i want to use ory hydrator mutator such that is i want to sent only specific user information from a cookie to my backend server? But i am getting this error: "message": "json: cannot unmarshal string into Go value of type authn.AuthenticationSession"

The idea behind the hydrator is to collect additional information (which is not available in the authentication information) from other sources. So, you actually cannot use it to provide a specific information to your backend in sense of authentication. 8f you wanna do that, these are the other mutators, which do that. Nevertheless, the implementation of the hydrator expects the response to adhere to a specific structure. It looks like your backend does not answer with that expected structure (take a closer look at the hydrator docs. The expected response structure is described there)