Claude Fable 5.1

Everything Fervor AI has published that touches Claude Fable 5.1 — 2 pieces, newest first.

• Preserved Thinking Splits the Claude API by Account Creation Date

  Anthropic now enforces its anti-distillation thinking-block check by API account creation date, so harness maintainers on older keys will ship code that breaks for every…

• Briefing · September 1, 2026 · afternoon

  Anthropic shipped two models today that are the same model, and everything around them moves the control surface off the weights and onto the account, so who you are now…