Where Does the Verbalizable Workspace Band Begin? An Audit of the Jacobian-Lens Instruments Across Open-Weight Families

Claims about where verbalizable representations occupy a model’s depth depend on an instrument that can locate the region and distinguish known capacities. We audit the Jacobian lens (J-lens), a map from intermediate states into vocabulary space, used by Gurnee et al. (2026) to place a mid-depth workspace band at roughly 38% of depth. We test five measurements, four band signatures and one occupancy estimator; one signature does not pass our validation conditions and one estimator depends on an unstated convention. One band signature, excess kurtosis, changes sign between two fits of one model differing only in fit corpus. Whether occupancy, under every operator we test, can separate 25 from 40 planted directions depends on an unembedding convention the original leaves unstated: it cannot on five of six open-model J-lens dictionaries uncentered, and can on all six centered. What survives is a J-lens effective-dimensionality transition, not a fixed workspace-band location: with reference-free averaged-Jacobian lenses and one rule fixed in advance, its onset ranges from 28.6% to 78.7% across seven families on 22 screened model–lens artifacts. Its location varies across model families and model–lens artifacts and depends on threshold and locator choice; open families begin both earlier and later than 38%, so comparison against that frontier-scale figure is uninformative.