Claude Fable 5.1
Fable 5.1 is the generally available release, and the gains sit in long tasks: steadier context, fewer mid-run detours. Check three migration risks before upgrading.
Fable 5.1 ships as the generally available model, and the changes sit in long-horizon work: context survives longer runs, mid-task course corrections drift less, and tool calls hold up across more round trips. It shares a base model with Mythos 5.1, so the difference is guardrail level, not capability. Check prompts that assumed the model loses the thread, retry logic tuned for short tasks, and code parsing the old output format.
