A very good question, and one where it is difficult to give a cut and dried answer. Certainly it is clear from statute and case law that the agenda must provide sufficient information for councillors to know what matters are to be decided at a meeting. Inadequate information would make decisions invalid and open to challenge. There is also a strong tendency in case law and advice towards a requirement for supporting documents to be available to both councillors and public. In the current climate, I would argue that the only sound way to proceed is for all agenda items to be specific, and for all but strictly confidential supporting papers to be made available in good time, along with the agenda, to both councillors and public.
If the agenda does not give sufficient advance information, then certainly councillors would be entitled to object on the grounds that decisions made would be invalid, and thus the matter cannot be decided at that time.