Breaking

Anthropic releases Claude Fable 5.1 with cheaper cache reads and fewer safeguard false positives

September 1, 2026 at 15:41 EDT

Foundation Models
Software Dev & Coding
AI Agents

Anthropic on Tuesday released Claude Fable 5.1, a generally available update to its most capable public model that the company says will cost an estimated 25% less than Claude Fable 5 on typical token-billed workloads and will trigger fewer false-positive blocks from its cybersecurity and biology safeguards.

September 1, 2026 · Anthropic

Cheaper long jobs. Fewer false blocks.

Claude Fable 5.1 keeps $10 / $50 token prices but cuts cache-read costs 75%, and it flags fewer benign cybersecurity and biology requests than Fable 5.

Cache-read price

Agent loops now pay a quarter of the old cache-read rate

$1.00

Fable 5
per MTok

$0.25

Fable 5.1
per MTok

4 stacked blocks vs 1: cache reads are 75% cheaper, or 4× lower, than on Fable 5.

~25%

Typical workload savings vs Fable 5

75%

Cut in cache-read price, to $0.25 / MTok

~60%

Fewer cyber false positives / session blocks

~45%

Peak savings on highly agentic jobs

Terminal-Bench-Science 0.1

Fable 5.1 more than doubles Fable 5

24.7%

Fable 5

29.0%

Opus 5

52.6%

Fable 5.1

22.4%

GPT-5.6 Sol

What improved

Partners are moving production coding traffic over. Cache-heavy agent jobs get cheaper, cyber classifiers fire less often on benign work, and the model can now be used to find software vulnerabilities.

What still bites

Code reviews ran slower in one independent test. Dual-use cyber tasks still fall back to Opus. Some users hit session limits in minutes, and forced tool use now returns an error.

Still redirected

Fable 5.1 is not a full unlock

Penetration testing, exploit generation, and binary scanning still go to Opus
Life-sciences R&D queries still go to Opus; Mythos 5.1 stays trusted-access only
Default Fable use still requires 30-day data retention

Continue reading

The rest of this article is for AI News Blitz readers. Choose an option below to keep reading.