Claude Fable 5.2 changelog

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.

Claude Code makes Claude Fable 5.2 the default Fable model

Claude Fable 5.2 has 1M context and costs $8/$40 per Mtok with $0.20/Mtok cache reads. It is now the default Fable model in Claude Code.

Who is affected:

Claude Code users who run the default Fable model get Claude Fable 5.2 in new sessions.

Suggested next step:

No action needed unless you want Fable 5.1, which you can select with /model claude-fable-5-1.