diff --git a/loopx/capabilities/benchmark_toolkit/native_codex_isolation.py b/loopx/capabilities/benchmark_toolkit/native_codex_isolation.py
index 4418d494af..f687f0e330 100644
--- a/loopx/capabilities/benchmark_toolkit/native_codex_isolation.py
+++ b/loopx/capabilities/benchmark_toolkit/native_codex_isolation.py
@@ -295,7 +295,10 @@ def _atomic_write_text(path: Path, text: str) -> None:
     )
     temporary_path = Path(temporary)
     try:
-        os.fchmod(descriptor, path.stat().st_mode & 0o777)
+        # Windows has no fchmod; the POSIX permission bits of the source file do
+        # not exist there either, so the copy keeps the platform default.
+        if hasattr(os, "fchmod"):
+            os.fchmod(descriptor, path.stat().st_mode & 0o777)
         with os.fdopen(descriptor, "w", encoding="utf-8") as stream:
             stream.write(text)
             stream.flush()
diff --git a/loopx/capabilities/decision_context/capture.py b/loopx/capabilities/decision_context/capture.py
index 6b3c64d747..5994517f77 100644
--- a/loopx/capabilities/decision_context/capture.py
+++ b/loopx/capabilities/decision_context/capture.py
@@ -41,7 +41,9 @@ def _open_spool(path: Path, *, goal_id: str, agent_id: str) -> sqlite3.Connectio
     descriptor = os.open(
         path, os.O_CREAT | os.O_RDWR | getattr(os, "O_NOFOLLOW", 0), 0o600
     )
-    os.fchmod(descriptor, 0o600)
+    # Windows has no fchmod; os.open already applied the mode above.
+    if hasattr(os, "fchmod"):
+        os.fchmod(descriptor, 0o600)
     os.close(descriptor)
     db = sqlite3.connect(path, timeout=1)
     db.row_factory = sqlite3.Row
diff --git a/loopx/control_plane/goals/botmux_runtime.py b/loopx/control_plane/goals/botmux_runtime.py
index 51b89cedc2..2f55c65fc8 100644
--- a/loopx/control_plane/goals/botmux_runtime.py
+++ b/loopx/control_plane/goals/botmux_runtime.py
@@ -163,7 +163,9 @@ def _write_private_json_atomic(path: Path, payload: Mapping[str, Any]) -> None:
     )
     temporary = Path(temporary_name)
     try:
-        os.fchmod(descriptor, 0o600)
+        # Windows has no fchmod; mkstemp already created the file for this user only.
+        if hasattr(os, "fchmod"):
+            os.fchmod(descriptor, 0o600)
         with os.fdopen(descriptor, "w", encoding="utf-8") as handle:
             json.dump(dict(payload), handle, ensure_ascii=False, indent=2)
             handle.write("\n")
diff --git a/loopx/extensions/lark/private_json.py b/loopx/extensions/lark/private_json.py
index c1f7b2b1e0..9ba1ea0ef9 100644
--- a/loopx/extensions/lark/private_json.py
+++ b/loopx/extensions/lark/private_json.py
@@ -21,7 +21,9 @@ def write_private_json_atomic(
     )
     temporary = Path(temporary_name)
     try:
-        os.fchmod(descriptor, 0o600)
+        # Windows has no fchmod; mkstemp already created the file for this user only.
+        if hasattr(os, "fchmod"):
+            os.fchmod(descriptor, 0o600)
         with os.fdopen(descriptor, "w", encoding="utf-8") as handle:
             json.dump(dict(payload), handle, ensure_ascii=False, indent=2)
             handle.write("\n")
diff --git a/tests/test_windows_atomic_writes.py b/tests/test_windows_atomic_writes.py
index 8816434be9..639d5fca50 100644
--- a/tests/test_windows_atomic_writes.py
+++ b/tests/test_windows_atomic_writes.py
@@ -10,7 +10,9 @@
 import pytest
 
 from loopx.capabilities.decision_context import private_state
+from loopx.capabilities.benchmark_toolkit import native_codex_isolation
 from loopx.control_plane.heartbeat import automation_upgrade
+from loopx.control_plane.goals import botmux_runtime
 from loopx.extensions import presentation
 from loopx.extensions.lark import private_json
 
@@ -22,7 +24,10 @@ def __init__(self, directory: Path) -> None:
         self._directory = directory
 
     def __getattr__(self, name: str) -> Any:
-        if name == "O_DIRECTORY":
+        # Native Windows Python has neither directory fsync nor fchmod; an
+        # attribute that exists only on POSIX must stay missing here, or the
+        # POSIX lane stops covering the Windows surface.
+        if name in {"O_DIRECTORY", "fchmod"}:
             raise AttributeError(name)
         return getattr(os, name)
 
@@ -48,6 +53,14 @@ def _write_heartbeat_automation(path: Path) -> None:
     automation_upgrade._atomic(path, 'prompt = "ready"\n')
 
 
+def _write_botmux_binding(path: Path) -> None:
+    botmux_runtime._write_private_json_atomic(path, {"status": "ready"})
+
+
+def _write_native_isolation(path: Path) -> None:
+    native_codex_isolation._atomic_write_text(path, "prompt = \"ready\"\n")
+
+
 @pytest.mark.parametrize(
     ("module", "writer", "expected"),
     [
@@ -75,6 +88,18 @@ def _write_heartbeat_automation(path: Path) -> None:
             'prompt = "ready"\n',
             id="heartbeat-automation",
         ),
+        pytest.param(
+            botmux_runtime,
+            _write_botmux_binding,
+            {"status": "ready"},
+            id="botmux-binding",
+        ),
+        pytest.param(
+            native_codex_isolation,
+            _write_native_isolation,
+            'prompt = "ready"\n',
+            id="native-isolation",
+        ),
     ],
 )
 def test_atomic_writers_skip_unsupported_windows_directory_fsync(