Unable to locate the specified class: Session.php

Pre-hooking controller construction means that you don't have anything loaded inside the controller object so the last suggestion of ciadmin will help you.

BTW instead of prehooking controllers I can suggest another tactic using MY_Controller which will be used from your controllers and will extend CI_Controller..
There you will have real control of the ci controller construct + you can add any custom common method for all controllers.

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