Currently, chain-of-thought (CoT) is a valuable tool for overseeing AI models. However, some architectural shifts could significantly reduce CoT monitorability. We have recently proposed that AI companies should transparently share ​​information about the degree to which their architectures may allow for latent reasoning and communication. To assist with this proposal, this document operationalizes a measure that serves as a proxy for the amount of unverbalized serial cognition a model can perform. Our measure is a specific instantiation of the notion of “opaque serial depth”, originally defined in a recent paper from GDM (Brown-Cohen et al, 2026).To measure the opaque serial depth of a computation, Brown-Cohen et al. propose measuring the longest path in the computational graph which doesn’t pass through some form of “interpretable bottleneck”. Centrally, if one considers CoT tokens as “interpretable” but transformer hidden states as “non-interpretable”, then the opaque serial depth o...