pallets/flask · error · TypeError

Cookies are disabled. Create a client with…

Error message

Cookies are disabled. Create a client with 'use_cookies=True'.

What it means

FlaskClient.session_transaction needs cookie support to bind the session to the request/response cycle. If the client was created with use_cookies=False (or cookies were explicitly disabled), self._cookies is None and the method raises TypeError immediately, before touching the session.

Solutions

Create the client with use_cookies=True: with app.test_client(use_cookies=True) as c: ...
Move session assertions to a route that reads/writes the session and assert on the response/cookie instead.
If using a custom session backend, drive it through real requests rather than session_transaction.

Example fix

// before
client = app.test_client(use_cookies=False)
with client.session_transaction() as sess:
    sess['k'] = 1  # raises

// after
client = app.test_client(use_cookies=True)
with client.session_transaction() as sess:
    sess['k'] = 1

Defensive patterns

Strategy: validation

Validate before calling

def require_cookies_enabled(client):
    if getattr(client, '_cookies', None) is None:
        raise TypeError('client has cookies disabled; pass use_cookies=True')

Type guard

def client_has_cookies(client) -> bool:
    return getattr(client, '_cookies', None) is not None

Prevention

Build test clients with use_cookies=True whenever sessions are involved.
Centralize client creation in a fixture with the correct flags.
Document use_cookies requirement for session_transaction in test helpers.

When it happens

Trigger: Calling client.session_transaction() on a client built with use_cookies=False; calling it on app.test_client(use_cookies=False); a custom test client subclass that nulls _cookies.

Common situations: Newer Flask (>=3.1) made use_cookies explicit and some test helpers default differently; tests written against an older Flask that always had cookies; API-only test clients that opted out of cookies but then try session_transaction.

Related errors

AI-assisted analysis of pallets/flask@3596b1ab61 (2026-08-11). Data as JSON: /api/errors/081d89dcc4e312ce. Report an issue: GitHub.

Appendix: source

Thrown at src/flask/testing.py:156

    ) -> t.Iterator[SessionMixin]:
        """When used in combination with a ``with`` statement this opens a
        session transaction.  This can be used to modify the session that
        the test client uses.  Once the ``with`` block is left the session is
        stored back.

        ::

            with client.session_transaction() as session:
                session['value'] = 42

        Internally this is implemented by going through a temporary test
        request context and since session handling could depend on
        request variables this function accepts the same arguments as
        :meth:`~flask.Flask.test_request_context` which are directly
        passed through.
        """
        if self._cookies is None:
            raise TypeError(
                "Cookies are disabled. Create a client with 'use_cookies=True'."
            )

        app = self.application
        ctx = app.test_request_context(*args, **kwargs)
        self._add_cookies_to_wsgi(ctx.request.environ)

        with ctx:
            sess = app.session_interface.open_session(app, ctx.request)

        if sess is None:
            raise RuntimeError("Session backend did not open a session.")

        yield sess
        resp = app.response_class()

        if app.session_interface.is_null_session(sess):
            return

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