A model can load correctly, repeat deterministically, and still fail the product claim. Those facts are worth preserving.

One checkpoint reproduced 98.44% top-1 on its own official shard and then recovered none of 23 predeclared critical tokens on continuous-ASL holdout clips. The model was real; the transfer assumption was wrong.

Sign First locks development and diagnostic evidence before evaluation, records the exact model and preprocessing path, and separates own-distribution performance from continuous-ASL utility.

A near-miss may identify useful representation signal. It cannot be tuned on the opened diagnostic and then presented as independent evidence.