AFAIK no provider guarantees compatibility of reasoning traces, even in the same model generation, and we've in practice seen most of the big LLM APIs throw errors indicating incompatibility (at least transiently) when switching models. The only stable solution right now is to just throw away reasoning traces whenever a model is switched.

Fair (I haven't been using the encrypted-reasoning systems, though this is common in open ones - I'm kinda surprised it's an option in encrypted ones too), though what they're doing here is cross-user replays in addition to cross-model.

There seems to be an obvious choice to make here, should you give the users to decrypt and use the COT that they did not generate themselves?

This is only required if you want users to be able to share things with everyone and you are going for the simplest implementation.

If not you could try to keep a record of keys associated with a user, then when a new request comes in look through to see if the user has a valid key to decrypt the COT.

For explicit shares, just add the key used in that one conversation to the users valid keys. For global shares use the global keys. But that's adding more complexity to the system.

I really don't understand why server-side storage of the trace isn't a viable approach here, with only a unique key flowing to the client and back. Does it have something to do with how backend load-balancing works?

100% guaranteed that this research just forced this to happen now.

Sucks.