<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:mn>3</mml:mn> <mml:mo>+</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> <mml:mi math

So far a major source of uncertainty in the study of heavy-ion collisions arises from the early time dynamics which includes initial state and preequilibrium dynamics. The state-of-the-art framework, KøMPøST [Kurkela ; Kurkela ], employs nonequilibrium Green’s functions to propagate the initial energy-momentum tensor to the hydrodynamic phase, yet currently only treats transverse plane dynamics under boost-invariant conditions. In this work, we extend KøMPøST to include nonboost-invariant responses to initial conditions, essential for accurately capturing the longitudinal structures observed in heavy-ion collisions. Nonboost-invariant fluctuations on top of a homogeneous background are evolved using <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mo stretchy="false">(</a:mo> <a:mn>3</a:mn> <a:mo>+</a:mo> <a:mn>1</a:mn> <a:mo stretchy="false">)</a:mo> <a:mi mathvariant="normal">D</a:mi> </a:mrow> </a:math> response functions calculated in kinetic theory. To assess kinetic theory’s transition towards hydrodynamic evolution, we systematically compare the out-of-equilibrium shear-stress tensor from KøMPøST-3D with estimates based on Navier-Stokes hydrodynamics. Subsequently, a comprehensive <f:math xmlns:f="http://www.w3.org/1998/Math/MathML" display="inline"> <f:mrow> <f:mo stretchy="false">(</f:mo> <f:mn>3</f:mn> <f:mo>+</f:mo> <f:mn>1</f:mn> <f:mo stretchy="false">)</f:mo> <f:mi mathvariant="normal">D</f:mi> </f:mrow> </f:math> framework, cipper + KøMPøST-3D + CLVisc + SMASH, is utilized to simulate the complete spacetime evolution of heavy-ion collisions. The sensitivity of key observables, including longitudinal structure of anisotropic flow, to variations in the hydrodynamic initialization time is thoroughly investigated.