Closed string deformations in open string field theory. Part I. Bosonic string

This is the first of a series three papers on open string field theories based Witten star product deformed with gauge invariant open/closed coupling. deformation tree-level tadpole which destabilizes initial perturbative vacuum. We discuss existence vacuum-shift solutions cancel and represent new configuration where D-brane system has adapted to change in closed background. As an example we consider bulk changes compactification radius and, order deformation, reproduce shift mass KK modes from kinetic operator after vacuum shift. also possibility taming degenerations propagator simplest amplitude corresponding two strings off disk.