diff --git a/CHANGELOG.md b/CHANGELOG.md
index eb34e3ce81..70116de2a6 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -19,6 +19,15 @@ This section accumulates work toward the **2.3.0** milestone
 
 ### Fixed
 
+- **gateway:** A caller-supplied thread ID that is a Windows reserved device
+  name (`NUL`, `CON`, `COM1`, ...) is now rejected at the shared
+  `validate_thread_id()` boundary. Such IDs previously passed validation but
+  failed workspace creation on Windows hosts with `FileNotFoundError`. The
+  check reuses `windows_incompatible_segment()` and applies on every platform,
+  so threads with reserved-name IDs created on Linux hosts will fail
+  validation on read/resume/upload routes after upgrade (deletion still
+  works). ([#7892])
+
 - **extensions:** Context-compaction observers now learn which task a
   compaction happened in. Inside a lead run or subagent they receive that
   task's store instead of a detached one, and `CompactionEvent` gains
@@ -16012,3 +16021,4 @@ with **180 merged pull requests** since the first 2.0 milestone tag.
 [#7781]: https://github.com/bytedance/deer-flow/pull/7781
 [#7792]: https://github.com/bytedance/deer-flow/pull/7792
 [#7831]: https://github.com/bytedance/deer-flow/pull/7831
+[#7892]: https://github.com/bytedance/deer-flow/pull/7892
diff --git a/backend/packages/harness/deerflow/utils/thread_id.py b/backend/packages/harness/deerflow/utils/thread_id.py
index 1bae01348a..cd35ad37e3 100644
--- a/backend/packages/harness/deerflow/utils/thread_id.py
+++ b/backend/packages/harness/deerflow/utils/thread_id.py
@@ -8,6 +8,8 @@ from typing import Annotated
 
 from pydantic import AfterValidator, StringConstraints
 
+from deerflow.utils.host_paths import windows_incompatible_segment
+
 THREAD_ID_PATTERN = r"^[A-Za-z0-9_-]{1,64}$"
 _THREAD_ID_RE = re.compile(THREAD_ID_PATTERN)
 
@@ -20,6 +22,15 @@ def validate_thread_id(thread_id: str) -> str:
     """
     if not isinstance(thread_id, str) or _THREAD_ID_RE.fullmatch(thread_id) is None:
         raise ValueError("Invalid thread_id: expected 1-64 ASCII letters, digits, hyphens, or underscores")
+    # Windows reserves device names such as NUL in every directory; creating a
+    # thread workspace named "NUL" fails with FileNotFoundError on Windows
+    # hosts. Reuse the canonical Windows-name check that uploads, sandbox
+    # writes and skill support files already go through, so thread IDs follow
+    # the same documented "rejected on every platform when a name is created"
+    # contract instead of keeping a second name list in sync.
+    reason = windows_incompatible_segment(thread_id)
+    if reason is not None:
+        raise ValueError(f"Invalid thread_id {thread_id!r}: {reason}")
     return thread_id
 
 
diff --git a/backend/tests/test_thread_id_validation.py b/backend/tests/test_thread_id_validation.py
index cb987360ac..4ef630f2e3 100644
--- a/backend/tests/test_thread_id_validation.py
+++ b/backend/tests/test_thread_id_validation.py
@@ -27,6 +27,18 @@ def test_validate_thread_id_accepts_canonical_ids(thread_id: str) -> None:
         "has space",
         "line\nbreak",
         "线程",
+        # Windows reserved device names break workspace creation on Windows
+        # hosts (e.g. Paths.ensure_thread_dirs raising FileNotFoundError for
+        # "NUL"), so they are rejected at the shared validation boundary via
+        # windows_incompatible_segment().
+        "NUL",
+        "nul",
+        "Nul",
+        "CON",
+        "PRN",
+        "AUX",
+        "COM1",
+        "LPT9",
     ],
 )
 def test_validate_thread_id_rejects_noncanonical_ids(thread_id: str) -> None:
@@ -36,6 +48,28 @@ def test_validate_thread_id_rejects_noncanonical_ids(thread_id: str) -> None:
         validate_thread_id(thread_id)
 
 
+@pytest.mark.parametrize(
+    "thread_id",
+    [
+        "NUL",
+        "nul",
+        "Nul",
+        "CON",
+        "PRN",
+        "AUX",
+        "COM1",
+        "LPT9",
+    ],
+)
+def test_validate_thread_id_rejects_windows_reserved_names(thread_id: str) -> None:
+    """Reserved device names hit the windows_incompatible_segment branch, not
+    just the character-pattern branch."""
+    from deerflow.utils.thread_id import validate_thread_id
+
+    with pytest.raises(ValueError, match="reserved Windows device name"):
+        validate_thread_id(thread_id)
+
+
 @pytest.mark.parametrize("thread_id", [1, {}, []])
 def test_validate_thread_id_rejects_non_strings(thread_id: object) -> None:
     from deerflow.utils.thread_id import validate_thread_id