Anthropic's September: Fable 5.1, Mythos 5.1, Opus 5.5 and Sonnet 5.5

The timeline

DateRelease
Sept 1, 2026Claude Fable 5.1 and Claude Mythos 5.1
Sept 22, 2026Claude Opus 5.5
Sept 28, 2026Claude Sonnet 5.5

Fable 5.1 and Mythos 5.1: one model, two safeguard levels

Anthropic describes Fable 5.1 and Mythos 5.1 as the same underlying model with different safeguards. Fable 5.1 is the widely available version. Mythos 5.1 has looser safeguards and is available only to vetted professionals through two programs:

  • Cyber Verification Program: for defensive security professionals.
  • Life Sciences Verification Program: for research and development teams, currently limited to US organizations.

Pricing is $10 per million input tokens and $50 per million output tokens. Anthropic says typical workloads cost about 25% less than on Fable 5, and highly agentic tasks up to 45% less.

Benchmarks Anthropic reports for Fable 5.1:

  • Terminal-Bench 4.0: 55.8% (Fable 5: 42.0%)
  • Terminal-Bench-Science 0.1: 52.6% (Fable 5: 24.7%)
  • Humanity's Last Exam: 60.9% without tools

Anthropic also says it now allows defensive vulnerability discovery, added anti-distillation measures, and implemented watermarking to comply with the EU AI Act.

Opus 5.5 (September 22)

Anthropic says Opus 5.5 performs at about the level of Fable 5.1 on most work while costing about 40% less than Opus 5 on typical workloads and producing output about 30% faster.

  • Price: $4 input / $20 output per million tokens (20% below Opus 5)
  • Availability: Claude Platform, AWS, Google Cloud and Microsoft Azure; API ID claude-opus-5-5
  • Reported benchmarks: Terminal-Bench 4.0 66.4%, FrontierCode v1.1 54.4%, OSWorld 2.1 81.8% partial success

Sonnet 5.5 (September 28)

Sonnet 5.5 is the fast, cheaper tier. Anthropic says it generates output more than 30% faster than Sonnet 5 and costs up to 30% less per task because it uses fewer tokens.

  • Price: $2 input / $10 output per million tokens
  • Availability: all platforms including AWS, Google Cloud and Azure; API ID claude-sonnet-5-5
  • Reported benchmarks: Terminal-Bench 4.0 70.6% (Sonnet 5: 10.3%), CursorBench 4.0 55.5% (Sonnet 5: 34.1%)

Anthropic positions Sonnet 5.5 for well-scoped everyday tasks and Opus 5.5 for complex judgment work.

Why it matters

Sonnet 5.5's price is the same as the reported price of OpenAI's GPT-6 Sol, released six days earlier. That puts the two mid-tier models in direct price competition.

Sources

This article was drafted with AI assistance and checked against the sources above. Benchmark figures are Anthropic's own.