Key takeaways
What happened
1, its most capable AI models yet. Along with gains in agentic coding and text quality, the company cuts costs by up to 45 percent. 1 models share the same base model but differ in safety guardrails. 1 is restricted to special access programs for cybersecurity and life sciences. They're also the first Claude models to ship with built-in watermarks.
1 generate 60 percent fewer false positives, and for biology-related queries, the filters fire 85 percent less often on harmless questions about basic biology and medicine. 1 can now identify software vulnerabilities for the first time, though not develop exploits. Penetration testing and exploit generation still get routed to the Opus models. Mythos is also part of Claude Security for defensive purposes.
1 is available immediately on all platforms, including AWS, Google Cloud, and Microsoft Azure. Developers can access it via the API using claude-fable-5-1. For enterprise customers, Anthropic is rolling out Enterprise Frontier Safeguards (EFS), which store customer data solely on the customer's own cloud infrastructure.
Why it matters
Anthropic is launching a detection API in private preview that lets regulators, media outlets, fact-checkers, and research institutions verify whether a text contains the watermark. The company plans to expand access over time, and interested parties can sign up here.
1 costs about 25 percent less than Fable 5 for typical workloads, with savings climbing to roughly 45 percent for heavily agentic tasks involving long, autonomous runs with many tool calls. 25 per million tokens. All other API prices remain unchanged at $10 per million input tokens and $50 per million output tokens.
For comparison, Opus 5 runs at half that price with $5 input and $25 output per million tokens. High cost was the biggest complaint about Fable 5, and it likely contributed to the model seeing low adoption among enterprise customers. The price pressure has been building since Opus 5 launched in late July, already matching or beating Fable 5 on most benchmarks at a lower price.
Like earlier Claude models, the new versions come with an effort-level system that controls compute usage. 1 should match Fable 5's results at lower cost. 1 posts major gains on agentic benchmarks. 4 percent. 6 Sol. Whether these gains translate to real-world use at the same scale will become clear over the coming weeks. 1 also improves its writing style.
1 dials that back while following style instructions more closely and sounding more natural overall. The safety filters for cybersecurity, biology, and medical questions are less aggressive than in earlier versions. 0 models, they were so sensitive that they triggered on well-intentioned requests too, producing false positives at a frustrating rate.
What to watch
1 is currently limited to US organizations through two programs: the Cyber Verification Program for defensive security work and the Life Sciences Verification Program, developed with the US government. Anthropic plans to expand access to international partners. Anthropic is also cracking down on distillation attacks, where a model's capabilities are systematically extracted through thousands of fake accounts.
New API accounts can no longer edit Claude's prior context in multi-turn conversations while keeping the thinking transcript, which according to Anthropic closes a documented distillation technique.