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Fable 5.1

Model ● Covered in 12 stories + Follow

Fable 5.1 is an Anthropic large language model positioned as faster and easier to talk to than its prior model, with updates that include a new system prompt restricting repeated song lyrics and certain copyrighted content. Recent coverage also highlights reduced API costs through a 75% cut in input caching costs, making usage about 25% cheaper. In benchmarks and adjacent comparisons, Fable 5.1 is repeatedly referenced as a strong baseline—e.g., used as a target for other releases and compared against models like GPT-6 Astra and Opus 5.5.

Updated 13 October 2026

Specifications

No specifications recorded yet.

Latest developments

[AINews] Opus 5.5 is good at explainer videos Latent Space · 2 weeks ago · 35
Claude Opus 5.5 Should Raise Your Ambitions Zvi (Don't Worry About the Vase) · 3 weeks ago · 5
Claude Opus 5.5: The System Card Zvi (Don't Worry About the Vase) · 3 weeks ago · 22
GPT-6-Astra Can Do Ambitious Things Zvi (Don't Worry About the Vase) · 1 month ago · 14
Cognition's SWE-2 Product Hunt · 1 month ago · 46
GPT-6 Astra: Too Good The Algorithmic Bridge · 1 month ago · 3
AI #184: Post Post Mortem Zvi (Don't Worry About the Vase) · 1 month ago · 16 Fable 5.1 Ben's Bites · 1 month ago · 16

Timeline

Month Quarter Year

2026

Anthropic released Claude Opus 5.5 and OpenAI released GPT-6 variants with lower prices, intensifying a model price war Model release

Anthropic released Claude Opus 5.5 with updated cybersecurity safeguards and testing controls Model release

OpenAI released GPT-6 Astra, a new model positioned as a major upgrade over prior generations, with specified per-token input and output pricing Model release

Cognition launched SWE-2, a post-trained coding model built on Kimi K3 and optimized with reinforcement learning for lower cost and improved coding performance Model release

OpenAI released GPT-6 Astra, its flagship AI model, along with evaluation results, a system card, and related updates to how the model is benchmarked and priced Model release

Anthropic released updated Claude models Fable 5.1 and Mythos 5.1 with lower cache-read pricing and modified access and safeguards Model release

Anthropic announced Enterprise Frontier Safeguards (EFS) to add customer-controlled zero data retention and automated misuse detection for its frontier models Feature update

Anthropic adjusted Claude Code usage limits effective September 14 and reported a reduction in Claude cache read costs Pricing change

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