On 1 September Anthropic released Claude Fable 5.1 for everyone and Mythos 5.1 only for vetted cybersecurity and life sciences teams. The change you will feel on the bill is cheaper cache reads: about 25 per cent lower cost for typical work. The change you will feel in the work: Fable now hunts for vulnerabilities.
- Cache reads get 75 per cent cheaper, down to $0.25 per million tokens; input and output stay at $10 and $50.
- The new cyber safeguards mean about 60 per cent fewer interventions per session in Claude Code. Fable 5.1 finds vulnerabilities but does not write exploits.
- Enterprise Frontier Safeguards keeps data in the customer's cloud; it arrives in phases from the autumn, and until then eligible customers get zero data retention.
A crash that happens once in a million runs and that nobody on the team had explained in four to five years. According to the investment firm Millennium, Fable 5.1 is the first model to find it: it disassembled a third-party library, matched it against the core dump and got to the bug.
Stories like this are the most interesting part of model announcements and the hardest to check. This one comes from a customer quoted by Anthropic. I include it because it is concrete, not because it is proof.
The benchmark table is long and it is theirs. I will not copy it.
The bill is the news
In highly agentic work cache reads make up most of the bill: the agent goes back to context already loaded. So a 75 per cent cut exactly there turns into about 25 per cent for ordinary work and up to 45 for agentic work. Input and output are untouched. What has been touched is where agents spend.
The second change is for security people. Until now Fable stopped many of your questions if they were about vulnerabilities. Now it will look for holes in your code. It will not write you an exploit, and the penetration test goes to Opus.
Paying for Fable by the token? Switch to 5.1 and the bill drops by itself. Dropped it because of security refusals? Try again with a real task. And if you have your own integrations that edit Claude's prior context, read their note on distillation before you open a new account.