Fooling around with encrypted reasoning blobs
A few experiments reveal that we can replay an unmodified older reasoning blocks, with no visible error at all.
Not only can we replay within sessions, this same idea also seems to work across different sessions. It even applies to sessions running in different accounts. That is: when we obtain reasoning blobs from a session running under one OpenAI or Anthropic account, we can replay them against a session in a different account altogether. For OpenAI specifically, we can even replay blobs across different models. (The Claudes got fussy about this.)
At a cryptographic level, this tells us something very simple: the providers are probably using a single global key to encrypt and authenticate all reasoning data sent to the client.