diff --git a/README.md b/README.md
index d899c6c851..cb21249f32 100644
--- a/README.md
+++ b/README.md
@@ -2124,6 +2124,9 @@ failing the upload; hidden staging files are left for the startup sweep.
 Uploads, new skill support files, and new local sandbox paths reject Windows
 reserved device names on every platform, including `COM¹`, `LPT²`, the console
 aliases `CONIN$` and `CONOUT$`, and names with extensions such as `com³.txt`.
+ASCII spaces before the extension do not make a device name portable:
+`NUL .txt` and `COM1  .log` are rejected too. Ordinary names such as
+`report .txt` are preserved unchanged.
 Rename these files before creating or uploading them so the same file tree
 remains usable on Windows.
 
diff --git a/backend/packages/harness/deerflow/utils/AGENTS.md b/backend/packages/harness/deerflow/utils/AGENTS.md
index b254801cf0..0d67eda661 100644
--- a/backend/packages/harness/deerflow/utils/AGENTS.md
+++ b/backend/packages/harness/deerflow/utils/AGENTS.md
@@ -94,6 +94,10 @@ every platform, including the `COM`/`LPT` aliases with superscript ¹, ² and ³
 The console aliases `CONIN$` and `CONOUT$` are reserved too; match them
 case-insensitively before the first dot, without rejecting longer ordinary
 names such as `CONIN$notes.txt`.
+Device-name comparison ignores ASCII spaces before the first dot, so
+`NUL .txt` and `COM1  .log` remain reserved. Trim the comparison stem only,
+not the supplied filename; ordinary names such as `report .txt` and
+non-ASCII whitespace remain unchanged.
 Do not normalize arbitrary Unicode digits into device numbers: names such as
 `COM⁴.txt` and `COM¹notes.txt` are ordinary portable names. Read/removal callers
 retain their existing portability exemptions.
diff --git a/backend/packages/harness/deerflow/utils/host_paths.py b/backend/packages/harness/deerflow/utils/host_paths.py
index b65553a74b..55f1b20511 100644
--- a/backend/packages/harness/deerflow/utils/host_paths.py
+++ b/backend/packages/harness/deerflow/utils/host_paths.py
@@ -52,7 +52,9 @@ def windows_incompatible_segment(segment: str) -> str | None:
         return None
     if segment.endswith((" ", ".")):
         return "trailing dot or space"
-    stem = segment.split(".", 1)[0]
+    # Windows ignores ASCII spaces before a device name's extension (NUL .txt).
+    # Trim only for comparison; ordinary filenames must retain their spaces.
+    stem = segment.split(".", 1)[0].rstrip(" ")
     if stem.upper() in _WINDOWS_RESERVED_NAMES:
         return "reserved Windows device name"
     return None
diff --git a/backend/tests/test_sandbox_tools_security.py b/backend/tests/test_sandbox_tools_security.py
index 47bb1223d6..0ab937e32e 100644
--- a/backend/tests/test_sandbox_tools_security.py
+++ b/backend/tests/test_sandbox_tools_security.py
@@ -519,6 +519,14 @@ def test_validate_local_tool_path_rejects_new_console_device_names(name: str) ->
     validate_local_tool_path(path, _THREAD_DATA, read_only=True)
 
 
+@pytest.mark.parametrize("name", ["NUL .txt", "con  .log", "COM1 .txt", "lpt²  .md", "CONIN$ .txt", "conout$  .log"])
+def test_validate_local_tool_path_rejects_new_padded_device_names(name: str) -> None:
+    path = f"{VIRTUAL_PATH_PREFIX}/workspace/{name}"
+    with pytest.raises(PermissionError, match="Access denied"):
+        validate_local_tool_path(path, _THREAD_DATA, read_only=False)
+    validate_local_tool_path(path, _THREAD_DATA, read_only=True)
+
+
 def test_validate_local_tool_path_rejects_traversal_in_user_data() -> None:
     """Path traversal via .. in user-data paths must be rejected."""
     with pytest.raises(PermissionError, match="path traversal"):
diff --git a/backend/tests/test_skill_storage_support_files.py b/backend/tests/test_skill_storage_support_files.py
index a269f6a368..94fbbcdd15 100644
--- a/backend/tests/test_skill_storage_support_files.py
+++ b/backend/tests/test_skill_storage_support_files.py
@@ -86,6 +86,12 @@ def test_ensure_safe_support_path_rejects_console_device_names(storage, skill_di
         storage.ensure_safe_support_path("demo-skill", path)
 
 
+@pytest.mark.parametrize("path", ["assets/NUL .txt", "scripts/con  .log", "assets/COM1 .txt", "assets/lpt²  .md", "assets/CONIN$ .txt", "assets/conout$  .log", "assets/COM1 .txt/icon.png"])
+def test_ensure_safe_support_path_rejects_padded_device_names(storage, skill_dir, path):
+    with pytest.raises(ValueError, match="reserved Windows device name"):
+        storage.ensure_safe_support_path("demo-skill", path)
+
+
 def test_read_text_or_none_decodes_text_and_returns_none_for_binary(tmp_path: Path):
     text = tmp_path / "t.md"
     text.write_text("héllo", encoding="utf-8")
diff --git a/backend/tests/test_uploads_manager.py b/backend/tests/test_uploads_manager.py
index 10c8e3806f..0b75362dbb 100644
--- a/backend/tests/test_uploads_manager.py
+++ b/backend/tests/test_uploads_manager.py
@@ -108,6 +108,15 @@ class TestNormalizeFilename:
         with pytest.raises(ValueError, match="reserved Windows device name"):
             normalize_filename(filename)
 
+    @pytest.mark.parametrize("filename", ["NUL .txt", "con  .log", "COM1 .txt", "lpt²  .md", "CONIN$ .txt", "conout$  .log"])
+    def test_rejects_windows_device_names_with_spaces_before_extension(self, filename):
+        with pytest.raises(ValueError, match="reserved Windows device name"):
+            normalize_filename(filename)
+
+    @pytest.mark.parametrize("filename", ["report .txt", "NULnotes .txt", "COM¹notes .txt", "NUL\u00a0.txt"])
+    def test_preserves_spaces_in_ordinary_filename_stems(self, filename):
+        assert normalize_filename(filename) == filename
+
     @pytest.mark.parametrize("filename", ["CONIN", "CONOUT", "CONIN$notes.txt", "CONOUT$notes.log"])
     def test_allows_names_resembling_console_devices(self, filename):
         assert normalize_filename(filename) == filename