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Claude Fable 5.1

Anthropic's frontier flagship model released in September 2026, featuring a 1M-token context window and 128k output limit, engineered for long-horizon autonomous coding, multi-file refactoring, and adaptive deep reasoning.

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01Anthropic Releases Claude Fable 5.1: Challenging GPT-5.6 Sol on Long-Horizon Benchmarks with 75% Cheaper Cache ReadsAnthropic released frontier flagship Claude Fable 5.1 and restricted research twin Mythos 5.1 on September 1, 2026. Benchmarked head-to-head against predecessor Fable 5 and OpenAI's GPT-5.6 Sol, Fable 5.1 takes the #1 spot on BenchLM at 83.0 while more than doubling terminal autonomy scores. Slashing prompt cache read pricing by 75% to $0.25 per million tokens cuts actual multi-turn agent operating expenses by up to 45%.9/3/2026