diff --git a/tests/tools/test_file_sync_back.py b/tests/tools/test_file_sync_back.py
index 5b29010795..5075dd3aea 100644
--- a/tests/tools/test_file_sync_back.py
+++ b/tests/tools/test_file_sync_back.py
@@ -457,3 +457,44 @@ class TestSyncBackSizeCap:
         # Default cap (2 GiB) is far above our tiny tar; extraction should proceed
         mgr.sync_back(hermes_home=tmp_path / ".hermes")
         assert Path(host_file).read_bytes() == b"remote_version"
+
+
+class TestSyncBackWindowsHost:
+    """#76267: sync_back on a Windows host. The staging tar must be reopenable for writing by
+    the backend (NamedTemporaryFile held an exclusive handle → PermissionError), and remote
+    keys/parents must stay POSIX (relpath/Path stringify with backslashes on Windows, so no
+    staged file ever matched its mapping and every edit was dropped as "no host mapping")."""
+
+    def test_backend_can_reopen_the_tar_path_for_writing(self, tmp_path):
+        """Contract on every host: the download callback receives a path nothing else holds open."""
+        seen = {}
+
+        def download(dest: Path) -> None:
+            buf = io.BytesIO()
+            with tarfile.open(fileobj=buf, mode="w") as tar:
+                info = tarfile.TarInfo(name="root/.hermes/skill.py")
+                info.size = 2
+                tar.addfile(info, io.BytesIO(b"v2"))
+            with open(dest, "wb") as fh:  # the SSH/Modal backends write exactly like this
+                fh.write(buf.getvalue())
+            seen["dest"] = dest
+
+        host_file = tmp_path / "host" / "skill.py"
+        _write_file(host_file, b"v1")
+        mgr = _make_manager(tmp_path, [(str(host_file), "/root/.hermes/skill.py")], bulk_download_fn=download)
+        mgr._pushed_hashes["/root/.hermes/skill.py"] = _sha256_bytes(b"v1")
+        mgr.sync_back(hermes_home=tmp_path / ".hermes")
+        assert host_file.read_bytes() == b"v2"
+        assert not seen["dest"].exists()  # staging tar removed after use
+
+    @pytest.mark.windows_only
+    def test_posix_remote_keys_match_on_windows(self, tmp_path):
+        host_file = tmp_path / "host" / "skill.py"
+        _write_file(host_file, b"v1")
+        mapping = [(str(host_file), "/root/.hermes/skills/a/skill.py")]
+        mgr = _make_manager(tmp_path, mapping, bulk_download_fn=_make_download_fn({
+            "root/.hermes/skills/a/skill.py": b"v2", "root/.hermes/skills/a/new.md": b"new"}))
+        mgr._pushed_hashes["/root/.hermes/skills/a/skill.py"] = _sha256_bytes(b"v1")
+        mgr.sync_back(hermes_home=tmp_path / ".hermes")
+        assert host_file.read_bytes() == b"v2"  # relpath key was 'root\\.hermes\\...' → skipped
+        assert (tmp_path / "host" / "new.md").read_bytes() == b"new"  # _infer_host_path parent match