diff --git a/packages/notte-browser/src/notte_browser/session.py b/packages/notte-browser/src/notte_browser/session.py
index 07e8d8a6f..722b7318e 100644
--- a/packages/notte-browser/src/notte_browser/session.py
+++ b/packages/notte-browser/src/notte_browser/session.py
@@ -129,6 +129,7 @@ def __init__(
         persona: BasePersona | None = None,
         window: BrowserWindow | None = None,
         keep_alive: bool = False,
+        open_viewer: bool = False,
         **data: Unpack[SessionStartRequestDict],
     ) -> None:
         if storage is not None and storage.is_remote:
diff --git a/tests/test_session.py b/tests/test_session.py
index 5dce6052e..ff1766ea6 100644
--- a/tests/test_session.py
+++ b/tests/test_session.py
@@ -242,6 +242,14 @@ async def alist_downloaded_files(self) -> list[FileInfo]:
         _ = NotteSession(storage=_FakeRemoteStorage())
 
 
+@pytest.mark.parametrize("open_viewer", [True, False])
+def test_open_viewer_does_not_raise_validation_error(open_viewer: bool):
+    """open_viewer must be consumed before **data reaches SessionStartRequest.model_validate()."""
+    session = NotteSession(open_viewer=open_viewer)
+    # If we get here, no ValidationError was raised
+    assert session is not None
+
+
 def test_captcha_solver_not_available_error():
     with pytest.raises(CaptchaSolverNotAvailableError):
         _ = NotteSession(solve_captchas=True, browser_type="chrome")