diff --git a/test/services/test_webui_task.py b/test/services/test_webui_task.py
index 4138493d5..6369db98e 100644
--- a/test/services/test_webui_task.py
+++ b/test/services/test_webui_task.py
@@ -156,6 +156,7 @@ def test_completed_task_renders_subject_named_video_download(
     selected_nodes = []
     target_names = {
         "_DOWNLOAD_FILENAME_INVALID_PATTERN",
+        "_WINDOWS_RESERVED_FILENAMES",
         "_build_video_download_name",
         "_normalize_task_state",
         "_render_generation_task_snapshot",
diff --git a/test/services/test_webui_task_history.py b/test/services/test_webui_task_history.py
index 56b33a820..5f1401892 100644
--- a/test/services/test_webui_task_history.py
+++ b/test/services/test_webui_task_history.py
@@ -16,6 +16,7 @@
 TASK_HISTORY_CONSTANTS = {
     "_FINAL_VIDEO_PATTERN",
     "_DOWNLOAD_FILENAME_INVALID_PATTERN",
+    "_WINDOWS_RESERVED_FILENAMES",
     "VOICE_MODE_TTS",
     "VOICE_MODE_UPLOAD",
     "VOICE_MODE_NONE",
@@ -90,6 +91,12 @@ def test_build_video_download_name_handles_empty_and_long_subjects():
     assert len(build_video_download_name("a" * 100, 1, 1)) == 84
 
 
+def test_build_video_download_name_avoids_windows_reserved_names():
+    assert build_video_download_name("CON", 1, 1) == "_CON.mp4"
+    assert build_video_download_name("aux.extra", 1, 1) == "_aux.extra.mp4"
+    assert build_video_download_name("lpt1", 1, 1) == "_lpt1.mp4"
+
+
 def test_restore_requirements_block_missing_uploaded_files():
     params = {
         "video_source": "local",
diff --git a/webui/Main.py b/webui/Main.py
index c20dddbfa..c092bdc82 100644
--- a/webui/Main.py
+++ b/webui/Main.py
@@ -138,6 +138,11 @@
     re.IGNORECASE,
 )
 _DOWNLOAD_FILENAME_INVALID_PATTERN = re.compile(r'[<>:"/\\|?*\x00-\x1f]')
+_WINDOWS_RESERVED_FILENAMES = frozenset(
+    {"CON", "PRN", "AUX", "NUL"}
+    | {f"COM{number}" for number in range(1, 10)}
+    | {f"LPT{number}" for number in range(1, 10)}
+)
 _RUNTIME_CONFIG_SECTIONS = {
     "app": config.app,
     "azure": config.azure,
@@ -990,6 +995,8 @@ def _build_video_download_name(subject, index, total):
     safe_subject = re.sub(r"\s+", " ", safe_subject).strip(" .")[:80].rstrip(" .")
     if not safe_subject:
         safe_subject = "video"
+    if safe_subject.split(".", 1)[0].upper() in _WINDOWS_RESERVED_FILENAMES:
+        safe_subject = f"_{safe_subject}"
 
     suffix = f"-{index}" if total > 1 else ""
     return f"{safe_subject}{suffix}.mp4"