I've felt for a little while now that steganography, in the sense of text that looks normal and monitorable to humans but actually allows LLMs to do general-purpose hidden reasoning, is a less likely failure mode of CoT monitoring than switching to recurrent latent reasoning (neuralese) or obvious and visible linguistic drift.

I think I have a new and fairly crisp argument for this: Using general-purpose steganographic encoding and decoding itself requires a lot of serial reasoning.

For example, one way you can do steganography is by partitioning the set of all tokens into disjoint groups, with synonyms placed in different sets, and every time you have multiple words to choose from with the same meaning, you encode some additional hidden meaning based on which set the word you output belongs to. To reliably output the right token to encode meaning by such a scheme, you probably need to think pretty carefully about how to follow some rules about which tokens are in which set and what you are trying to encode. Similarly, extracting the hidden message from the text requires carefully pulling out the hidden bits and then piecing together what they mean. So either the serial reasoning required for encoding and decoding should be easily monitorable, or there is some non-steganographic reason why that serial reasoning is unmonitorable. Either way, steganography isn't the first reason why CoT monitoring fails.

I would guess this is true of all general-purpose steganographic encoding and decoding schemes because it seems hard to hide arbitrary messages in normal-looking text without thinking hard about what to write, but I'm not super confident about that.