Claude Fable 5.1
Claude Fable 5.1 is Anthropic's September 1, 2026 generally available flagship for demanding reasoning and long-horizon agentic coding, research, and document work.
Provider
Anthropic
Model family
Anthropic Claude
Multimodal LLM
Cost tier
Fable
Status
Current
Release Sep 1, 2026
Why teams choose it
Anthropic estimates typical token-billed workloads cost about 25% less than Fable 5
and highly agentic work up to about 45%, because cache reads dropped 75%.
Platform docs say start with Claude Opus 5 for most workloads
use Fable 5.1 when Opus 5 at higher effort still falls short.
Breaking changes vs Fable 5 include forced tool use errors and thinking-block compatibility
effort and turn-scoped system messages are additive betas.
Tradeoffs to know
- Cyber offense such as exploit generation still redirects; Fable 5.1 may discover vulnerabilities but not develop exploits.
- Vendor Terminal-Bench and science scores use Anthropic's harness; reproduce before replacing Opus 5 or Fable 5.
- Default effort is high in Claude Code and medium on Claude.ai—token use can jump if you leave defaults on.
Benchmarks
- Gdpval Aa V2
- 1,853
- Terminal Bench 4 0
- 55.8
- Osworld 2 0 Partial
- 77.9
- Terminal Bench Science 0 1
- 52.6
Technical specs
- Inputs
- text, image
- Outputs
- text
- License
- Proprietary API
- Hle No Tools
- 60.9
- Automationbench
- 31.4
- Cursorbench 3 2 0
- 73.4
- Capabilities
- long-horizon coding
- agentic research
- adaptive thinking
- computer use
- document work
- tool use
- long context
Anthropic Claude family lineup
Current models
Previous versions
Related models
Claude Fable 5.1 FAQ
What is Claude Fable 5.1?
Claude Fable 5.1 is Anthropic's September 1, 2026 generally available flagship for demanding reasoning and long-horizon agentic coding, research, and document work. The API ID is claude-fable-5-1, with a 1M-token context window, 128K max output, adaptive thinking, and default high effort. Anthropic keeps the $10/$50 input/output list price of Fable 5 while cutting cache reads to $0.25 per million tokens.
When does Claude Fable 5.1 fit best?
Long-running Claude Code and research agents
What should teams watch out for with Claude Fable 5.1?
Cyber offense such as exploit generation still redirects; Fable 5.1 may discover vulnerabilities but not develop exploits.
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