What is Fable 5?

Anthropic expects 85% fewer refusals from Claude Opus 5 compared to Fable 5, attributing the difference to a distinct set of guardrails on Opus.

“As a result, Anthropic is using a different set of guardrails on Opus than they do on Fable 5, which they believe will lead to 85% fewer refusals.”

Claude Opus 5 scored 43.3% on Frontier Bench, roughly 10 points above Fable 5 and 9 points above GPT-5-6-Soul on the same benchmark.

“It scored a 43.3% on that one that I just mentioned, Frontier Bench, which is a more difficult version of Terminal Bench, which is about 10 points higher than Fable 5 and around 9 points higher than GPT-5-6-Soul.”

The US government used an export control directive to demand Anthropic suspend Fable 5 and Mythos 5 access for foreign nationals, a novel application of export controls to frontier AI model access.

“The US government used an export control directive to demand that Anthropic suspend Fable 5 and Mythos 5 access for foreign nationals.”

GLM 5.2 from ZAI ranks number one on Bridgebench and reasoning, beating Fable 5 at 1/10 the cost and 300 tokens per second throughput.

“GLM 5.2 is number one on Bridgebench and number one on reasoning beating Fable 5 at 1/10 of the cost and 300 tokens per second.”

GLM 5.2 from ZAI tops Bridgebench and reasoning benchmarks while undercutting Fable 5 on price by 10x, positioning ZAI as a serious frontier challenger.

“GLM 5.2 is number one on Bridgebench and number one on reasoning beating Fable 5 at 1/10 of the cost and 300 tokens per second.”

Anthropic removed 80% of the system prompt for Claude Opus 5, Fable 5, and Claude Code with zero change to coding benchmark results.

“Anthropic had removed 80% of the system prompt for Opus 5 and Fable 5 and Claude Code. He said this resulted in zero change to their coding benchmarks.”

GLM-5.2 is worth watching as an open-weight model that beats Fable 5 in website design benchmarks per Designer Arena, signaling credible frontier competition from Chinese open-weight models.

“Designer Arena wrote a blog post on X about how GLM-52 beat Fable 5 at website.”

Nathaniel Whittemore on model embargoes and their effect on development speed.

“Stopping models like Fable 5 or Mythos 5 from being served to the public does nothing to slow down development. In fact, it probably speeds it up slightly by freeing up resources.”

Claude Opus 5 beats Fable 5 on many benchmarks but is described as nowhere near Fable 5 in practical use, illustrating a widening gap between benchmark rankings and real-world utility.

“Opus 5 is nowhere near Fable in practical use, not even close. Anyone who's used it meaningfully can tell this very quickly after a few tasks. Yet, Opus beats Fable on many.”

Tariq on Fable 5 shifting the performance bottleneck from model capability to user clarity.

“Fable is the first model where I find the quality of the work is bottlenecked by my ability to clarify its unknowns.”