PSA: Many reasoning models lose access to their CoT between turns.

I was looking into how chat templates render multi-turn interactions, and came across the (surprising to me) fact that it's common practice for reasoning models to discard CoT from prior assistant turns once a new user turn comes in. The DeepSeek documentation has a nice illustration of how this works:

In the Claude API documentation:

Thinking block context removal

• On earlier Opus/Sonnet models and all Haiku models, thinking blocks from previous turns are removed from context, which can affect cache breakpoints. On Opus 4.5+ and Sonnet 4.6+, they are kept by default.

And the OpenAI documentation:

Input and output tokens from each step are carried over, while reasoning tokens are discarded.

Other OS models (e.g. Nemotron, Qwen) do this too, as can been seen by inspecting their chat template. My understanding is that CoT is typically not dropped between tool calls, but only between user turns.

I found it surprising that I don't think I've ever heard anybody mention this fact in the context of AI safety, even though it seems relevant to CoT monitoring, control, and steganography.

It also might play a contributing part in why models are often so verbose in their outputs -- anything not in the output will be lost!

Some quick preliminary investigations suggest that the CC harness does discard CoTs^[1], even for later models. When I asked Opus 4.8 how they felt about this, I got this response:

1. ^{^}

   When I ask Opus 4.8 directly whether it can see CoT from prior turns, it responds with something like "I am genuinely uncertain about whether I can see the CoT". When you push back and point out that it should be able to tell either way, it says that it can't see CoT from earlier turns. If you ask it to solve a problem in its CoT, and then on the next turn ask for the solution, it claims that it can't retrieve the solution from the CoT. This experiment is not particularly rigorous, and I don't trust Claude's claims about how Claude works, but it suggests to me that CC is indeed discarding CoTs (alternatively, Claude might just be sycophantic and think that's the conclusion I expect to reach). I would appreciate more rigorous evidence here.