whats-new-fable-5-1 changedmodels/fable-5-1/whats-new-fable-5-1

## Models ## Breaking changes ### Forced tool use is not supported ### Only Claude Fable 5.1 and Claude Mythos 5.1 read Claude Fable 5.1 thinking blocks ### Editing earlier turns invalidates thinking blocks ### Some advisor tool pairings are not supported ## New features ### Change effort mid-conversation (beta) ### Turn-scoped system messages (beta) ### Progress updates between tool calls (beta) ### Content provenance ## Behavior differences ### Changed from Claude Fable 5 ### Unchanged from Claude Fable 5 ## Capability improvements ## Refusals, fallback, and billing ## Pricing ## Availability ## Migrate from Claude Fable 5 ## Next steps