A. Soulain, B. Lopez, A. Matter, F. Lykou, P. Boley, M. Scheuck, R. van Boekel, J. -C. Augereau, M. leTessier, J. Bouvier, P. Berio, P. Ábrahám, N. Anugu, J. -P. Berger, R. Burn, W. -C. Danchi, W. J. de Wit, F. Drewes, V. Fleury, V. Hocdé, W. Jaffe, Á Kóspál, E. Koumpia, J. -B. Lebouquin, J. S. Martin, H. Meheut, F. Millour, N. Nardetto, E. Pantin, K. Perraut, R. Petrov, L. N. A van Haastere, J. Varga, G. Weigelt, S. Wolf

The dynamics of the inner regions of young stellar objects (YSOs) is driven by a variety of physical phenomena, from magnetospheres and accretion to the dust sublimation rim and inner disk flows. These inner environments evolve on timescales of hours to days, exactly when bursts, dips, and rapid structural changes carry the most valuable information about star and planet formations, but remain hardly reachable with current facilities. A better reactive infrastructure with six or more telescopes, combined with alerts from large time-domain surveys (e.g., at the era of LSST/Rubin type facilities), and equipped with instruments spanning from the V-band to the thermal infrared (N), would provide the instantaneous uv-coverage and spectral diagnostics needed to unambiguously interpret and image these events as they happen. Such a world's first time-domain interferometric observatory would enable qualitatively new science: directly linking optical and infrared variability to spatially resolved changes in magnetospheric accretion, inner-disk geometry, and dust and gas dynamics in the innermost astronomical unit. Crucially, connecting these processes to outer-scale unresolved information from JWST, ALMA, and the ELT would yield a complete tomography of the planet-forming region.