Claude Opus 5.5 (Adaptive Reasoning, Max Effort, Default Fallback)

Claude Opus 5.5 is the first model in Anthropic's Claude 5.5 family. It performs at Claude Fable 5.1 level on most work at about 40% lower cost than Claude Opus 5, and targets long-running agentic coding and knowledge work. It accepts text and image input with text output, supports multilingual and vision workloads, tool use, and adaptive thinking that is always on (API default effort medium). Context window is 1M tokens with 128K max output on the sync Messages API; reliable knowledge and training-data cutoffs are June 2026. First-party pricing is $4/$20 per million input/output tokens, with cache reads at $0.20 and 5-minute cache writes at $5 per million tokens. Fast mode is available in Claude Code and on the Claude Platform at $8/$40 per million input/output tokens with up to 2.5x speed (same model id; not a separate deployment). Capability scores are self-reported from the Claude Opus 5.5 System Card (Table 8.1.A and §8) and the launch announcement. Available on the Claude API as `claude-opus-5-5`.