Anthropic just released Claude Fable 5 — their most powerful model ever made publicly available. Here’s what you need to know:
🔬 What it is:
Fable 5 runs on the same underlying weights as Mythos — the model that shocked the cybersecurity world in April. The difference? Built-in safety classifiers that reroute high-risk queries (cyber, biology, chemistry) to Claude Opus 4.8 automatically. Less than 5% of sessions are affected.
📊 Benchmark highlights (vs GPT-5.5):
→ SWE-Bench Pro: 80.3% vs 58.6%
→ FrontierCode Diamond: 29.3% vs 5.7%
→ CursorBench: 72.9% — 8pts above previous best
→ Frontier physics research: matched 4 days of GPT-5.5 work in 36 hours, using ⅓ the reasoning tokens
🏗️ Real-world proof:
Stripe used it to migrate a 50-million-line Ruby codebase in a single day — a job that would take a full engineering team over two months.
🧬 The science angle is underrated:
Mythos 5 (the restricted version for vetted partners) autonomously ran genomics research over a week, trained a custom ML model across 138 animal species, and outperformed a recently published model in Science — while being 100× smaller.
⚠️ Two things worth knowing before you deploy:
<ol>
<li>Safety filters are deliberately broad — some benign security tasks may get rerouted</li>
<li>New 30-day data retention policy applies to ALL users, including enterprise customers with previous zero-retention agreements (not for training, but for safety monitoring)</li>
</ol>
💰 Pricing: $10/1M input · $50/1M output
Free on Pro/Max/Team plans until June 22, then usage credits required.
This isn’t just a faster model. It’s the first AI that genuinely improves the longer it runs — designed for hours or days of autonomous work, not just single exchanges.
The capability bar just moved. Is your team ready to use it — and use it responsibly?
🔗 Full announcement: <a href="http://anthropic.com/news/claude-fable-5-mythos-5" target="_blank" rel="noopener nofollow">anthropic.com/news/claude-fable-5-mythos-5</a>
#AI #Anthropic #ClaudeFable5 #LLM #SoftwareEngineering #AISafety #TechNews #FutureOfWork #MachineLearning

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Dev World

Anthropic has released Claude Fable 5, built on the same weights as the restricted Mythos model but with built-in safety classifiers that reroute high-risk queries to Claude Opus 4.8 (affecting under 5% of sessions). Benchmark highlights show strong coding performance: 80.3% on SWE-Bench Pro vs GPT-5.5's 58.6%, and 72.9% on CursorBench. Real-world use includes Stripe migrating a 50-million-line Ruby codebase in a single day. The restricted Mythos 5 variant autonomously conducted genomics research, trained a custom ML model across 138 species, and outperformed a published Science paper model while being 100× smaller. Pricing is $10/1M input and $50/1M output, with free access on Pro/Max/Team plans until June 22. A new 30-day data retention policy applies to all users including enterprise customers with prior zero-retention agreements.

Claude Fable 5 and Claude Mythos 5