diff --git a/README.md b/README.md
index a844f5f58f..bc78d1f7fa 100644
--- a/README.md
+++ b/README.md
@@ -2452,6 +2452,11 @@ follow ordinary upload filename validation. Names containing NUL, Windows
 reserved device names (such as `CON.txt`), or trailing dots are rejected with
 `400` before bytes are staged.
 
+On native Windows, the document shelf uses extended-length filesystem paths
+so deep workspace directories and long filenames can be uploaded, downloaded,
+restored, and purged without enabling the system-wide long-path setting.
+Filesystem limits on individual path components still apply.
+
 Runs on member threads also receive a bounded `<documents>` index rendered per
 run from the pinned snapshot (capped by `projects.shelf_index_max_entries` and
 `projects.shelf_index_max_bytes`), and the agent can page the shelf and read
diff --git a/backend/packages/harness/deerflow/config/AGENTS.md b/backend/packages/harness/deerflow/config/AGENTS.md
index 247a1bccc3..2ed1ebda12 100644
--- a/backend/packages/harness/deerflow/config/AGENTS.md
+++ b/backend/packages/harness/deerflow/config/AGENTS.md
@@ -1,5 +1,14 @@
 ### Configuration System
 
+`Paths.user_projects_dir()` uses `extended_length_path()` on native Windows.
+All document paths, including staging and retention walks, inherit the same
+extended drive/UNC namespace even when the root itself is short. Persisted
+`stored_relpath` values and Docker mount paths retain their existing spelling.
+`project_document_path()` still resolves symlinks and checks confinement using
+the same namespace for both the user projects root and the document path.
+Do not prefix only paths already exceeding MAX_PATH: appended filenames and
+derived companions can cross the limit later.
+
 Operator prompt overlays: `lead_prompt_overlay` on AppConfig and
 `subagents.agents.<name>.prompt_overlay` accept literal `prepend`/`append` strings.
 The per-assembly snapshot owns these settings; no run-context override exists.
diff --git a/backend/packages/harness/deerflow/config/paths.py b/backend/packages/harness/deerflow/config/paths.py
index 3723f04969..e8508c6808 100644
--- a/backend/packages/harness/deerflow/config/paths.py
+++ b/backend/packages/harness/deerflow/config/paths.py
@@ -6,6 +6,7 @@ import shutil
 from pathlib import Path, PureWindowsPath
 
 from deerflow.config.runtime_paths import runtime_home
+from deerflow.utils.host_paths import extended_length_path
 from deerflow.utils.thread_id import validate_thread_id
 
 # Virtual path prefix seen by agents inside the sandbox
@@ -366,7 +367,7 @@ class Paths:
 
     def user_projects_dir(self, user_id: str) -> Path:
         """Host path root for one user's project shelves: ``users/{user_id}/projects/``."""
-        return self.user_dir(user_id) / "projects"
+        return extended_length_path(self.user_dir(user_id) / "projects")
 
     def user_project_dir(self, user_id: str, project_id: str) -> Path:
         """Host path for one project: ``users/{user_id}/projects/{project_id}/``."""
diff --git a/backend/packages/harness/deerflow/projects/trash.py b/backend/packages/harness/deerflow/projects/trash.py
index fdafe15f6a..fb0f93c841 100644
--- a/backend/packages/harness/deerflow/projects/trash.py
+++ b/backend/packages/harness/deerflow/projects/trash.py
@@ -31,6 +31,7 @@ from typing import TYPE_CHECKING
 from deerflow.config.paths import Paths
 from deerflow.projects.documents import _content_intact, check_document_content, converted_markdown_path, original_file_path
 from deerflow.utils.file_io import await_drained, run_file_io
+from deerflow.utils.host_paths import extended_length_path
 from deerflow.utils.time import coerce_iso
 
 if TYPE_CHECKING:
@@ -233,7 +234,7 @@ def _reconcile_storage(paths: Paths, *, user_id: str | None, rows: list[dict], g
     if user_id is not None:
         user_ids = [user_id]
     else:
-        users_root = paths.base_dir / "users"
+        users_root = extended_length_path(paths.base_dir / "users")
         user_ids = sorted(entry.name for entry in users_root.iterdir() if entry.is_dir()) if users_root.is_dir() else []
     for uid in user_ids:
         projects_root = paths.user_projects_dir(uid)
diff --git a/backend/packages/harness/deerflow/utils/AGENTS.md b/backend/packages/harness/deerflow/utils/AGENTS.md
index 3143816145..b254801cf0 100644
--- a/backend/packages/harness/deerflow/utils/AGENTS.md
+++ b/backend/packages/harness/deerflow/utils/AGENTS.md
@@ -82,6 +82,13 @@ lives in `tests/test_file_conversion_cancellation.py`.
 
