test_main_reports_invalid_thread_id_without_traceback(tmp_path, capsys):
     assert "Traceback" not in captured.err
 
 
+def test_main_reports_directory_out_without_traceback(tmp_path, capsys):
+    project_root = tmp_path / "project"
+    project_root.mkdir()
+    out_dir = tmp_path / "bundles"
+    out_dir.mkdir()
+
+    exit_code = support_bundle.main(["--project-root", str(project_root), "--out", str(out_dir)])
+
+    captured = capsys.readouterr()
+    assert exit_code == 2
+    assert "--out must be a file path, not a directory" in captured.err
+    assert "Traceback" not in captured.err
+
+
 def test_main_prints_reporter_next_steps_and_optional_upload(tmp_path, capsys):
     project_root = tmp_path / "project"
     project_root.mkdir()
diff --git a/scripts/support_bundle.py b/scripts/support_bundle.py
index fdb3461fb9..5e3bdeadd3 100644
--- a/scripts/support_bundle.py
+++ b/scripts/support_bundle.py
@@ -899,6 +899,10 @@ def create_support_bundle(
     config_path = (config_path or project_root / "config.yaml").resolve()
     extensions_config_path = (extensions_config_path or project_root / "extensions_config.json").resolve()
     out_path = (out_path or _default_out_path(project_root)).resolve()
+    # `--out` names the zip, so a directory there would only fail inside
+    # zipfile with a raw PermissionError/IsADirectoryError.
+    if out_path.is_dir():
+        raise ValueError(f"--out must be a file path, not a directory: {out_path}")
     out_path.parent.mkdir(parents=True, exist_ok=True)
     if thread_id:
         _validate_thread_id(thread_id)
@@ -964,7 +968,7 @@ def parse_args(argv: list[str] | None = None) -> argparse.Namespace:
     parser.add_argument("--config", type=Path, default=None, help="Path to config.yaml")
     parser.add_argument("--extensions-config", type=Path, default=None, help="Path to extensions_config.json")
     parser.add_argument("--thread-id", default=None, help="Optional thread id to include file manifests for")
-    parser.add_argument("--out", type=Path, default=None, help="Output zip path")
+    parser.add_argument("--out", type=Path, default=None, help="Output zip file path")
     parser.add_argument("--include-doctor", action="store_true", help="Include redacted make doctor output")
     return parser.parse_args(argv)