diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 858d9d42..9c9d083b 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -1,3 +1,8 @@
+## 2026-07-25 - [Cross-platform upload basename normalization]
+**Behavior:** Upload metadata now interprets both forward slashes and backslashes as path separators before extracting a basename.
+**Learning:** On POSIX systems, `pathlib.Path(filename).name` retains backslashes because they are ordinary characters there. That caused inconsistent manifest and converter filenames for Windows-style client paths. The upload itself is still written inside a trusted temporary workspace, and batch archive entry names are generated outputs; this change does not establish a filesystem traversal or archive-entry escape.
+**Prevention:** Normalize client path separators before extracting a basename, retain the existing empty/`.`/`..` fallback, and test the persisted source name and manifest metadata. Treat the normalization as cross-platform consistency and defense in depth, not as evidence of a demonstrated Zip Slip exploit.
+
 ## 2026-05-28 - [Sentinel Fixes: Temp Files & Injection]
 **Vulnerability:** Predictable Temp Files (CWE-377) and Insecure Default Permissions (CWE-276), plus Command Injection via FFmpeg Filtergraph (CWE-20).
 **Learning:** Python's `Path.with_name` plus a suffix string to make a temp file opens a race condition because it's predictable and the permissions default to system `umask` which might expose secret `0600` data. Additionally, interpolating variables directly into FFmpeg filtergraph strings allows arbitrary filter injection.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4cca1ced..9313538b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,3 +11,4 @@
 
 ### Fixed
 - 단일·일괄 대상 크기 입력을 비웠을 때 이전 custom validity와 `aria-invalid` 상태를 즉시 초기화해 현재 필수 입력 상태를 정확히 전달합니다.
+- 업로드 파일명의 경로 구분자를 정규화하여 POSIX에서도 Windows 형식의 클라이언트 경로가 일관된 basename으로 기록되도록 수정했습니다.
diff --git a/saas_web.py b/saas_web.py
index 0ef95a1e..63265e94 100644
--- a/saas_web.py
+++ b/saas_web.py
@@ -486,7 +486,7 @@ def _persist_upload(file: UploadFile) -> tuple[Path, Path, Path, Path]:
         input_dir.mkdir()
         output_dir.mkdir()
 
-        safe_filename = Path(file.filename).name
+        safe_filename = Path((file.filename or "").replace("\\", "/")).name
         if not safe_filename or safe_filename in (".", ".."):
             safe_filename = "upload.tmp"
 
@@ -619,7 +619,7 @@ def shrink_media_batch(
     try:
         with zipfile.ZipFile(zip_path, "w", compression=zipfile.ZIP_STORED) as archive:
             for index, upload in enumerate(files):
-                safe_filename = Path(upload.filename or "").name
+                safe_filename = Path((upload.filename or "").replace("\\", "/")).name
                 if not safe_filename or safe_filename in (".", ".."):
                     safe_filename = "upload.tmp"
                 entry = {
diff --git a/tests/test_saas_web.py b/tests/test_saas_web.py
index cd45dbc3..f574a3cb 100644
--- a/tests/test_saas_web.py
+++ b/tests/test_saas_web.py
@@ -560,6 +560,32 @@ def test_shrink_batch_handles_archive_failure(self, _mock_zipfile):
         self.assertEqual(response.status_code, 500)
         self.assertEqual(response.json(), {"error": "Upload processing failed"})
 
+    @patch("saas_web.media_shrinker.convert_file")
+    def test_shrink_batch_uses_safe_fallback_filename_with_backslashes(self, mock_convert_file):
+        mock_convert_file.return_value = []
+
+        response = saas_web.shrink_media_batch(
+            BackgroundTasks(),
+            files=[
+                SimpleNamespace(
+                    filename="..\\..\\windows.ini",
+                    content_type="audio/wav",
+                    file=io.BytesIO(b"dummy"),
+                )
+            ],
+            target_bytes=10000,
+        )
+
+        try:
+            with zipfile.ZipFile(response.path) as archive:
+                manifest = json.loads(archive.read("results.json"))
+            self.assertEqual(manifest["results"][0]["filename"], "windows.ini")
+            self.assertEqual(
+                mock_convert_file.call_args.kwargs["source"].name, "windows.ini"
+            )
+        finally:
+            saas_web.cleanup_temp_dir(Path(response.path).parent)
+
     @patch("saas_web.media_shrinker.convert_file")
     def test_shrink_batch_uses_safe_fallback_filename(self, mock_convert_file):
         mock_convert_file.return_value = []
@@ -576,13 +602,15 @@ def test_shrink_batch_uses_safe_fallback_filename(self, mock_convert_file):
             target_bytes=10000,
         )
 
-        archive = zipfile.ZipFile(response.path)
-        manifest = json.loads(archive.read("results.json"))
-        self.assertEqual(manifest["results"][0]["filename"], "upload.tmp")
-        self.assertEqual(
-            mock_convert_file.call_args.kwargs["source"].name, "upload.tmp"
-        )
-        saas_web.cleanup_temp_dir(Path(response.path).parent)
+        try:
+            with zipfile.ZipFile(response.path) as archive:
+                manifest = json.loads(archive.read("results.json"))
+            self.assertEqual(manifest["results"][0]["filename"], "upload.tmp")
+            self.assertEqual(
+                mock_convert_file.call_args.kwargs["source"].name, "upload.tmp"
+            )
+        finally:
+            saas_web.cleanup_temp_dir(Path(response.path).parent)
 
     def test_get_ui_includes_batch_upload_form(self):
         response = client.get("/")