The protection of national security has traditionally been an exception to general norms of public accountability, based on prerogative powers. The last three decades have seen efforts to bring national security closer to the normal constitutional control mechanisms of parliament and the courts. The design of and changes to mechanisms of accountability have, however, been accepted without discussion of the often narrower purposes for which they were first established (most notably for oversight of surveillance), the extent of their departure from constitutional principles, or their impact in embedding new forms of exceptionalism in the constitutional framework. This article critically assesses these developments, prompted for example by the Law Commission's recommendations to reform official secrets laws, which adopted trusted intermediary and indirect accountability models without full consideration of historical and contemporary concerns or the exceptionalism on which they were based. Though focused on the UK, our account provides a cautionary tale for national security law reform in any modern democracy.