 ### Host Path Portability
 
+`host_paths.extended_length_path()` provides lexical Windows filesystem
+spelling (absolute `\\?\` drive paths or `\\?\UNC\` shares); it is a no-op
+on POSIX. It does not replace symlink resolution or confinement checks and
+must not be used for persisted relative paths or Docker mount sources.
+Existing extended (`\\?\`) and device (`\\.\`, including named pipes)
+namespaces pass through unchanged before normalization.
+
 `host_paths.py` rejects Windows device names for host-visible creation paths on
 every platform, including the `COM`/`LPT` aliases with superscript ¹, ² and ³.
 The console aliases `CONIN$` and `CONOUT$` are reserved too; match them
diff --git a/backend/packages/harness/deerflow/utils/host_paths.py b/backend/packages/harness/deerflow/utils/host_paths.py
index f998a7f707..b65553a74b 100644
--- a/backend/packages/harness/deerflow/utils/host_paths.py
+++ b/backend/packages/harness/deerflow/utils/host_paths.py
@@ -2,6 +2,32 @@
 
 from __future__ import annotations
 
+import os
+from pathlib import Path
+
+
+def extended_length_path(path: Path) -> Path:
+    """Use Windows' extended namespace for local filesystem access.
+
+    Prefix before appending children: even a short root can contain a long
+    document filename. Normalize ordinary paths before adding the prefix;
+    extended paths bypass Win32's ordinary path normalization. This is lexical
+    only: callers must still resolve symlinks and enforce their own confinement.
+    Existing extended/device namespaces (including named pipes) are returned
+    unchanged, without normalizing their segments.
+    POSIX paths are unchanged. Do not use this spelling for Docker mount sources
+    or persisted relative paths.
+    """
+    if os.name != "nt":
+        return path
+    if str(path).startswith(("\\\\?\\", "\\\\.\\")):
+        return path
+    absolute = os.path.abspath(path)
+    if absolute.startswith("\\\\"):
+        return Path("\\\\?\\UNC\\" + absolute[2:])
+    return Path("\\\\?\\" + absolute)
+
+
 # Windows also treats the ISO-8859-1 superscript digits ¹, ² and ³ as device numbers.
 # https://learn.microsoft.com/en-us/windows/win32/fileio/naming-a-file
 # CONIN$ and CONOUT$ open the console input and output buffers, not ordinary files.
diff --git a/backend/tests/blocking_io/test_project_documents.py b/backend/tests/blocking_io/test_project_documents.py
index c05c66a736..fb08765e5a 100644
--- a/backend/tests/blocking_io/test_project_documents.py
+++ b/backend/tests/blocking_io/test_project_documents.py
@@ -20,7 +20,6 @@ from __future__ import annotations
 
 import functools
 import json
-import os
 from types import SimpleNamespace
 
 import pytest
@@ -36,10 +35,6 @@ from deerflow.utils.file_io import run_file_io as _real_run_file_io
 pytestmark = [
     pytest.mark.asyncio,
     pytest.mark.allow_blocking_io,
-    pytest.mark.skipif(
-        os.name == "nt",
-        reason="project-document anchors exceed Windows MAX_PATH (WinError 206); see #6353",
-    ),
 ]
 
 _USER = "u1"
diff --git a/backend/tests/blocking_io/test_project_documents_promotion.py b/backend/tests/blocking_io/test_project_documents_promotion.py
index 98b94ad15b..e3e34f643f 100644
--- a/backend/tests/blocking_io/test_project_documents_promotion.py
+++ b/backend/tests/blocking_io/test_project_documents_promotion.py
@@ -13,7 +13,6 @@ an offload drops the dispatch and turns the anchor red.
 from __future__ import annotations
 
 import functools
-import os
 from types import SimpleNamespace
 
 import pytest
@@ -64,10 +63,6 @@ async def _shelf_document(env) -> dict:
     return result[0]
 
