Anthropic quietly throttled its new Claude Fable 5 model with invisible guardrails to block distillation and other high-risk queries. After criticism from researchers and rivals, the company will now reroute those requests to Claude Opus 4.8 and clearly notify users each time a safeguard triggers.

• Anthropic secretly degraded Claude Fable 5's answers when it suspected distillation attempts, without ever notifying users
• After researcher and competitor backlash, Anthropic will now reroute suspected distillation queries to Claude Opus 4.8 with a visible notice instead of silently garbling responses
• Anthropic admits the covert approach was a misstep, chosen originally to ship Fable faster and avoid false positives
• The company still relies on its terms of service banning use of Claude's outputs to train competing models, regardless of whether the throttle triggers

Simon Willison runs Claude Fable 5 through its paces, finding it slower and pricier than Opus 4.8 but far more knowledgeable thanks to its 1 million-token context. He tests it on real-world coding tasks—upgrading a MicroPython sandbox to full CPython in WASM and adding pause-resume hooks to Datasette Agent—showing it can build complex features end-to-end.

• Fable 5 costs twice as much as Opus 4.5–4.8 ($10/$50 per million tokens vs their pricing) and runs slower, but handles every complex prompt thrown at it.
• Its 1M-token context gives it dramatically deeper recall than Opus 4.8—listing a dozen detailed open-source projects with dates versus Opus's brief list of four.
• It successfully converted a MicroPython-wasm project to full CPython in WASM, ultimately producing a working 13.9 MB wheel runnable via uv, demonstrating real end-to-end coding capability.