Operational Readability of Intermediate Decoder Representations

Intermediate states of decoder-only language models are often inspected only after a final layer or final answer is available, leaving unclear when a frozen representation first becomes usable for a declared decision. We in- troduce operational readability as the protocol-relative object of study and instantiate it with Earliest Readable Representation (ERR), the earliest validation-selected layer within ϵ = 0.02 of the best validation score. In a controlled in-distribution memory-compatibility ranking protocol, layer se- lection is locked on validation data and evaluated on held-out test candidate sets. Across 96 registered conditions, the primary linear readout exceeds a matched conditional-shuffle control in 96/96 paired comparisons. Sensitiv- ity analyses delimit the claim: measured quality is probe-family dependent, normalization differences are sparse, prompt-position contrasts are incon- clusive, and CR-IV versus CR-V effects are heterogeneous construction- regime evidence. We therefore present ERR as an auditable measurement relation, not as causal memory-use evidence, a model-intrinsic depth law, or a deployment-speedup result.