diff --git a/loopx/control_plane/coordination/sqlite_backup.ts b/loopx/control_plane/coordination/sqlite_backup.ts
index 79462655b0..22cd48f280 100644
--- a/loopx/control_plane/coordination/sqlite_backup.ts
+++ b/loopx/control_plane/coordination/sqlite_backup.ts
@@ -28,7 +28,9 @@ export async function snapshotSqliteDatabase(source: string, destination: string
         throw new Error("SQLite backup integrity check failed");
       }
     } finally { copy.close(); }
-    const handle = await open(destination, "r");
+    // Windows flush (FlushFileBuffers) requires write access; the snapshot bytes
+    // are already complete, so update mode only permits the durability sync.
+    const handle = await open(destination, "r+");
     try { await handle.sync(); } finally { await handle.close(); }
     return info;
   } catch (error) {
diff --git a/loopx/state_backup.py b/loopx/state_backup.py
index 0365494e41..d35f6dab38 100644
--- a/loopx/state_backup.py
+++ b/loopx/state_backup.py
@@ -619,7 +619,9 @@ def execute_state_backup_plan(payload: dict[str, Any]) -> dict[str, Any]:
         staged_manifest.write_text(json.dumps(updated, ensure_ascii=False, indent=2) + "\n", encoding="utf-8")
         for path in (staged_archive, staged_manifest):
             path.chmod(0o600)
-            with path.open("rb") as handle:
+            # Windows requires write access for fsync; update mode only permits
+            # the durability flush.
+            with path.open("rb+") as handle:
                 os.fsync(handle.fileno())
         os.replace(staged_archive, archive_path)
         os.replace(staged_manifest, manifest_path)
diff --git a/tests/control_plane_ts/sqlite_backup.test.ts b/tests/control_plane_ts/sqlite_backup.test.ts
index c5aa55ba7d..bf8e1c26e0 100644
--- a/tests/control_plane_ts/sqlite_backup.test.ts
+++ b/tests/control_plane_ts/sqlite_backup.test.ts
@@ -34,7 +34,8 @@ test("online backup preserves committed metadata, schema and blobs, excluding an
     } finally { copy.close(); }
     writer.exec("ROLLBACK");
     assert.equal(writer.prepare("SELECT metadata FROM facts").get()?.metadata, metadata);
-    assert.equal((await stat(target)).mode & 0o777, 0o600);
+    // Windows stat reports 0o666 for every writable file and has no owner-only mode.
+    if (process.platform !== "win32") assert.equal((await stat(target)).mode & 0o777, 0o600);
   } finally { writer.close(); }
 });
 
diff --git a/tests/test_state_backup.py b/tests/test_state_backup.py
index 8908d95e97..d45741816f 100644
--- a/tests/test_state_backup.py
+++ b/tests/test_state_backup.py
@@ -106,8 +106,10 @@ def test_duplicate_targets_reuse_snapshot_and_preserve_ordinary_files(tmp_path):
     assert (restored / "runtime-root/database-link").is_symlink()
     assert "runtime-root/notes-wal" in names
     assert (restored / "runtime-root/not-a-database.sqlite").read_text() == "ordinary text"
-    assert (Path(payload["archive_path"]).stat().st_mode & 0o777) == 0o600
-    assert (Path(payload["manifest_path"]).stat().st_mode & 0o777) == 0o600
+    if sys.platform != "win32":
+        # Windows stat reports 0o666 for every writable file and has no owner-only mode.
+        assert (Path(payload["archive_path"]).stat().st_mode & 0o777) == 0o600
+        assert (Path(payload["manifest_path"]).stat().st_mode & 0o777) == 0o600
 
 
 def test_backup_does_not_walk_or_archive_junction_targets(tmp_path, monkeypatch):