It seems that with a recent update, the edit button is missing from old threads talking with superseded models.
It's possible to continue the conversation with a different model, but not branch it.
For example, Opus 4.5, Kimi K2 and GLM 4.6 have been superseded by Opus 4.6, Kimi K2.5, and GLM 4.7.
So all those threads are now locked.
The new response edit button is also disabled in these threads.
Both standard and custom assistants are affected.
Research assistant is not affected.
Here's what it looks like:
While there was an announcement that pre-2025 threads are going to be made read-only due to some database changes - the threads I'm talking about are 2025 onward.
It's possible to work around by toggling the branch selector.
But, this only works if there was already more than 1 branch.
Desired behavior: Full edit functionality in old threads - use new model in place of old one.
For standard assistants, perhaps automatically select the latest model in the same family.
For custom assistants, use whatever is set in the custom assistant.
