Arena comparison: GPT-6 Astra and Claude Fable 5.1 cost efficiency

Arena.ai @arenaOriginally posted 2026.09.17

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We compared the top model from each family by net improvement score and median cost per task. Family is defined as variants within the same model base name. GPT-6 Astra and Claude Fable 5.1 stand out: both cost far more than other models from their respective labs despite relatively close performance. By @OpenAI: - GPT-6 Astra (Max): +11.7% | $3.94/task - GPT-5.6 Sol (xHigh): +7.0% | $1.03/task By @AnthropicAI: - Claude Fable 5.1 (Max): +13.7% | $4.40/task - Claude Opus 5 (High): +10.2% | $2.07/task