Yamato as both battle-line weapon and deterrent

Were Yamato and Musashi intended only as secret battle-line surprises rather than deterrent assets?

No; Japanese naval factions assigned them different purposes, including a deterrent role meant to compensate for inferior numbers.

Yamato and Musashi were not conceived only as an unpleasant surprise for an American battle line. Japanese naval factions assigned the ships different purposes and fought bitterly over strategy. One camp emphasized decisive battle, while figures associated with Mitsumasa Yonai treated oversized battleships as a way to offset American numerical superiority and deter war. That deterrent logic mattered because many Japanese naval leaders understood that a prolonged war with the United States was not one Japan could win.