diff --git a/src/telemetry_window_demo/io.py b/src/telemetry_window_demo/io.py
index 902ed0f..b56418e 100644
--- a/src/telemetry_window_demo/io.py
+++ b/src/telemetry_window_demo/io.py
@@ -247,8 +247,7 @@ def _require_text_columns(
 
 
 def write_table(frame: pd.DataFrame, path: str | Path) -> Path:
-    output_path = Path(path)
-    _ensure_output_directory(output_path.parent)
+    output_path = ensure_output_file_path(path)
 
     export = frame.copy()
     for column in export.columns:
@@ -264,8 +263,7 @@ 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)
-    _ensure_output_directory(output_path.parent)
+    output_path = ensure_output_file_path(path)
     output_path.write_text(
         json.dumps(payload, indent=2) + "\n",
         encoding="utf-8",
@@ -279,6 +277,14 @@ def ensure_output_directory(path: str | Path) -> Path:
     return output_dir
 
 
+def ensure_output_file_path(path: str | Path) -> Path:
+    output_path = Path(path)
+    if output_path.exists() and output_path.is_dir():
+        raise ValueError(f"Output file path is a directory: {output_path}")
+    _ensure_output_directory(output_path.parent)
+    return output_path
+
+
 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}")
diff --git a/src/telemetry_window_demo/visualize.py b/src/telemetry_window_demo/visualize.py
index 0503e29..7f826ed 100644
--- a/src/telemetry_window_demo/visualize.py
+++ b/src/telemetry_window_demo/visualize.py
@@ -5,7 +5,7 @@
 import matplotlib
 import pandas as pd
 
-from .io import ensure_output_directory
+from .io import ensure_output_directory, ensure_output_file_path
 
 matplotlib.use("Agg")
 import matplotlib.pyplot as plt
@@ -51,6 +51,7 @@ def _plot_metric(
     ylabel: str,
     alerts: pd.DataFrame | None = None,
 ) -> Path:
+    output_path = ensure_output_file_path(output_path)
     figure, axis = plt.subplots(figsize=(11, 4.5))
     if features.empty:
         axis.text(0.5, 0.5, "No feature windows generated", ha="center", va="center")
@@ -95,6 +96,7 @@ def _plot_metric(
 
 
 def _plot_alert_timeline(alerts: pd.DataFrame, output_path: Path) -> Path:
+    output_path = ensure_output_file_path(output_path)
     figure, axis = plt.subplots(figsize=(11, 4.5))
     if alerts.empty:
         axis.text(0.5, 0.5, "No alerts triggered", ha="center", va="center")
diff --git a/tests/test_cli_errors.py b/tests/test_cli_errors.py
index 6c2d762..7c41d29 100644
--- a/tests/test_cli_errors.py
+++ b/tests/test_cli_errors.py
@@ -102,6 +102,37 @@ def test_main_reports_file_output_dir_without_traceback(tmp_path, capsys) -> Non
     assert "Traceback" not in stderr
 
 
+def test_main_reports_directory_output_artifact_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_dir = tmp_path / "processed"
+    (output_dir / "features.csv").mkdir(parents=True)
+    config_path = tmp_path / "directory-output-artifact.yaml"
+    config_path.write_text(
+        yaml.safe_dump(
+            {
+                "input_path": str(input_path),
+                "output_dir": str(output_dir),
+            }
+        ),
+        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 file path is a directory" in stderr
+    assert "features.csv" in stderr
+    assert "Traceback" not in stderr
+
+
 def test_main_reports_bad_summarize_timestamp_column_without_traceback(
     tmp_path,
     capsys,
@@ -222,3 +253,41 @@ def test_main_reports_file_plot_output_dir_without_traceback(tmp_path, capsys) -
     assert stderr.startswith("error: ")
     assert "Output directory path is not a directory" in stderr
     assert "Traceback" not in stderr
+
+
+def test_main_reports_directory_plot_artifact_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_dir = tmp_path / "plots"
+    (output_dir / "event_count_timeline.png").mkdir(parents=True)
+
+    with pytest.raises(SystemExit) as excinfo:
+        main(
+            [
+                "plot",
+                "--features",
+                str(features_path),
+                "--alerts",
+                str(alerts_path),
+                "--output-dir",
+                str(output_dir),
+            ]
+        )
+
+    assert excinfo.value.code == 1
+    stderr = capsys.readouterr().err
+    assert stderr.startswith("error: ")
+    assert "Output file path is a directory" in stderr
+    assert "event_count_timeline.png" in stderr
+    assert "Traceback" not in stderr