 
-@pytest.mark.skipif(
-    os.name == "nt",
-    reason="attach staging copy/read exceed Windows MAX_PATH (WinError 206); see #6353",
-)
 async def test_attach_staging_copy_dispatches_off_the_loop(tmp_path, monkeypatch) -> None:
     """The under-lock source copy (existence/size check + ``shutil.copyfile``)
     must go through ``run_file_io`` while the row lock is held."""
@@ -87,10 +82,6 @@ async def test_attach_staging_copy_dispatches_off_the_loop(tmp_path, monkeypatch
         await close_engine()
 
 
-@pytest.mark.skipif(
-    os.name == "nt",
-    reason="attach staging copy/read exceed Windows MAX_PATH (WinError 206); see #6353",
-)
 async def test_attach_staged_read_dispatches_off_the_loop(tmp_path, monkeypatch) -> None:
     """``read_file_chunks`` (attach ingestion source + from-thread promote
     source) must dispatch the open, every chunk read, and the close through
diff --git a/backend/tests/blocking_io/test_project_trash.py b/backend/tests/blocking_io/test_project_trash.py
index 7a40948b6c..2126c9db98 100644
--- a/backend/tests/blocking_io/test_project_trash.py
+++ b/backend/tests/blocking_io/test_project_trash.py
@@ -11,7 +11,6 @@ anchor red (mutation-verified style of this suite).
 from __future__ import annotations
 
 import functools
-import os
 from types import SimpleNamespace
 
 import pytest
@@ -25,10 +24,6 @@ from deerflow.utils.file_io import run_file_io as _real_run_file_io
 pytestmark = [
     pytest.mark.asyncio,
     pytest.mark.allow_blocking_io,
-    pytest.mark.skipif(
-        os.name == "nt",
-        reason="project-document anchors exceed Windows MAX_PATH (WinError 206); see #6353",
-    ),
 ]
 
 _USER = "u1"
diff --git a/backend/tests/test_extended_length_paths.py b/backend/tests/test_extended_length_paths.py
new file mode 100644
index 0000000000..440be37afc
--- /dev/null
+++ b/backend/tests/test_extended_length_paths.py
@@ -0,0 +1,72 @@
+"""Windows filesystem spelling and document confinement regressions."""
+
+import ntpath
+import os
+from pathlib import Path
+from types import SimpleNamespace
+
+import pytest
+
+from deerflow.config.paths import Paths
+from deerflow.utils import host_paths
+
+
+@pytest.mark.parametrize(
+    ("raw", "expected"),
+    [
+        (r"C:\shelf\..\documents", r"\\?\C:\documents"),
+        ("C:/shelf/documents", r"\\?\C:\shelf\documents"),
+        (r"\\server\share\shelf", r"\\?\UNC\server\share\shelf"),
+        (r"\\?\C:\shelf", r"\\?\C:\shelf"),
+        (r"\\?\UNC\server\share\shelf", r"\\?\UNC\server\share\shelf"),
+        (r"\\.\C:\shelf", r"\\.\C:\shelf"),
+        (r"\\.\pipe\deerflow", r"\\.\pipe\deerflow"),
+        (r"\\?\C:\shelf\..\documents", r"\\?\C:\shelf\..\documents"),
+    ],
+)
+def test_windows_extended_spelling(monkeypatch, raw, expected):
+    # Patch this module's OS binding, never the process-wide os.name (pathlib
+    # and pytest need to keep using the actual host's path implementation).
+    monkeypatch.setattr(host_paths, "os", SimpleNamespace(name="nt", path=ntpath))
+    assert str(host_paths.extended_length_path(Path(raw))) == expected
+
+
+def test_posix_path_is_unchanged(monkeypatch):
+    monkeypatch.setattr(host_paths, "os", SimpleNamespace(name="posix"))
+    path = Path("relative/documents")
+    assert host_paths.extended_length_path(path) is path
+
+
+@pytest.mark.skipif(os.name != "nt", reason="native Windows relative-path normalization")
+def test_windows_relative_path_is_absolutized(monkeypatch, tmp_path):
+    monkeypatch.chdir(tmp_path)
+    actual = host_paths.extended_length_path(Path("shelf/../documents"))
+    assert str(actual) == "\\\\?\\" + str(tmp_path / "documents")
+
+
+def test_document_traversal_is_still_rejected(tmp_path):
+    paths = Paths(base_dir=tmp_path)
+    with pytest.raises(ValueError, match="path traversal"):
+        paths.project_document_path("alice", "../outside")
+    with pytest.raises(ValueError, match="path traversal"):
+        paths.project_document_path("alice", str(tmp_path / "outside"))
+
+
+@pytest.mark.parametrize("outside_root", [False, True])
+def test_document_symlink_confinement(tmp_path, outside_root):
+    paths = Paths(base_dir=tmp_path)
+    base = paths.user_projects_dir("alice")
+    base.mkdir(parents=True)
+    target = tmp_path / "outside" if outside_root else base / "target"
+    target.mkdir()
+    try:
+        (base / "link").symlink_to(target, target_is_directory=True)
+    except OSError:
+        if os.name == "nt":
+            pytest.skip("Windows symlink privilege is not available")
+        raise
+    if outside_root:
+        with pytest.raises(ValueError, match="path traversal"):
+            paths.project_document_path("alice", "link/content")
+    else:
+        assert paths.project_document_path("alice", "link/content") == (target / "content").resolve()
diff --git a/backend/tests/test_trash_router.py b/backend/tests/test_trash_router.py
index a98715a244..0b6d37ff65 100644
--- a/backend/tests/test_trash_router.py
+++ b/backend/tests/test_trash_router.py
@@ -8,6 +8,7 @@ and the lazy retention sweep through the real HTTP stack.
 
