Cosmic parallax watches the sky for the position drifts that anisotropic expansion would superimpose on the aberration baseline our own acceleration produces (detected at the predicted microarcseconds per year). Gaia's current bounds are consistent with isotropy but orders above the metric-test targets, and the model treats the young null as settled confirmation while its anomaly families, dipole excess, directional H₀, axis alignments, all hint at exactly the directional structure this instrument exists to measure.
Expansion isotropy is an FLRW axiom, not a measurement: the model predicts identically zero cosmic parallax forever, at every sensitivity, in every direction. It therefore has nothing to gain from the coming precision and everything to lose, an asymmetry it manages by not discussing the test.
SCT predicts the drift the axiom forbids. An environment-dependent Λ_eff field makes expansion genuinely anisotropic at the few-percent level across the local gigaparsec (P17, P19, the same field the Migkas anisotropy and dipole excess read), and anisotropic expansion is precisely what cosmic parallax measures kinematically: coherent position-drift patterns, microarcsecond-scale over decade baselines, with a quadrupolar-plus-dipolar structure oriented by the local Λ_eff gradient and the inherited frame geometry (P63, P48). The prediction comes with its address: the drift pattern's axes should align with the established anomaly family, the dipole-excess direction, the bulk-flow axis, the Migkas gradient, because all are projections of one field. A null at current Gaia sensitivity costs nothing; a null at Theia-class sensitivity would cost the mechanism its kinematic existence.
The test's purity is its value: no ladders, no calibrations, no population models, just positions over time, the same character that makes the sirens decisive for the radial profile. SCT's environmental program will end in instruments like this one, measured drift or measured silence. Foundations in Paper 1 (P17, P19, P63) with the falsification register of Paper 10, From Chaos To Confirming Falsifiability.
No new premises: the standing field, observed through the one instrument that bypasses every intermediary.
The kill is clean and scheduled: end-of-mission Gaia plus next-generation astrometry (Theia-class, microarcsecond VLBI) finding cosmic parallax consistent with zero at sensitivity reaching percent-level gigaparsec anisotropy would falsify the Λ_eff field's kinematic reality, the most direct execution available, since no modeling layer intervenes. A detected drift misaligned with the anomaly-axis family would equally break the one-field reading while leaving isotropy violated.