diff --git a/pylti1p3/contrib/flask/cookie.py b/pylti1p3/contrib/flask/cookie.py
index 775855f..ee67156 100644
--- a/pylti1p3/contrib/flask/cookie.py
+++ b/pylti1p3/contrib/flask/cookie.py
@@ -31,4 +31,5 @@ def update_response(self, response):
 
             if self._request.is_secure():
                 cookie_kwargs["samesite"] = "None"
+            cookie_kwargs["partitioned"] = True
             response.set_cookie(**cookie_kwargs)
diff --git a/pylti1p3/cookies_allowed_check.py b/pylti1p3/cookies_allowed_check.py
index 388d924..5d384d0 100644
--- a/pylti1p3/cookies_allowed_check.py
+++ b/pylti1p3/cookies_allowed_check.py
@@ -89,7 +89,7 @@ def get_js_block(self) -> str:
         function checkCookiesAllowed() {
             var cookie = "lti1p3_test_cookie=1; path=/";
             if (siteProtocol === 'https') {
-                cookie = cookie + '; SameSite=None; secure';
+                cookie = cookie + '; SameSite=None; secure; partitioned';
             }
             document.cookie = cookie;
             var res = document.cookie.indexOf("lti1p3_test_cookie") !== -1;
diff --git a/pylti1p3/deep_link_resource.py b/pylti1p3/deep_link_resource.py
index 131606f..0574f29 100644
--- a/pylti1p3/deep_link_resource.py
+++ b/pylti1p3/deep_link_resource.py
@@ -65,7 +65,6 @@ def to_dict(self) -> t.Dict[str, object]:
             "type": self._type,
             "title": self._title,
             "url": self._url,
-            "custom": self._custom_params,
         }
         if self._lineitem:
             line_item: t.Dict[str, object] = {
@@ -93,4 +92,7 @@ def to_dict(self) -> t.Dict[str, object]:
         if self._icon_url:
             res["icon"] = {"url": self._icon_url}
 
+        if self._custom_params:
+            res["custom"] = self._custom_params
+
         return res
diff --git a/pylti1p3/message_launch.py b/pylti1p3/message_launch.py
index 4143900..3718525 100644
--- a/pylti1p3/message_launch.py
+++ b/pylti1p3/message_launch.py
@@ -775,7 +775,7 @@ def set_launch_data_storage(
             if session_id:
                 data_storage.set_session_id(session_id)
             else:
-                raise LtiException(f"Missing %s cookie {session_cookie_name}")
+                raise LtiException(f"Missing {session_cookie_name} cookie")
         self._session_service.set_data_storage(data_storage)
         return self
 
diff --git a/pylti1p3/oidc_login.py b/pylti1p3/oidc_login.py
index 6d23e01..7a6574f 100644
--- a/pylti1p3/oidc_login.py
+++ b/pylti1p3/oidc_login.py
@@ -134,7 +134,10 @@ def _prepare_redirect_url(self, launch_url: str) -> str:
             # LTI message hint to identify LTI context within the platform
             auth_params["lti_message_hint"] = lti_message_hint
 
-        auth_login_return_url = auth_login_url + "?" + urlencode(auth_params)
+        if "?" in auth_login_url:
+            auth_login_return_url = auth_login_url + "&" + urlencode(auth_params)
+        else:
+            auth_login_return_url = auth_login_url + "?" + urlencode(auth_params)
         return auth_login_return_url
 
     def _prepare_redirect(self, launch_url: str) -> Redirect[RED]: