Claude Fable 5.2 and Mythos 5.2 return 400 errors when the prefix changes

Both models enforce the prefix check for every account. Requests that edit the system prompt, tools, or an earlier message, then send back a thinking block from a later turn, return a 400 error.

Who is affected:

Apps that edit earlier context while reusing thinking blocks will fail on these models.

Suggested next step:

Check whether your code edits the prefix before you switch models.