Product communication from **Anthropic** published on **September 1, 2026** on anthropic.com (~4,000 words, six sections, 22 testimonials from early-access partners). It announces **Claude Fable 5.1** (general availability) and **Claude Mythos 5.1** (verified access): *the same model, but with different levels of safeguards*.
#Claude Fable 5.1#Claude Mythos 5.1#foundation model
Anthropic — communication produit publiée sur anthropic.com · sans signature individuelle.
Benchmark announcement from **Artificial Analysis** (independent AI model evaluation platform, via X/Twitter + model page): **GLM-5.2** from **Z.ai** (Zhipu AI, @Zai_org) becomes **the leading open weights model** and climbs to **#3 in the overall ranking** of **GDPval-AA**, a real-world benchmark for *economically valuable knowledge work* (long-horizon, multi-turn, agentic tasks). GLM-5.2 scores **1524 Elo**, behind only **Claude Fable 5 (1783)** and **Claude Opus 4.8 (1615)**, and on par with **GPT-5.5 (xhigh, 1509)**. It leads the next-best open model (**MiniMax-M3, 1408**) by a wide margin, along with numerous proprietary models: **Gemini 3.5 Flash (1357)**, **Qwen 3.7 Max (1289)**, **Muse Spark (1158)**. The tasks are genuinely agentic: **~31 turns per task** on average across **1,999 matches**. The same ranking holds on the **Artificial Analysis Intelligence Index** (1st among open weights), the **Agentic Index** (#3) and **AA-Briefcase** (#3, ahead of GPT-5.5 xhigh, behind only Fable 5). Notable highlight: an **open weights** model under **MIT license**, **MoE with 753B parameters / 40B active**, **1M-token context**, priced at **$1.40/$4.40 per 1M tokens** input/output, rivals the proprietary frontier on agentic work — a real step forward for open models.