Claude Opus 5.5
Anthropic's recommended default for long-running agentic coding and knowledge work — Fable 5.1-level performance on most tasks at Opus pricing, and the first release of the 5.5 wave
Claude Opus 5.5
Anthropic • September 2026
Training Data
Up to June 2026
Claude Opus 5.5
September 2026
Parameters
Not disclosed
Training Method
Advanced Constitutional AI with always-on adaptive thinking (medium effort by default) and preserved-thinking anti-distillation safeguards
Context Window
1,000,000 tokens
Knowledge Cutoff
June 2026
Key Features
Fable-level Work at ~40% Lower Cost • 30%+ Faster Output • 1M Context / 128K Output
Capabilities
Agentic Coding: Outstanding (66.4% Terminal-Bench 4.0)
Knowledge Work: Outstanding (1846 GDPval-AA Elo)
Alignment: Outstanding
What's New in This Version
Surpasses Fable 5.1 on Terminal-Bench 4.0 (66.4% vs 55.8%) and GDPval-AA v2.1 (1846 vs 1735 Elo) while costing about 40% less to run than Opus 5 ($4/$20 per 1M tokens, cache reads 60% cheaper) and generating output over 30% faster; Anthropic's strongest result yet on its automated behavioral audit
Anthropic's recommended default for long-running agentic coding and knowledge work — Fable 5.1-level performance on most tasks at Opus pricing, and the first release of the 5.5 wave
What's New in This Version
Surpasses Fable 5.1 on Terminal-Bench 4.0 (66.4% vs 55.8%) and GDPval-AA v2.1 (1846 vs 1735 Elo) while costing about 40% less to run than Opus 5 ($4/$20 per 1M tokens, cache reads 60% cheaper) and generating output over 30% faster; Anthropic's strongest result yet on its automated behavioral audit
Technical Specifications
Key Features
Capabilities
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