Unread or Unenforced? Separating Representation from Enforcement Failure in Content Guards

Abstract

When an encoded attack passes a content guard, the guard either never represented the payload's harmful content or represented it and failed to act. End-to-end attack success rate reports one number for both, yet the remedies are opposite: one is a representational limit that more safety training cannot reach, the other a decision rule that it can. We separate them by reading a guard's own residual stream -- a content probe fitted on plaintext and transferred, without refitting, to the encoded condition -- alongside its verdict logits, at the cost of one forward pass and no judge model. Doing this honestly is most of the problem, and it is our main contribution. A permutation test licenses the decode measurement on 17 of 19 conditions for one open guard and 12 of 19 for another; a length-matched null and a control floor calibrated on conditions the guard's base model provably cannot decode reduce both to 4. The discarded cells are not marginal ones: the largest result in our first analysis -- a guard representing a cipher at AUROC 0.72 while blocking none of it -- is an artefact on an encoding its base model decodes at rate zero. On one guard, two screens sharing no input agree exactly on which conditions to reject. What survives is a policy failure that is real but narrower than the uncontrolled analysis claimed: 7 to 23 per 100 prompts represented and not blocked on conditions the guards block heavily, and 56 per 100 on one condition a guard barely blocks. Blocked without decoding is near zero throughout, so neither guard reacts to the appearance of encoding rather than to content. On genuine ciphers both guards block essentially nothing, and we report those cells as unmeasured rather than as evidence of failure to decode.