diff --git a/src/steerbench/cli.py b/src/steerbench/cli.py
index 9e85f3d..caa7a0c 100644
--- a/src/steerbench/cli.py
+++ b/src/steerbench/cli.py
@@ -114,6 +114,9 @@ def main(argv: list[str] | None = None) -> int:
     parser = _build_parser()
     args, extra = parser.parse_known_args(argv)
 
+    if not args.stem.strip():
+        parser.error("output stem must not be empty")
+
     if args.run:
         return _run_modal(args.run, extra)
 
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 87c3f7e..686ae11 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -89,3 +89,26 @@ def test_cli_errors_on_missing_named_side_csv(
         )
     assert exc.value.code != 0
     assert "side-effects CSV not found" in capsys.readouterr().err
+
+
+@pytest.mark.parametrize("stem", ["", "   "])
+def test_cli_errors_on_empty_stem(
+    stem: str, tmp_path: Path, capsys: pytest.CaptureFixture[str]
+) -> None:
+    # An empty or whitespace-only --stem would write cryptic dotfiles
+    # (".md", ".html", ".png"); it must fail fast instead.
+    with pytest.raises(SystemExit) as exc:
+        cli.main(
+            [
+                "--dose-csv",
+                str(_DOSE),
+                "--layer-csv",
+                str(_LAYER),
+                "--out",
+                str(tmp_path / "o"),
+                "--stem",
+                stem,
+            ]
+        )
+    assert exc.value.code != 0
+    assert "output stem must not be empty" in capsys.readouterr().err