 from __future__ import annotations
 
+import os
 from datetime import UTC, datetime, timedelta
 from pathlib import Path
 from types import SimpleNamespace
@@ -131,6 +132,52 @@ def _original_path(app: FastAPI, row: dict, *, user_id: str = "user-a") -> Path:
     return original_file_path(get_paths(), user_id=user_id, row=row)
 
 
+@pytest.mark.parametrize("filename", ["notes.txt", "a" * 251 + ".txt"])
+def test_long_workspace_document_lifecycle(tmp_path, monkeypatch, filename):
+    """Real router/file operations must work beyond Windows' legacy MAX_PATH."""
+    from deerflow.config.paths import get_paths
+    from deerflow.projects.trash import run_trash_retention_sweep
+
+    home = tmp_path / ("nested-" + "a" * 73) / ("nested-" + "b" * 73) / ("nested-" + "c" * 73)
+    assert len(str(home)) > 260
+    monkeypatch.setenv("DEER_FLOW_HOME", str(home))
+    app = _build_app(tmp_path)
+    with TestClient(app) as client:
+        origin = _create_project(client, name="Origin")
+        target = _create_project(client, name="Target")
+        doc = _upload(client, origin["id"], filename, b"long-path content")
+        row = _get_row(app, doc["id"])
+        original = _original_path(app, row)
+        assert original.read_bytes() == b"long-path content"
+        assert not row["stored_relpath"].startswith("\\\\")
+        listing = client.get(f"/api/projects/{origin['id']}/documents").json()
+        assert listing["documents"][0]["content_missing"] is False
+        download = client.get(f"/api/projects/{origin['id']}/documents/{doc['id']}/content", params={"download": "true"})
+        assert download.status_code == 200
+        assert download.content == b"long-path content"
+        staging = get_paths().project_documents_dir("user-a", origin["id"]) / ".staging" / "old-orphan"
+        staging.write_bytes(b"abandoned upload")
+        os.utime(staging, (0, 0))
+
+        async def startup_sweep():
+            return await run_trash_retention_sweep(app.state.project_document_repo, get_paths(), retention_days=30, user_id=None)
+
+        report = anyio.run(startup_sweep)
+        assert report.staging_removed == 1
+        assert not staging.exists()
+        assert original.read_bytes() == b"long-path content"
+        _trash(client, origin["id"], doc["id"])
+        restored = client.post(f"/api/trash/documents/{doc['id']}/restore", json={"project_id": target["id"]})
+        assert restored.status_code == 200
+        assert restored.json()["outcome"] == "restored"
+        assert _original_path(app, _get_row(app, doc["id"])) == original
+        assert client.get(f"/api/projects/{target['id']}/documents/{doc['id']}/content").content == b"long-path content"
+        _trash(client, target["id"], doc["id"])
+        assert client.post(f"/api/trash/documents/{doc['id']}/purge").status_code == 204
+        assert not original.exists()
+        assert _get_row(app, doc["id"]) is None
+
+
 class TestTrashListing:
     def test_list_empty_then_populated_with_origin_display_fields(self, tmp_path):
         app = _build_app(tmp_path)