Long-running AI agents can become expensive because they repeatedly read the same instructions, files, code, and conversation history while completing a task. Anthropic is targeting that cost with Claude Fable 5.1, its latest model for coding, research and complex knowledge work.
Released in early September, Fable 5.1 is designed to handle projects that can run for hours and involve several applications. Anthropic estimates that the model costs around 25% less than Fable 5 for typical workloads and up to around 45% less for highly agentic tasks.
However, the reduction does not come from lower standard input and output prices. Instead, Anthropic has cut the cost of reading previously processed information from a cache, which is especially important for agents that must return to the same material many times.
How the 45% Cost Reduction Works
Fable 5.1 still costs $10 per million input tokens and $50 per million output tokens, the same rates charged for Fable 5. However, cache reads now cost $0.25 per million tokens, down from $1, representing a 75% reduction.
The AI safety company based its broader savings estimates on four weeks of actual usage in August across Claude Enterprise, Claude Code, and its API. The company said the 45% figure applies to context-heavy and tool-heavy tasks where cached information accounts for most of the bill. However, workloads that make little use of caching will not receive the same level of savings.
This change could make Anthropic’s most capable public model more practical for developers building agents that review large codebases, conduct research, or complete multi-stage business processes.
Cognition co-founder Walden Yan said the new cache price makes a Fable-class model “finally economical” for workloads the company had kept on the cheaper Opus model.
Better Results on Long Tasks
Anthropic says Fable 5.1 improves on its predecessor across coding, computer use, and business workflows. In the company’s tests, the model scored 55.8% on Terminal-Bench 4.0, compared with 42% for Fable 5. It also recorded 31.4% on AutomationBench, up from 17.1% for the previous model.
While these are vendor-reported results and do not independently prove how the model will perform in every workplace, they still support Anthropic’s main pitch for Fable 5.1, which is that businesses can assign it longer and more difficult work while spending less on repeated context.
Access Comes With Added Safeguards
Fable 5.1 is available to Pro, Max, Team, and Enterprise users. Developers can access it through Anthropic’s API, Amazon Web Services, Google Cloud, and Microsoft Azure.
Anthropic also released Claude Mythos 5.1, the same underlying model with more permissive safeguards for vetted cybersecurity and life sciences organisations.
The performance difference between the two versions was visible on Terminal-Bench 4.0, where Mythos 5.1 scored 60.9% compared with Fable 5.1’s 55.8%. Anthropic said the gap reflects tasks where the public model’s cybersecurity safeguards intervened. WhileFable 5.1 can still help identify software vulnerabilities, requests involving exploit generation and penetration testing may be redirected to less capable Opus models.
For companies building AI agents, this release shifts attention from the price of a single prompt to the cost of completing an entire job. And Claude Fable 5.1 seems to be a viable option.
