diff --git a/src/telemetry_window_demo/io.py b/src/telemetry_window_demo/io.py
index 614ae4e..902ed0f 100644
--- a/src/telemetry_window_demo/io.py
+++ b/src/telemetry_window_demo/io.py
@@ -248,9 +248,9 @@ def _require_text_columns(
 
 def write_table(frame: pd.DataFrame, path: str | Path) -> Path:
     output_path = Path(path)
-    output_path.parent.mkdir(parents=True, exist_ok=True)
-
-    export = frame.copy()
+    _ensure_output_directory(output_path.parent)
+
+    export = frame.copy()
     for column in export.columns:
         dtype = export[column].dtype
         if pd.api.types.is_datetime64_any_dtype(dtype) or isinstance(
@@ -265,15 +265,27 @@ def write_table(frame: pd.DataFrame, path: str | Path) -> Path:
 
 def write_json(payload: dict[str, Any], path: str | Path) -> Path:
     output_path = Path(path)
-    output_path.parent.mkdir(parents=True, exist_ok=True)
+    _ensure_output_directory(output_path.parent)
     output_path.write_text(
         json.dumps(payload, indent=2) + "\n",
         encoding="utf-8",
     )
     return output_path
-
-
-def format_timestamp(value: Any) -> str:
+
+
+def ensure_output_directory(path: str | Path) -> Path:
+    output_dir = Path(path)
+    _ensure_output_directory(output_dir)
+    return output_dir
+
+
+def _ensure_output_directory(output_dir: Path) -> None:
+    if output_dir.exists() and not output_dir.is_dir():
+        raise ValueError(f"Output directory path is not a directory: {output_dir}")
+    output_dir.mkdir(parents=True, exist_ok=True)
+
+
+def format_timestamp(value: Any) -> str:
     if pd.isna(value):
         return ""
     timestamp = pd.Timestamp(value)
diff --git a/src/telemetry_window_demo/visualize.py b/src/telemetry_window_demo/visualize.py
index a9d968f..0503e29 100644
--- a/src/telemetry_window_demo/visualize.py
+++ b/src/telemetry_window_demo/visualize.py
@@ -5,6 +5,8 @@
 import matplotlib
 import pandas as pd
 
+from .io import ensure_output_directory
+
 matplotlib.use("Agg")
 import matplotlib.pyplot as plt
 
@@ -14,8 +16,7 @@ def plot_outputs(
     alerts: pd.DataFrame,
     output_dir: str | Path,
 ) -> list[Path]:
-    target_dir = Path(output_dir)
-    target_dir.mkdir(parents=True, exist_ok=True)
+    target_dir = ensure_output_directory(output_dir)
 
     plt.style.use("seaborn-v0_8-whitegrid")
     paths = [
diff --git a/tests/test_cli_errors.py b/tests/test_cli_errors.py
index 02cb04b..6c2d762 100644
--- a/tests/test_cli_errors.py
+++ b/tests/test_cli_errors.py
@@ -72,6 +72,36 @@ def test_main_reports_directory_input_without_traceback(tmp_path, capsys) -> Non
     assert "Traceback" not in stderr
 
 
+def test_main_reports_file_output_dir_without_traceback(tmp_path, capsys) -> None:
+    input_path = tmp_path / "events.csv"
+    input_path.write_text(
+        "timestamp,event_type,source,target,status\n"
+        "2026-03-10T10:00:00Z,login_success,user_a,auth,ok\n",
+        encoding="utf-8",
+    )
+    output_path = tmp_path / "processed"
+    output_path.write_text("not a directory\n", encoding="utf-8")
+    config_path = tmp_path / "file-output-dir.yaml"
+    config_path.write_text(
+        yaml.safe_dump(
+            {
+                "input_path": str(input_path),
+                "output_dir": str(output_path),
+            }
+        ),
+        encoding="utf-8",
+    )
+
+    with pytest.raises(SystemExit) as excinfo:
+        main(["run", "--config", str(config_path)])
+
+    assert excinfo.value.code == 1
+    stderr = capsys.readouterr().err
+    assert stderr.startswith("error: ")
+    assert "Output directory path is not a directory" in stderr
+    assert "Traceback" not in stderr
+
+
 def test_main_reports_bad_summarize_timestamp_column_without_traceback(
     tmp_path,
     capsys,
@@ -155,3 +185,40 @@ def test_main_reports_missing_default_alert_table_without_traceback(tmp_path, ca
     assert stderr.startswith("error: ")
     assert "Alert table not found" in stderr
     assert "Traceback" not in stderr
+
+
+def test_main_reports_file_plot_output_dir_without_traceback(tmp_path, capsys) -> None:
+    features_path = tmp_path / "features.csv"
+    features_path.write_text(
+        "window_start,window_end,event_count,error_rate\n"
+        "2026-03-10T10:00:00Z,2026-03-10T10:01:00Z,10,0.25\n",
+        encoding="utf-8",
+    )
+    alerts_path = tmp_path / "alerts.csv"
+    alerts_path.write_text(
+        "alert_time,window_start,window_end,rule_name,severity\n"
+        "2026-03-10T10:01:00Z,2026-03-10T10:00:00Z,"
+        "2026-03-10T10:01:00Z,high_error_rate,medium\n",
+        encoding="utf-8",
+    )
+    output_path = tmp_path / "plots"
+    output_path.write_text("not a directory\n", encoding="utf-8")
+
+    with pytest.raises(SystemExit) as excinfo:
+        main(
+            [
+                "plot",
+                "--features",
+                str(features_path),
+                "--alerts",
+                str(alerts_path),
+                "--output-dir",
+                str(output_path),
+            ]
+        )
+
+    assert excinfo.value.code == 1
+    stderr = capsys.readouterr().err
+    assert stderr.startswith("error: ")
+    assert "Output directory path is not a directory" in stderr
+    assert "Traceback" not in stderr