This is resolved using queryParams on the frontend side.

I was able to set the queryParams like authToken and userEmail inisde the NextJs App
e.g:

const queryParameters = {
      parameters: {
        authToken: { stringValue: authToken },
        userEmail: { stringValue: userEmail },
      },
    };

dfMessenger.setQueryParameters(queryParameters);

and read them at the agent level inside vertex ai playbook instructions as session params.